ASP.NET Core Blazor file uploads

This article explains how to upload files in Blazor with the InputFile component.

File uploads

Warning

Always follow security best practices when permitting users to upload files. For more information, see Upload files in ASP.NET Core.

Use the InputFile component to read browser file data into .NET code. The InputFile component renders an HTML <input> element of type file for single file uploads. Add the multiple attribute to permit the user to upload multiple files at once.

File selection isn't cumulative when using an InputFile component or its underlying HTML <input type="file">, so you can't add files to an existing file selection. The component always replaces the user's initial file selection, so file references from prior selections aren't available.

The following InputFile component executes the LoadFiles method when the OnChange (change) event occurs. An InputFileChangeEventArgs provides access to the selected file list and details about each file:

<InputFile OnChange="LoadFiles" multiple />

@code {
    private void LoadFiles(InputFileChangeEventArgs e)
    {
        ...
    }
}

Rendered HTML:

<input multiple="" type="file" _bl_2="">

Note

In the preceding example, the <input> element's _bl_2 attribute is used for Blazor's internal processing.

To read data from a user-selected file, call IBrowserFile.OpenReadStream on the file and read from the returned stream. For more information, see the File streams section.

OpenReadStream enforces a maximum size in bytes of its Stream. Reading one file or multiple files larger than 500 KB results in an exception. This limit prevents developers from accidentally reading large files into memory. The maxAllowedSize parameter of OpenReadStream can be used to specify a larger size if required.

If you need access to a Stream that represents the file's bytes, use IBrowserFile.OpenReadStream. Avoid reading the incoming file stream directly into memory all at once. For example, don't copy all of the file's bytes into a MemoryStream or read the entire stream into a byte array all at once. These approaches can result in degraded app performance and potential Denial of Service (DoS) risk, especially for server-side components. Instead, consider adopting either of the following approaches:

• Copy the stream directly to a file on disk without reading it into memory. Note that Blazor apps executing code on the server aren't able to access the client's file system directly.
• Upload files from the client directly to an external service. For more information, see the Upload files to an external service section.

In the following examples, browserFile represents the uploaded file and implements IBrowserFile. Working implementations for IBrowserFile are shown in the file upload components later in this article.

✔️Supported: The following approach is recommended because the file's Stream is provided directly to the consumer, a FileStream that creates the file at the provided path:

await using FileStream fs = new(path, FileMode.Create);
await browserFile.OpenReadStream().CopyToAsync(fs);

✔️Supported: The following approach is recommended for Microsoft Azure Blob Storage because the file's Stream is provided directly to UploadBlobAsync:

await blobContainerClient.UploadBlobAsync(
    trustedFileName, browserFile.OpenReadStream());

❌Not recommended: The following approach is NOT recommended because the file's Stream content is read into a String in memory (reader):

var reader = 
    await new StreamReader(browserFile.OpenReadStream()).ReadToEndAsync();

❌Not recommended: The following approach is NOT recommended for Microsoft Azure Blob Storage because the file's Stream content is copied into a MemoryStream in memory (memoryStream) before calling UploadBlobAsync:

var memoryStream = new MemoryStream();
await browserFile.OpenReadStream().CopyToAsync(memoryStream);
await blobContainerClient.UploadBlobAsync(
    trustedFileName, memoryStream));

A component that receives an image file can call the BrowserFileExtensions.RequestImageFileAsync convenience method on the file to resize the image data within the browser's JavaScript runtime before the image is streamed into the app. Use cases for calling RequestImageFileAsync are most appropriate for Blazor WebAssembly apps.

File size read and upload limits

Server-side or client-side, there's no file read or upload size limit specifically for the InputFile component. However, client-side Blazor reads the file's bytes into a single JavaScript array buffer when marshalling the data from JavaScript to C#, which is limited to 2 GB or to the device's available memory. Large file uploads (> 250 MB) may fail for client-side uploads using the InputFile component. For more information, see the following discussions:

• The Blazor InputFile Component should handle chunking when the file is uploaded (dotnet/runtime #84685)
• Request Streaming upload via http handler (dotnet/runtime #36634)

For large client-side file uploads that fail when attempting to use the InputFile component, we recommend chunking large files with a custom component using multiple HTTP range requests instead of using the InputFile component.

Work is currently scheduled for .NET 9 (late 2024) to address the client-side file size upload limitation.

Examples

The following examples demonstrate multiple file upload in a component. InputFileChangeEventArgs.GetMultipleFiles allows reading multiple files. Specify the maximum number of files to prevent a malicious user from uploading a larger number of files than the app expects. InputFileChangeEventArgs.File allows reading the first and only file if the file upload doesn't support multiple files.

InputFileChangeEventArgs is in the Microsoft.AspNetCore.Components.Forms namespace, which is typically one of the namespaces in the app's _Imports.razor file. When the namespace is present in the _Imports.razor file, it provides API member access to the app's components.

Namespaces in the _Imports.razor file aren't applied to C# files (.cs). C# files require an explicit using directive at the top of the class file:

using Microsoft.AspNetCore.Components.Forms;

For testing file upload components, you can create test files of any size with PowerShell:

$out = new-object byte[] {SIZE}; (new-object Random).NextBytes($out); [IO.File]::WriteAllBytes('{PATH}', $out)

In the preceding command:

• The {SIZE} placeholder is the size of the file in bytes (for example, 2097152 for a 2 MB file).
• The {PATH} placeholder is the path and file with file extension (for example, D:/test_files/testfile2MB.txt).

