Call a web API from ASP.NET Core Blazor

Note

This isn't the latest version of this article. For the current release, see the ASP.NET Core 8.0 version of this article.

This article describes how to call a web API from a Blazor app.

Package

The System.Net.Http.Json package provides extension methods for System.Net.Http.HttpClient and System.Net.Http.HttpContent that perform automatic serialization and deserialization using System.Text.Json. The package is provided by the .NET shared framework and doesn't require adding a package reference to the app.

Sample app

See the sample app in the dotnet/blazor-samples GitHub repository.

Currently, one sample app is available: Standalone Blazor WebAssembly Call web API (BlazorWebAssemblyCallWebApi, .NET 8 or later).

One or more additional sample apps will be added soon, focusing first on adding a Blazor Web App with Auto components sample.

Server-side scenarios

Server-based components call web APIs using HttpClient instances, typically created using IHttpClientFactory. For guidance that applies to server-side apps, see Make HTTP requests using IHttpClientFactory in ASP.NET Core.

A server-side app doesn't include an HttpClient service by default. Provide an HttpClient to the app using the HttpClient factory infrastructure.

In the Program file:

builder.Services.AddHttpClient();

The following Razor component makes a request to a web API for GitHub branches similar to the Basic Usage example in the Make HTTP requests using IHttpClientFactory in ASP.NET Core article.

CallWebAPI.razor:

@page "/call-web-api"
@using System.Text.Json
@using System.Text.Json.Serialization
@inject IHttpClientFactory ClientFactory

<h1>Call web API from a Blazor Server Razor component</h1>

@if (getBranchesError || branches is null)
{
    <p>Unable to get branches from GitHub. Please try again later.</p>
}
else
{
    <ul>
        @foreach (var branch in branches)
        {
            <li>@branch.Name</li>
        }
    </ul>
}

@code {
    private IEnumerable<GitHubBranch>? branches = Array.Empty<GitHubBranch>();
    private bool getBranchesError;
    private bool shouldRender;

    protected override bool ShouldRender() => shouldRender;

    protected override async Task OnInitializedAsync()
    {
        var request = new HttpRequestMessage(HttpMethod.Get,
            "https://api.github.com/repos/dotnet/AspNetCore.Docs/branches");
        request.Headers.Add("Accept", "application/vnd.github.v3+json");
        request.Headers.Add("User-Agent", "HttpClientFactory-Sample");

        var client = ClientFactory.CreateClient();

        var response = await client.SendAsync(request);

        if (response.IsSuccessStatusCode)
        {
            using var responseStream = await response.Content.ReadAsStreamAsync();
            branches = await JsonSerializer.DeserializeAsync
                <IEnumerable<GitHubBranch>>(responseStream);
        }
        else
        {
            getBranchesError = true;
        }

        shouldRender = true;
    }

    public class GitHubBranch
    {
        [JsonPropertyName("name")]
        public string? Name { get; set; }
    }
}

For an additional working example, see the server-side file upload example that uploads files to a web API controller in the ASP.NET Core Blazor file uploads article.

Client-side services for HttpClient fail during prerendering

This section only applies to prerendered WebAssembly components or Auto components during server-side rendering (SSR) in Blazor Web Apps.

Blazor Web Apps normally prerender client-side WebAssembly components, and Auto components render on the server during static or interactive server-side rendering (SSR). HttpClient services aren't registered by default in a Blazor Web App's main project. If the app is run with only the HttpClient services registered in the .Client project, as described in the Add the HttpClient service section, executing the app results in a runtime error:

InvalidOperationException: Cannot provide a value for property 'Http' on type '...{COMPONENT}'. There is no registered service of type 'System.Net.Http.HttpClient'.

Use either of the following approaches to resolve this problem:

  • Add the HttpClient services to the main project to make them available during SSR. Use the following service registration in the main project's Program file:

    builder.Services.AddHttpClient();
    

    No explicit package reference is required for the main project because HttpClient services are provided by the shared framework.

  • If prerendering isn't required for a WebAssembly component, disable prerendering by following the guidance in ASP.NET Core Blazor render modes. If you adopt this approach, you don't need to add HttpClient services to the main project of the Blazor Web App.

For more information, see Client-side services fail to resolve during prerendering.

Add the HttpClient service

The guidance in this section applies to client-side scenarios.

Client-side components call web APIs using a preconfigured HttpClient service, which is focused on making requests back to the server of origin. Additional HttpClient service configurations for other web APIs can be created in developer code. Requests are composed using Blazor JSON helpers or with HttpRequestMessage. Requests can include Fetch API option configuration.

The configuration examples in this section are only useful when a single web API is called for a single HttpClient instance in the app. When the app must call multiple web APIs, each with its own base address and configuration, you can adopt the following approaches, which are covered later in this article:

In the Program file, add an HttpClient service if it isn't already present from a Blazor project template used to create the app:

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.

