Work with images in ASP.NET Core Blazor

This article describes common scenarios for working with images in Blazor apps.

Dynamically set an image source

The following example demonstrates how to dynamically set an image's source with a C# field.

For the example in this section:

• Obtain three images from any source or right-click each of the following images to save them locally. Name the images image1.png, image2.png, and image3.png.

       

• Place the images in a new folder named images in the app's web root (wwwroot). The use of the images folder is only for demonstration purposes. You can organize images in any folder layout that you prefer, including serving the images directly from the wwwroot folder.

In the following ShowImage1 component:

• The image's source (src) is dynamically set to the value of imageSource in C#.
• The ShowImage method updates the imageSource field based on an image id argument passed to the method.
• Rendered buttons call the ShowImage method with an image argument for each of the three available images in the images folder. The file name is composed using the argument passed to the method and matches one of the three images in the images folder.

ShowImage1.razor:

@page "/show-image-1"

<PageTitle>Show Image 1</PageTitle>

<h1>Show Image Example 1</h1>

@if (imageSource is not null)
{
    <p>
        <img src="@imageSource" />
    </p>
}

@for (var i = 1; i <= 3; i++)
{
    var imageId = i;
    <button @onclick="() => ShowImage(imageId)">
        Image @imageId
    </button>
}

@code {
    private string? imageSource;

    private void ShowImage(int id)
    {
        imageSource = $"images/image{id}.png";
    }
}

The preceding example uses a C# field to hold the image's source data, but you can also use a C# property to hold the data.

Note

Avoid using a loop variable directly in a lambda expression, such as i in the preceding for loop example. Otherwise, the same variable is used by all lambda expressions, which results in use of the same value in all lambdas. Capture the variable's value in a local variable. In the preceding example:

• The loop variable i is assigned to imageId.
• imageId is used in the lambda expression.

Alternatively, use a foreach loop with Enumerable.Range, which doesn't suffer from the preceding problem:

@foreach (var imageId in Enumerable.Range(1,3))
{
    <button @onclick="() => ShowImage(imageId)">
        Image @imageId
    </button>
}

For more information, see ASP.NET Core Blazor event handling.

Stream image data

An image can be directly sent to the client using Blazor's streaming interop features instead of hosting the image at a public URL.

The example in this section streams image source data using JavaScript (JS) interop. The following setImage JS function accepts the <img> tag id and data stream for the image. The function performs the following steps:

• Reads the provided stream into an ArrayBuffer.
• Creates a Blob to wrap the ArrayBuffer.
• Creates an object URL to serve as the address for the image to be shown.
• Updates the <img> element with the specified imageElementId with the object URL just created.
• To prevent memory leaks, the function calls revokeObjectURL to dispose of the object URL when the component is finished working with an image.

<script>
  window.setImage = async (imageElementId, imageStream) => {
    const arrayBuffer = await imageStream.arrayBuffer();
    const blob = new Blob([arrayBuffer]);
    const url = URL.createObjectURL(blob);
    const image = document.getElementById(imageElementId);
    image.onload = () => {
      URL.revokeObjectURL(url);
    }
    image.src = url;
  }
</script>

Note

For general guidance on JS location and our recommendations for production apps, see JavaScript location in ASP.NET Core Blazor apps.

The following ShowImage2 component:

• Injects services for an System.Net.Http.HttpClient and Microsoft.JSInterop.IJSRuntime.
• Includes an <img> tag to display an image.
• Has a GetImageStreamAsync C# method to retrieve a Stream for an image. A production app may dynamically generate an image based on the specific user or retrieve an image from storage. The following example retrieves the .NET avatar for the dotnet GitHub repository.
• Has a SetImageAsync method that's triggered on the button's selection by the user. SetImageAsync performs the following steps:
  • Retrieves the Stream from GetImageStreamAsync.
  • Wraps the Stream in a DotNetStreamReference, which allows streaming the image data to the client.
  • Invokes the setImage JavaScript function, which accepts the data on the client.

Note

Server-side apps use a dedicated HttpClient service to make requests, so no action is required by the developer of a server-side Blazor app to register an HttpClient service. Client-side apps have a default HttpClient service registration when the app is created from a Blazor project template. If an HttpClient service registration isn't present in the Program file of a client-side app, provide one by adding builder.Services.AddHttpClient();. For more information, see Make HTTP requests using IHttpClientFactory in ASP.NET Core.

ShowImage2.razor:

@page "/show-image-2"
@inject HttpClient Http
@inject IJSRuntime JS

<PageTitle>Show Image 2</PageTitle>

<h1>Show Image Example 2</h1>

<p>
    <img id="image" />
</p>

<button @onclick="SetImageAsync">
    Set Image
</button>

@code {
    private async Task<Stream> GetImageStreamAsync()
    {
        return await Http.GetStreamAsync(
            "https://avatars.githubusercontent.com/u/9141961");
    }

    private async Task SetImageAsync()
    {
        var imageStream = await GetImageStreamAsync();
        var dotnetImageStream = new DotNetStreamReference(imageStream);
        await JS.InvokeVoidAsync("setImage", "image", dotnetImageStream);
    }
}

Additional resources

• ASP.NET Core Blazor file uploads
• File uploads: Upload image preview
• ASP.NET Core Blazor file downloads
• Call .NET methods from JavaScript functions in ASP.NET Core Blazor
• Call JavaScript functions from .NET methods in ASP.NET Core Blazor
• Blazor samples GitHub repository (dotnet/blazor-samples) (how to download)