Server-side file upload example

To use the following code, create a Development/unsafe_uploads folder at the root of the app running in the Development environment.

Because the example uses the app's environment as part of the path where files are saved, additional folders are required if other environments are used in testing and production. For example, create a Staging/unsafe_uploads folder for the Staging environment. Create a Production/unsafe_uploads folder for the Production environment.

Warning

The example saves files without scanning their contents, and the guidance in this article doesn't take into account additional security best practices for uploaded files. On staging and production systems, disable execute permission on the upload folder and scan files with an anti-virus/anti-malware scanner API immediately after upload. For more information, see Upload files in ASP.NET Core.

FileUpload1.razor:

@page "/file-upload-1"
@using System 
@using System.IO
@using Microsoft.AspNetCore.Hosting
@inject ILogger<FileUpload1> Logger
@inject IWebHostEnvironment Environment

<PageTitle>File Upload 1</PageTitle>

<h1>File Upload Example 1</h1>

<p>
    <label>
        Max file size:
        <input type="number" @bind="maxFileSize" />
    </label>
</p>

<p>
    <label>
        Max allowed files:
        <input type="number" @bind="maxAllowedFiles" />
    </label>
</p>

<p>
    <label>
        Upload up to @maxAllowedFiles of up to @maxFileSize bytes:
        <InputFile OnChange="LoadFiles" multiple />
    </label>
</p>

@if (isLoading)
{
    <p>Uploading...</p>
}
else
{
    <ul>
        @foreach (var file in loadedFiles)
        {
            <li>
                <ul>
                    <li>Name: @file.Name</li>
                    <li>Last modified: @file.LastModified.ToString()</li>
                    <li>Size (bytes): @file.Size</li>
                    <li>Content type: @file.ContentType</li>
                </ul>
            </li>
        }
    </ul>
}

@code {
    private List<IBrowserFile> loadedFiles = new();
    private long maxFileSize = 1024 * 15;
    private int maxAllowedFiles = 3;
    private bool isLoading;

    private async Task LoadFiles(InputFileChangeEventArgs e)
    {
        isLoading = true;
        loadedFiles.Clear();

        foreach (var file in e.GetMultipleFiles(maxAllowedFiles))
        {
            try
            {
                var trustedFileName = Path.GetRandomFileName();
                var path = Path.Combine(Environment.ContentRootPath,
                    Environment.EnvironmentName, "unsafe_uploads",
                    trustedFileName);

                await using FileStream fs = new(path, FileMode.Create);
                await file.OpenReadStream(maxFileSize).CopyToAsync(fs);

                loadedFiles.Add(file);

                Logger.LogInformation(
                    "Unsafe Filename: {UnsafeFilename} File saved: {Filename}",
                    file.Name, trustedFileName);
            }
            catch (Exception ex)
            {
                Logger.LogError("File: {Filename} Error: {Error}", 
                    file.Name, ex.Message);
            }
        }

        isLoading = false;
    }
}

Client-side file upload example

The following example processes file bytes and doesn't send files to a destination outside of the app. For an example of a Razor component that sends a file to a server or service, see the following sections:

• Upload files to a server with client-side rendering (CSR)
• Upload files to an external service

The component assumes that the Interactive WebAssembly render mode (InteractiveWebAssembly) is inherited from a parent component or applied globally to the app.

@page "/file-upload-1"
@inject ILogger<FileUpload1> Logger

<PageTitle>File Upload 1</PageTitle>

<h1>File Upload Example 1</h1>

<p>
    <label>
        Max file size:
        <input type="number" @bind="maxFileSize" />
    </label>
</p>

<p>
    <label>
        Max allowed files:
        <input type="number" @bind="maxAllowedFiles" />
    </label>
</p>

<p>
    <label>
        Upload up to @maxAllowedFiles of up to @maxFileSize bytes:
        <InputFile OnChange="LoadFiles" multiple />
    </label>
</p>

@if (isLoading)
{
    <p>Uploading...</p>
}
else
{
    <ul>
        @foreach (var file in loadedFiles)
        {
            <li>
                <ul>
                    <li>Name: @file.Name</li>
                    <li>Last modified: @file.LastModified.ToString()</li>
                    <li>Size (bytes): @file.Size</li>
                    <li>Content type: @file.ContentType</li>
                </ul>
            </li>
        }
    </ul>
}

@code {
    private List<IBrowserFile> loadedFiles = new();
    private long maxFileSize = 1024 * 15;
    private int maxAllowedFiles = 3;
    private bool isLoading;

    private void LoadFiles(InputFileChangeEventArgs e)
    {
        isLoading = true;
        loadedFiles.Clear();

        foreach (var file in e.GetMultipleFiles(maxAllowedFiles))
        {
            try
            {
                loadedFiles.Add(file);
            }
            catch (Exception ex)
            {
                Logger.LogError("File: {FileName} Error: {Error}", 
                    file.Name, ex.Message);
            }
        }

        isLoading = false;
    }
}

IBrowserFile returns metadata exposed by the browser as properties. Use this metadata for preliminary validation.

• Name
• Size
• LastModified
• ContentType

Never trust the values of the preceding properties, especially the Name property for display in the UI. Treat all user-supplied data as a significant security risk to the app, server, and network. For more information, see Upload files in ASP.NET Core.