The most common use cases for using the client's own base address are:

  • The client project (.Client) of a Blazor Web App (.NET 8 or later) makes web API calls from WebAssembly components or code that runs on the client in WebAssembly to APIs in the server app.
  • The client project (Client) of a hosted Blazor WebAssembly app makes web API calls to the server project (Server). Note that the hosted Blazor WebAssembly project template is no longer available in .NET 8 or later. However, hosted Blazor WebAssembly apps remain supported for .NET 8.

If you're calling an external web API (not in the same URL space as the client app), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddScoped(sp => 
    new HttpClient
    {
        BaseAddress = new Uri("https://localhost:5001")
    });

JSON helpers

HttpClient is available as a preconfigured service for making requests back to the origin server.

HttpClient and JSON helpers (System.Net.Http.Json.HttpClientJsonExtensions) are also used to call third-party web API endpoints. HttpClient is implemented using the browser's Fetch API and is subject to its limitations, including enforcement of the same-origin policy, which is discussed later in this article in the Cross-Origin Resource Sharing (CORS) section.

The client's base address is set to the originating server's address. Inject an HttpClient instance into a component using the @inject directive:

@using System.Net.Http
@inject HttpClient Http

Use the System.Net.Http.Json namespace for access to HttpClientJsonExtensions, including GetFromJsonAsync, PutAsJsonAsync, and PostAsJsonAsync:

@using System.Net.Http.Json

GET from JSON (GetFromJsonAsync)

GetFromJsonAsync sends an HTTP GET request and parses the JSON response body to create an object.

In the following component code, the todoItems are displayed by the component. GetFromJsonAsync is called when the component is finished initializing (OnInitializedAsync).

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

todoItems = await Http.GetFromJsonAsync<TodoItem[]>("todoitems");

POST as JSON (PostAsJsonAsync)

PostAsJsonAsync sends a POST request to the specified URI containing the value serialized as JSON in the request body.

In the following component code, newItemName is provided by a bound element of the component. The AddItem method is triggered by selecting a <button> element.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

await Http.PostAsJsonAsync("todoitems", addItem);

PostAsJsonAsync returns an HttpResponseMessage. To deserialize the JSON content from the response message, use the ReadFromJsonAsync extension method. The following example reads JSON weather data as an array:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ??
    Array.Empty<WeatherForecast>();

In the preceding example, an empty array is created if no weather data is returned by the method, so content isn't null after the statement executes.

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>();

PUT as JSON (PutAsJsonAsync)

PutAsJsonAsync sends an HTTP PUT request with JSON-encoded content.

In the following component code, editItem values for Name and IsCompleted are provided by bound elements of the component. The item's Id is set when the item is selected in another part of the UI (not shown) and EditItem is called. The SaveItem method is triggered by selecting the <button> element. The following example doesn't show loading todoItems for brevity. See the GET from JSON (GetFromJsonAsync) section for an example of loading items.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

await Http.PutAsJsonAsync($"todoitems/{editItem.Id}", editItem);

PutAsJsonAsync returns an HttpResponseMessage. To deserialize the JSON content from the response message, use the ReadFromJsonAsync extension method. The following example reads JSON weather data as an array:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ??
    Array.Empty<WeatherForecast>();

In the preceding example, an empty array is created if no weather data is returned by the method, so content isn't null after the statement executes.

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>();

PATCH as JSON (PatchAsJsonAsync)

Note

This section's example isn't currently demonstrated in the Standalone Blazor WebAssembly Call web API sample app (BlazorWebAssemblyCallWebApi sample, .NET 8 or later). The example will be added to the sample app soon.

PatchAsJsonAsync sends an HTTP PATCH request with JSON-encoded content.

This section's examples are based on a TodoItem class that stores the following todo item data:

  • ID (Id, long): Unique ID of the item.
  • Name (Name, string): Name of the item.
  • Status (IsComplete, bool): Indication if the todo item is finished.

TodoItem class:

public class TodoItem
{
    public long Id { get; set; }
    public string? Name { get; set; }
    public bool IsComplete { get; set; }
}

In the following component code:

  • incompleteTodoItems is an array of incomplete TodoItem. The following example doesn't show loading incompleteTodoItems for brevity. Loading items is covered in the GET from JSON (GetFromJsonAsync) section.
  • The UpdateItem method is triggered by selecting the <button> element.
  • The PATCH document is provided as a plain text string. The web API described in the Tutorial: Create a web API with ASP.NET Core article doesn't handle PATCH requests by default. To make the PATCH example in this section work with the tutorial's web API, implement a PATCH controller action in the web API following the guidance in JsonPatch in ASP.NET Core web API. Later, this section demonstrates an example controller action and shows how to compose PATCH documents for ASP.NET Core web API apps that use .NET JSON PATCH support.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

@using System.Text.Json
@using System.Text.Json.Serialization
@inject HttpClient Http

<ul>
    @foreach (var item in incompleteTodoItems)
    {
        <li>
            @item.Name 
            <button @onclick="_ => UpdateItem(item.Id)">
                Mark 'Complete'
            </button>
        </li>
    }
</ul>