Upload files to a server with server-side rendering

This section applies to Interactive Server components in Blazor Web Apps or Blazor Server apps.

The following example demonstrates uploading files from a server-side app to a backend web API controller in a separate app, possibly on a separate server.

In the server-side app's Program file, add IHttpClientFactory and related services that allow the app to create HttpClient instances:

builder.Services.AddHttpClient();

For more information, see Make HTTP requests using IHttpClientFactory in ASP.NET Core.

For the examples in this section:

• The web API runs at the URL: https://localhost:5001
• The server-side app runs at the URL: https://localhost:5003

For testing, the preceding URLs are configured in the projects' Properties/launchSettings.json files.

The following UploadResult class maintains the result of an uploaded file. When a file fails to upload on the server, an error code is returned in ErrorCode for display to the user. A safe file name is generated on the server for each file and returned to the client in StoredFileName for display. Files are keyed between the client and server using the unsafe/untrusted file name in FileName.

UploadResult.cs:

public class UploadResult
{
    public bool Uploaded { get; set; }
    public string? FileName { get; set; }
    public string? StoredFileName { get; set; }
    public int ErrorCode { get; set; }
}

A security best practice for production apps is to avoid sending error messages to clients that might reveal sensitive information about an app, server, or network. Providing detailed error messages can aid a malicious user in devising attacks on an app, server, or network. The example code in this section only sends back an error code number (int) for display by the component client-side if a server-side error occurs. If a user requires assistance with a file upload, they provide the error code to support personnel for support ticket resolution without ever knowing the exact cause of the error.

The following LazyBrowserFileStream class defines a custom stream type that lazily calls OpenReadStream just before the first bytes of the stream are requested. The stream isn't transmitted from the browser to the server until reading the stream begins in .NET.

LazyBrowserFileStream.cs:

using Microsoft.AspNetCore.Components.Forms;
using System.Diagnostics.CodeAnalysis;

namespace BlazorSample;

internal sealed class LazyBrowserFileStream(IBrowserFile file, int maxAllowedSize) 
    : Stream
{
    private readonly IBrowserFile file = file;
    private readonly int maxAllowedSize = maxAllowedSize;
    private Stream? underlyingStream;
    private bool isDisposed;

    public override bool CanRead => true;

    public override bool CanSeek => false;

    public override bool CanWrite => false;

    public override long Length => file.Size;

    public override long Position
    {
        get => underlyingStream?.Position ?? 0;
        set => throw new NotSupportedException();
    }

    public override void Flush() => underlyingStream?.Flush();

    public override Task<int> ReadAsync(byte[] buffer, int offset, int count, 
        CancellationToken cancellationToken)
    {
        EnsureStreamIsOpen();

        return underlyingStream.ReadAsync(buffer, offset, count, cancellationToken);
    }

    public override ValueTask<int> ReadAsync(Memory<byte> buffer, 
        CancellationToken cancellationToken = default)
    {
        EnsureStreamIsOpen();
        return underlyingStream.ReadAsync(buffer, cancellationToken);
    }

    [MemberNotNull(nameof(underlyingStream))]
    private void EnsureStreamIsOpen() => 
        underlyingStream ??= file.OpenReadStream(maxAllowedSize);

    protected override void Dispose(bool disposing)
    {
        if (isDisposed)
        {
            return;
        }

        underlyingStream?.Dispose();
        isDisposed = true;

        base.Dispose(disposing);
    }

    public override int Read(byte[] buffer, int offset, int count)
        => throw new NotSupportedException();

    public override long Seek(long offset, SeekOrigin origin)
        => throw new NotSupportedException();

    public override void SetLength(long value)
        => throw new NotSupportedException();

    public override void Write(byte[] buffer, int offset, int count)
        => throw new NotSupportedException();
}

The following FileUpload2 component:

• Permits users to upload files from the client.
• Displays the untrusted/unsafe file name provided by the client in the UI. The untrusted/unsafe file name is automatically HTML-encoded by Razor for safe display in the UI.

Warning

Don't trust file names supplied by clients for:

• Saving the file to a file system or service.
• Display in UIs that don't encode file names automatically or via developer code.

For more information on security considerations when uploading files to a server, see Upload files in ASP.NET Core.

FileUpload2.razor:

@page "/file-upload-2"
@using System.Net.Http.Headers
@using System.Text.Json
@inject IHttpClientFactory ClientFactory
@inject ILogger<FileUpload2> Logger

<PageTitle>File Upload 2</PageTitle>

<h1>File Upload Example 2</h1>

<p>
    This example requires a backend server API to function. For more information, 
    see the <em>Upload files to a server</em> section 
    of the <em>ASP.NET Core Blazor file uploads</em> article.
</p>

<p>
    <label>
        Upload up to @maxAllowedFiles files:
        <InputFile OnChange="OnInputFileChange" multiple />
    </label>
</p>

@if (files.Any())
{
    <div class="card">
        <div class="card-body">
            <ul>
                @foreach (var file in files)
                {
                    <li>
                        File: @file.Name
                        <br>
                        @if (FileUpload(uploadResults, file.Name, Logger,
                           out var result))
                        {
                            <span>
                                Stored File Name: @result.StoredFileName
                            </span>
                        }
                        else
                        {
                            <span>
                                There was an error uploading the file
                                (Error: @result.ErrorCode).
                            </span>
                        }
                    </li>
                }
            </ul>
        </div>
    </div>
}

@code {
    private List<File> files = new();
    private List<UploadResult> uploadResults = new();
    private int maxAllowedFiles = 3;
    private bool shouldRender;

    protected override bool ShouldRender() => shouldRender;

    private async Task OnInputFileChange(InputFileChangeEventArgs e)
    {
        shouldRender = false;
        int maxFileSize = 1024 * 15;
        var upload = false;

        using var content = new MultipartFormDataContent();

        foreach (var file in e.GetMultipleFiles(maxAllowedFiles))
        {
            if (uploadResults.SingleOrDefault(
                f => f.FileName == file.Name) is null)
            {
                try
                {
                    files.Add(new() { Name = file.Name });

                    var stream = new LazyBrowserFileStream(file, maxFileSize);
                    var fileContent = new StreamContent(stream);

                    fileContent.Headers.ContentType = 
                        new MediaTypeHeaderValue(file.ContentType);

                    content.Add(
                        content: fileContent,
                        name: "\"files\"",
                        fileName: file.Name);

                    upload = true;
                }
                catch (Exception ex)
                {
                    Logger.LogInformation(
                        "{FileName} not uploaded (Err: 6): {Message}", 
                        file.Name, ex.Message);

                    uploadResults.Add(
                        new()
                        {
                            FileName = file.Name, 
                            ErrorCode = 6, 
                            Uploaded = false
                        });
                }
            }
        }

        if (upload)
        {
            var client = ClientFactory.CreateClient();

            var response = 
                await client.PostAsync("https://localhost:5001/Filesave", 
                content);

            if (response.IsSuccessStatusCode)
            {
                var options = new JsonSerializerOptions
                {
                    PropertyNameCaseInsensitive = true,
                };

                using var responseStream =
                    await response.Content.ReadAsStreamAsync();

                var newUploadResults = await JsonSerializer
                    .DeserializeAsync<IList<UploadResult>>(responseStream, options);

                if (newUploadResults is not null)
                {
                    uploadResults = uploadResults.Concat(newUploadResults).ToList();
                }
            }
        }

        shouldRender = true;
    }

    private static bool FileUpload(IList<UploadResult> uploadResults,
        string? fileName, ILogger<FileUpload2> logger, out UploadResult result)
    {
        result = uploadResults.SingleOrDefault(f => f.FileName == fileName) ?? new();

        if (!result.Uploaded)
        {
            logger.LogInformation("{FileName} not uploaded (Err: 5)", fileName);
            result.ErrorCode = 5;
        }

        return result.Uploaded;
    }

    private class File
    {
        public string? Name { get; set; }
    }
}

If the component limits file uploads to a single file at a time or if the component only adopts interactive client-side rendering (CSR, InteractiveWebAssembly), the component can avoid the use of the LazyBrowserFileStream and use a Stream. The following demonstrates the changes for the FileUpload2 component:

- var stream = new LazyBrowserFileStream(file, maxFileSize);
- var fileContent = new StreamContent(stream);
+ var fileContent = new StreamContent(file.OpenReadStream(maxFileSize));

Remove the LazyBrowserFileStream class (LazyBrowserFileStream.cs), as it isn't used.

The following controller in the web API project saves uploaded files from the client.

Important

The controller in this section is intended for use in a separate web API project from the Blazor app. The web API should mitigate Cross-Site Request Forgery (XSRF/CSRF) attacks if file upload users are authenticated.

To use the following code, create a Development/unsafe_uploads folder at the root of the web API project for the app running in the Development environment.

Because the example uses the app's environment as part of the path where files are saved, additional folders are required if other environments are used in testing and production. For example, create a Staging/unsafe_uploads folder for the Staging environment. Create a Production/unsafe_uploads folder for the Production environment.

Warning

The example saves files without scanning their contents, and the guidance in this article doesn't take into account additional security best practices for uploaded files. On staging and production systems, disable execute permission on the upload folder and scan files with an anti-virus/anti-malware scanner API immediately after upload. For more information, see Upload files in ASP.NET Core.

Controllers/FilesaveController.cs:

using System.Net;
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("[controller]")]
public class FilesaveController(
    IHostEnvironment env, ILogger<FilesaveController> logger) 
    : ControllerBase
{
    [HttpPost]
    public async Task<ActionResult<IList<UploadResult>>> PostFile(
        [FromForm] IEnumerable<IFormFile> files)
    {
        var maxAllowedFiles = 3;
        long maxFileSize = 1024 * 15;
        var filesProcessed = 0;
        var resourcePath = new Uri($"{Request.Scheme}://{Request.Host}/");
        List<UploadResult> uploadResults = [];

        foreach (var file in files)
        {
            var uploadResult = new UploadResult();
            string trustedFileNameForFileStorage;
            var untrustedFileName = file.FileName;
            uploadResult.FileName = untrustedFileName;
            var trustedFileNameForDisplay =
                WebUtility.HtmlEncode(untrustedFileName);

            if (filesProcessed < maxAllowedFiles)
            {
                if (file.Length == 0)
                {
                    logger.LogInformation("{FileName} length is 0 (Err: 1)",
                        trustedFileNameForDisplay);
                    uploadResult.ErrorCode = 1;
                }
                else if (file.Length > maxFileSize)
                {
                    logger.LogInformation("{FileName} of {Length} bytes is " +
                        "larger than the limit of {Limit} bytes (Err: 2)",
                        trustedFileNameForDisplay, file.Length, maxFileSize);
                    uploadResult.ErrorCode = 2;
                }
                else
                {
                    try
                    {
                        trustedFileNameForFileStorage = Path.GetRandomFileName();
                        var path = Path.Combine(env.ContentRootPath,
                            env.EnvironmentName, "unsafe_uploads",
                            trustedFileNameForFileStorage);

                        await using FileStream fs = new(path, FileMode.Create);
                        await file.CopyToAsync(fs);

                        logger.LogInformation("{FileName} saved at {Path}",
                            trustedFileNameForDisplay, path);
                        uploadResult.Uploaded = true;
                        uploadResult.StoredFileName = trustedFileNameForFileStorage;
                    }
                    catch (IOException ex)
                    {
                        logger.LogError("{FileName} error on upload (Err: 3): {Message}",
                            trustedFileNameForDisplay, ex.Message);
                        uploadResult.ErrorCode = 3;
                    }
                }

                filesProcessed++;
            }
            else
            {
                logger.LogInformation("{FileName} not uploaded because the " +
                    "request exceeded the allowed {Count} of files (Err: 4)",
                    trustedFileNameForDisplay, maxAllowedFiles);
                uploadResult.ErrorCode = 4;
            }

            uploadResults.Add(uploadResult);
        }

        return new CreatedResult(resourcePath, uploadResults);
    }
}

In the preceding code, GetRandomFileName is called to generate a secure file name. Never trust the file name provided by the browser, as a cyberattacker may choose an existing file name that overwrites an existing file or send a path that attempts to write outside of the app.

The server app must register controller services and map controller endpoints. For more information, see Routing to controller actions in ASP.NET Core.

Upload files to a server with client-side rendering (CSR)

This section applies to client-side rendered (CSR) components in Blazor Web Apps or Blazor WebAssembly apps.

The following example demonstrates uploading files to a backend web API controller in a separate app, possibly on a separate server, from a component in a Blazor Web App that adopts CSR or a component in a Blazor WebAssembly app.

The following UploadResult class maintains the result of an uploaded file. When a file fails to upload on the server, an error code is returned in ErrorCode for display to the user. A safe file name is generated on the server for each file and returned to the client in StoredFileName for display. Files are keyed between the client and server using the unsafe/untrusted file name in FileName.

UploadResult.cs:

public class UploadResult
{
    public bool Uploaded { get; set; }
    public string? FileName { get; set; }
    public string? StoredFileName { get; set; }
    public int ErrorCode { get; set; }
}

Note

The preceding UploadResult class can be shared between client- and server-based projects. When client and server projects share the class, add an import to each project's _Imports.razor file for the shared project. For example:

@using BlazorSample.Shared

The following FileUpload2 component:

• Permits users to upload files from the client.
• Displays the untrusted/unsafe file name provided by the client in the UI. The untrusted/unsafe file name is automatically HTML-encoded by Razor for safe display in the UI.

A security best practice for production apps is to avoid sending error messages to clients that might reveal sensitive information about an app, server, or network. Providing detailed error messages can aid a malicious user in devising attacks on an app, server, or network. The example code in this section only sends back an error code number (int) for display by the component client-side if a server-side error occurs. If a user requires assistance with a file upload, they provide the error code to support personnel for support ticket resolution without ever knowing the exact cause of the error.

Warning

Don't trust file names supplied by clients for:

• Saving the file to a file system or service.
• Display in UIs that don't encode file names automatically or via developer code.

For more information on security considerations when uploading files to a server, see Upload files in ASP.NET Core.

In the Blazor Web App main project, add IHttpClientFactory and related services in the project's Program file:

builder.Services.AddHttpClient();

The HttpClient services must be added to the main project because the client-side component is prerendered on the server. If you disable prerendering for the following component, you aren't required to provide the HttpClient services in the main app and don't need to add the preceding line to the main project.

For more information on adding HttpClient services to an ASP.NET Core app, see Make HTTP requests using IHttpClientFactory in ASP.NET Core.

The client project (.Client) of a Blazor Web App must also register an HttpClient for HTTP POST requests to a backend web API controller. Confirm or add the following to the client project's Program file:

builder.Services.AddScoped(sp => 
    new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });

The preceding example sets the base address with builder.HostEnvironment.BaseAddress (IWebAssemblyHostEnvironment.BaseAddress), which gets the base address for the app and is typically derived from the <base> tag's href value in the host page. If you're calling an external web API, set the URI to the web API's base address.

Specify the Interactive WebAssembly render mode attribute at the top of the following component in a Blazor Web App:

@rendermode InteractiveWebAssembly

FileUpload2.razor:

@page "/file-upload-2"
@using System.Linq
@using System.Net.Http.Headers
@inject HttpClient Http
@inject ILogger<FileUpload2> Logger

<PageTitle>File Upload 2</PageTitle>

<h1>File Upload Example 2</h1>

<p>
    <label>
        Upload up to @maxAllowedFiles files:
        <InputFile OnChange="OnInputFileChange" multiple />
    </label>
</p>

@if (files.Count > 0)
{
    <div class="card">
        <div class="card-body">
            <ul>
                @foreach (var file in files)
                {
                    <li>
                        File: @file.Name
                        <br>
                        @if (FileUpload(uploadResults, file.Name, Logger,
                           out var result))
                        {
                            <span>
                                Stored File Name: @result.StoredFileName
                            </span>
                        }
                        else
                        {
                            <span>
                                There was an error uploading the file
                                (Error: @result.ErrorCode).
                            </span>
                        }
                    </li>
                }
            </ul>
        </div>
    </div>
}

@code {
    private List<File> files = new();
    private List<UploadResult> uploadResults = new();
    private int maxAllowedFiles = 3;
    private bool shouldRender;

    protected override bool ShouldRender() => shouldRender;

    private async Task OnInputFileChange(InputFileChangeEventArgs e)
    {
        shouldRender = false;
        long maxFileSize = 1024 * 15;
        var upload = false;

        using var content = new MultipartFormDataContent();

        foreach (var file in e.GetMultipleFiles(maxAllowedFiles))
        {
            if (uploadResults.SingleOrDefault(
                f => f.FileName == file.Name) is null)
            {
                try
                {
                    files.Add(new() { Name = file.Name });

                    var fileContent = new StreamContent(file.OpenReadStream(maxFileSize));

                    fileContent.Headers.ContentType = 
                        new MediaTypeHeaderValue(file.ContentType);

                    content.Add(
                        content: fileContent,
                        name: "\"files\"",
                        fileName: file.Name);

                    upload = true;
                }
                catch (Exception ex)
                {
                    Logger.LogInformation(
                        "{FileName} not uploaded (Err: 6): {Message}", 
                        file.Name, ex.Message);

                    uploadResults.Add(
                        new()
                        {
                            FileName = file.Name, 
                            ErrorCode = 6, 
                            Uploaded = false
                        });
                }
            }
        }

        if (upload)
        {
            var response = await Http.PostAsync("/Filesave", content);

            var newUploadResults = await response.Content
                .ReadFromJsonAsync<IList<UploadResult>>();

            if (newUploadResults is not null)
            {
                uploadResults = uploadResults.Concat(newUploadResults).ToList();
            }
        }

        shouldRender = true;
    }

    private static bool FileUpload(IList<UploadResult> uploadResults,
        string? fileName, ILogger<FileUpload2> logger, out UploadResult result)
    {
        result = uploadResults.SingleOrDefault(f => f.FileName == fileName) ?? new();

        if (!result.Uploaded)
        {
            logger.LogInformation("{FileName} not uploaded (Err: 5)", fileName);
            result.ErrorCode = 5;
        }

        return result.Uploaded;
    }

    private class File
    {
        public string? Name { get; set; }
    }
}

The following controller in the server-side project saves uploaded files from the client.

To use the following code, create a Development/unsafe_uploads folder at the root of the server-side project for the app running in the Development environment.

Because the example uses the app's environment as part of the path where files are saved, additional folders are required if other environments are used in testing and production. For example, create a Staging/unsafe_uploads folder for the Staging environment. Create a Production/unsafe_uploads folder for the Production environment.

Warning

The example saves files without scanning their contents, and the guidance in this article doesn't take into account additional security best practices for uploaded files. On staging and production systems, disable execute permission on the upload folder and scan files with an anti-virus/anti-malware scanner API immediately after upload. For more information, see Upload files in ASP.NET Core.

In the following example, update the shared project's namespace to match the shared project if a shared project is supplying the UploadResult class.

Controllers/FilesaveController.cs:

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using BlazorSample.Shared;

[ApiController]
[Route("[controller]")]
public class FilesaveController(
    IHostEnvironment env, ILogger<FilesaveController> logger) 
    : ControllerBase
{
    [HttpPost]
    public async Task<ActionResult<IList<UploadResult>>> PostFile(
        [FromForm] IEnumerable<IFormFile> files)
    {
        var maxAllowedFiles = 3;
        long maxFileSize = 1024 * 15;
        var filesProcessed = 0;
        var resourcePath = new Uri($"{Request.Scheme}://{Request.Host}/");
        List<UploadResult> uploadResults = [];

        foreach (var file in files)
        {
            var uploadResult = new UploadResult();
            string trustedFileNameForFileStorage;
            var untrustedFileName = file.FileName;
            uploadResult.FileName = untrustedFileName;
            var trustedFileNameForDisplay =
                WebUtility.HtmlEncode(untrustedFileName);

            if (filesProcessed < maxAllowedFiles)
            {
                if (file.Length == 0)
                {
                    logger.LogInformation("{FileName} length is 0 (Err: 1)",
                        trustedFileNameForDisplay);
                    uploadResult.ErrorCode = 1;
                }
                else if (file.Length > maxFileSize)
                {
                    logger.LogInformation("{FileName} of {Length} bytes is " +
                        "larger than the limit of {Limit} bytes (Err: 2)",
                        trustedFileNameForDisplay, file.Length, maxFileSize);
                    uploadResult.ErrorCode = 2;
                }
                else
                {
                    try
                    {
                        trustedFileNameForFileStorage = Path.GetRandomFileName();
                        var path = Path.Combine(env.ContentRootPath,
                            env.EnvironmentName, "unsafe_uploads",
                            trustedFileNameForFileStorage);

                        await using FileStream fs = new(path, FileMode.Create);
                        await file.CopyToAsync(fs);

                        logger.LogInformation("{FileName} saved at {Path}",
                            trustedFileNameForDisplay, path);
                        uploadResult.Uploaded = true;
                        uploadResult.StoredFileName = trustedFileNameForFileStorage;
                    }
                    catch (IOException ex)
                    {
                        logger.LogError("{FileName} error on upload (Err: 3): {Message}",
                            trustedFileNameForDisplay, ex.Message);
                        uploadResult.ErrorCode = 3;
                    }
                }

                filesProcessed++;
            }
            else
            {
                logger.LogInformation("{FileName} not uploaded because the " +
                    "request exceeded the allowed {Count} of files (Err: 4)",
                    trustedFileNameForDisplay, maxAllowedFiles);
                uploadResult.ErrorCode = 4;
            }

            uploadResults.Add(uploadResult);
        }

        return new CreatedResult(resourcePath, uploadResults);
    }
}

In the preceding code, GetRandomFileName is called to generate a secure file name. Never trust the file name provided by the browser, as a cyberattacker may choose an existing file name that overwrites an existing file or send a path that attempts to write outside of the app.

The server app must register controller services and map controller endpoints. For more information, see Routing to controller actions in ASP.NET Core.

Cancel a file upload

A file upload component can detect when a user has cancelled an upload by using a CancellationToken when calling into the IBrowserFile.OpenReadStream or StreamReader.ReadAsync.

Create a CancellationTokenSource for the InputFile component. At the start of the OnInputFileChange method, check if a previous upload is in progress.

If a file upload is in progress:

• Call Cancel on the previous upload.
• Create a new CancellationTokenSource for the next upload and pass the CancellationTokenSource.Token to OpenReadStream or ReadAsync.

Upload files server-side with progress

The following example demonstrates how to upload files in a server-side app with upload progress displayed to the user.

To use the following example in a test app:

• Create a folder to save uploaded files for the Development environment: Development/unsafe_uploads.
• Configure the maximum file size (maxFileSize, 15 KB in the following example) and maximum number of allowed files (maxAllowedFiles, 3 in the following example).
• Set the buffer to a different value (10 KB in the following example), if desired, for increased granularity in progress reporting. We don't recommended using a buffer larger than 30 KB due to performance and security concerns.

Warning

The example saves files without scanning their contents, and the guidance in this article doesn't take into account additional security best practices for uploaded files. On staging and production systems, disable execute permission on the upload folder and scan files with an anti-virus/anti-malware scanner API immediately after upload. For more information, see Upload files in ASP.NET Core.

FileUpload3.razor:

@page "/file-upload-3"
@inject ILogger<FileUpload3> Logger
@inject IWebHostEnvironment Environment

<PageTitle>File Upload 3</PageTitle>

<h1>File Upload Example 3</h1>

<p>
    <label>
        Max file size:
        <input type="number" @bind="maxFileSize" />
    </label>
</p>

<p>
    <label>
        Max allowed files:
        <input type="number" @bind="maxAllowedFiles" />
    </label>
</p>

<p>
    <label>
        Upload up to @maxAllowedFiles of up to @maxFileSize bytes:
        <InputFile OnChange="LoadFiles" multiple />
    </label>
</p>

@if (isLoading)
{
    <p>Progress: @string.Format("{0:P0}", progressPercent)</p>
}
else
{
    <ul>
        @foreach (var file in loadedFiles)
        {
            <li>
                <ul>
                    <li>Name: @file.Name</li>
                    <li>Last modified: @file.LastModified.ToString()</li>
                    <li>Size (bytes): @file.Size</li>
                    <li>Content type: @file.ContentType</li>
                </ul>
            </li>
        }
    </ul>
}

@code {
    private List<IBrowserFile> loadedFiles = new();
    private long maxFileSize = 1024 * 15;
    private int maxAllowedFiles = 3;
    private bool isLoading;
    private decimal progressPercent;

    private async Task LoadFiles(InputFileChangeEventArgs e)
    {
        isLoading = true;
        loadedFiles.Clear();
        progressPercent = 0;

        foreach (var file in e.GetMultipleFiles(maxAllowedFiles))
        {
            try
            {
                var trustedFileName = Path.GetRandomFileName();
                var path = Path.Combine(Environment.ContentRootPath,
                    Environment.EnvironmentName, "unsafe_uploads", trustedFileName);

                await using FileStream writeStream = new(path, FileMode.Create);
                using var readStream = file.OpenReadStream(maxFileSize);
                var bytesRead = 0;
                var totalRead = 0;
                var buffer = new byte[1024 * 10];

                while ((bytesRead = await readStream.ReadAsync(buffer)) != 0)
                {
                    totalRead += bytesRead;
                    await writeStream.WriteAsync(buffer, 0, bytesRead);
                    progressPercent = Decimal.Divide(totalRead, file.Size);
                    StateHasChanged();
                }

                loadedFiles.Add(file);

                Logger.LogInformation(
                    "Unsafe Filename: {UnsafeFilename} File saved: {Filename}",
                    file.Name, trustedFileName);
            }
            catch (Exception ex)
            {
                Logger.LogError("File: {FileName} Error: {Error}", 
                    file.Name, ex.Message);
            }
        }

        isLoading = false;
    }
}

For more information, see the following API resources:

• FileStream: Provides a Stream for a file, supporting both synchronous and asynchronous read and write operations.
• FileStream.ReadAsync: The preceding FileUpload3 component reads the stream asynchronously with ReadAsync. Reading a stream synchronously with Read isn't supported in Razor components.

File streams

With server interactivity, file data is streamed over the SignalR connection into .NET code on the server as the file is read.

For a WebAssembly-rendered component, file data is streamed directly into the .NET code within the browser.

Upload image preview

For an image preview of uploading images, start by adding an InputFile component with a component reference and an OnChange handler:

<InputFile @ref="inputFile" OnChange="ShowPreview" />

Add an image element with an element reference, which serves as the placeholder for the image preview:

<img @ref="previewImageElem" />

Add the associated references:

@code {
    private InputFile? inputFile;
    private ElementReference previewImageElem;
}

In JavaScript, add a function called with an HTML input and img element that performs the following:

• Extracts the selected file.
• Creates an object URL with createObjectURL.
• Sets an event listener to revoke the object URL with revokeObjectURL after the image is loaded, so memory isn't leaked.
• Sets the img element's source to display the image.

window.previewImage = (inputElem, imgElem) => {
  const url = URL.createObjectURL(inputElem.files[0]);
  imgElem.addEventListener('load', () => URL.revokeObjectURL(url), { once: true });
  imgElem.src = url;
}

Finally, use an injected IJSRuntime to add the OnChange handler that calls the JavaScript function:

@inject IJSRuntime JS

...

@code {
    ...

    private async Task ShowPreview() => await JS.InvokeVoidAsync(
        "previewImage", inputFile!.Element, previewImageElem);
}

The preceding example is for uploading a single image. The approach can be expanded to support multiple images.

The following FileUpload4 component shows the complete example.

FileUpload4.razor:

@page "/file-upload-4"
@inject IJSRuntime JS

<h1>File Upload Example</h1>

<InputFile @ref="inputFile" OnChange="ShowPreview" />

<img style="max-width:200px;max-height:200px" @ref="previewImageElem" />

@code {
    private InputFile? inputFile;
    private ElementReference previewImageElem;

    private async Task ShowPreview() => await JS.InvokeVoidAsync(
        "previewImage", inputFile!.Element, previewImageElem);
}

Upload files to an external service

Instead of an app handling file upload bytes and the app's server receiving uploaded files, clients can directly upload files to an external service. The app can safely process the files from the external service on demand. This approach hardens the app and its server against malicious attacks and potential performance problems.

Consider an approach that uses Azure Files, Azure Blob Storage, or a third-party service with the following potential benefits:

• Upload files from the client directly to an external service with a JavaScript client library or REST API. For example, Azure offers the following client libraries and APIs:
  • Azure Storage File Share client library
  • Azure Files REST API
  • Azure Storage Blob client library for JavaScript
  • Blob service REST API
• Authorize user uploads with a user-delegated shared-access signature (SAS) token generated by the app (server-side) for each client file upload. For example, Azure offers the following SAS features:
  • Azure Storage File Share client library for JavaScript: with SAS Token
  • Azure Storage Blob client library for JavaScript: with SAS Token
• Provide automatic redundancy and file share backup.
• Limit uploads with quotas. Note that Azure Blob Storage's quotas are set at the account level, not the container level. However, Azure Files quotas are at the file share level and might provide better control over upload limits. For more information, see the Azure documents linked earlier in this list.
• Secure files with server-side encryption (SSE).

For more information on Azure Blob Storage and Azure Files, see the Azure Storage documentation.

Server-side SignalR message size limit

File uploads may fail even before they start, when Blazor retrieves data about the files that exceeds the maximum SignalR message size.

SignalR defines a message size limit that applies to every message Blazor receives, and the InputFile component streams files to the server in messages that respect the configured limit. However, the first message, which indicates the set of files to upload, is sent as a unique single message. The size of the first message may exceed the SignalR message size limit. The issue isn't related to the size of the files, it's related to the number of files.

The logged error is similar to the following:

Error: Connection disconnected with error 'Error: Server returned an error on close: Connection closed with an error.'. e.log @ blazor.server.js:1

When uploading files, reaching the message size limit on the first message is rare. If the limit is reached, the app can configure HubOptions.MaximumReceiveMessageSize with a larger value.

For more information on SignalR configuration and how to set MaximumReceiveMessageSize, see ASP.NET Core Blazor SignalR guidance.

Maximum parallel invocations per client hub setting

Blazor relies on MaximumParallelInvocationsPerClient set to 1, which is the default value.

Increasing the value leads to a high probability that CopyTo operations throw System.InvalidOperationException: 'Reading is not allowed after reader was completed.'. For more information, see MaximumParallelInvocationsPerClient > 1 breaks file upload in Blazor Server mode (dotnet/aspnetcore #53951).

Troubleshoot

The line that calls IBrowserFile.OpenReadStream throws a System.TimeoutException:

System.TimeoutException: Did not receive any data in the allotted time.

Possible causes:

• Using the Autofac Inversion of Control (IoC) container instead of the built-in ASP.NET Core dependency injection container. To resolve the issue, set DisableImplicitFromServicesParameters to true in the server-side circuit handler hub options. For more information, see FileUpload: Did not receive any data in the allotted time (dotnet/aspnetcore #38842).

• Not reading the stream to completion. This isn't a framework issue. Trap the exception and investigate it further in your local environment/network.

• Using server-side rendering and calling OpenReadStream on multiple files before reading them to completion. To resolve the issue, use the LazyBrowserFileStream class and approach described in the Upload files to a server with server-side rendering section of this article.

Additional resources

• ASP.NET Core Blazor file downloads
• Upload files in ASP.NET Core
• ASP.NET Core Blazor forms overview
• Blazor samples GitHub repository (dotnet/blazor-samples) (how to download)