@code {
    private async Task UpdateItem(long id) =>
        await Http.PatchAsJsonAsync(
            $"todoitems/{id}", 
            "[{\"operationType\":2,\"path\":\"/IsComplete\",\"op\":\"replace\",\"value\":true}]");
}

PatchAsJsonAsync returns an HttpResponseMessage. To deserialize the JSON content from the response message, use the ReadFromJsonAsync extension method. The following example reads JSON weather data as an array. An empty array is created if no weather data is returned by the method, so content isn't null after the statement executes:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ??
    Array.Empty<WeatherForecast>();

PatchAsJsonAsync receives a JSON PATCH document for the PATCH request. The preceding UpdateItem method called PatchAsJsonAsync with a PATCH document as a string with escaped quotes. Laid out with indentation, spacing, and non-escaped quotes, the unencoded PATCH document appears as the following JSON:

[
  {
    "operationType": 2,
    "path": "/IsComplete",
    "op": "replace",
    "value": true
  }
]

To simplify the creation of PATCH documents in the app issuing PATCH requests, an app can use .NET JSON PATCH support, as the following guidance demonstrates.

Install the Microsoft.AspNetCore.JsonPatch NuGet package and use the API features of the package to compose a JsonPatchDocument for a PATCH request.

Note

For guidance on adding packages to .NET apps, see the articles under Install and manage packages at Package consumption workflow (NuGet documentation). Confirm correct package versions at NuGet.org.

Add an @using directive for the Microsoft.AspNetCore.JsonPatch namespace to the top of the Razor component:

@using Microsoft.AspNetCore.JsonPatch

Compose the JsonPatchDocument for a TodoItem with IsComplete set to true using the Replace method:

var patchDocument = new JsonPatchDocument<TodoItem>()
    .Replace(p => p.IsComplete, true);

Pass the document's operations (patchDocument.Operations) to the PatchAsJsonAsync call. The following example shows how to make the call:

private async Task UpdateItem(long id)
{
    await Http.PatchAsJsonAsync(
        $"todoitems/{id}", 
        patchDocument.Operations, 
        new JsonSerializerOptions()
        {
            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
            WriteIndented = true
        });
}

JsonSerializerOptions.DefaultIgnoreCondition is set to JsonIgnoreCondition.WhenWritingDefault to ignore a property only if it equals the default value for its type.

JsonSerializerOptions.WriteIndented is used merely to present the JSON payload in a pleasant format for this article. Writing indented JSON has no bearing on processing PATCH requests and isn't typically performed in production apps for web API requests.

Next, follow the guidance in the JsonPatch in ASP.NET Core web API article to add a PATCH controller action to the web API.

Add a package reference for the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package to the web API app.

Note

There's no need to add a package reference for the Microsoft.AspNetCore.JsonPatch package to the app because the reference to the Microsoft.AspNetCore.Mvc.NewtonsoftJson package automatically transitively adds a package reference for Microsoft.AspNetCore.JsonPatch.

Add a custom JSON PATCH input formatter to the web API app.

JSONPatchInputFormatter.cs:

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Formatters;
using Microsoft.Extensions.Options;

public static class JSONPatchInputFormatter
{
    public static NewtonsoftJsonPatchInputFormatter Get()
    {
        var builder = new ServiceCollection()
            .AddLogging()
            .AddMvc()
            .AddNewtonsoftJson()
            .Services.BuildServiceProvider();

        return builder
            .GetRequiredService<IOptions<MvcOptions>>()
            .Value
            .InputFormatters
            .OfType<NewtonsoftJsonPatchInputFormatter>()
            .First();
    }
}

Configure the web API's controllers to use the Microsoft.AspNetCore.Mvc.NewtonsoftJson package and process PATCH requests with the JSON PATCH input formatter. Insert the JSONPatchInputFormatter in the first position of MVC's input formatter collection so that it processes requests prior to any other input formatter.

In the Program file modify the call to AddControllers:

builder.Services.AddControllers(options =>
{
    options.InputFormatters.Insert(0, JSONPatchInputFormatter.Get());
}).AddNewtonsoftJson();

In Controllers/TodoItemsController.cs, add a using statement for the Microsoft.AspNetCore.JsonPatch namespace:

using Microsoft.AspNetCore.JsonPatch;

In Controllers/TodoItemsController.cs, add the following PatchTodoItem action method:

[HttpPatch("{id}")]
public async Task<IActionResult> PatchTodoItem(long id, 
    JsonPatchDocument<TodoItem> patchDoc)
{
    if (patchDoc == null)
    {
        return BadRequest();
    }

    var todoItem = await _context.TodoItems.FindAsync(id);

    if (todoItem == null)
    {
        return NotFound();
    }

    patchDoc.ApplyTo(todoItem);

    _context.Entry(todoItem).State = EntityState.Modified;

    try
    {
        await _context.SaveChangesAsync();
    }
    catch (DbUpdateConcurrencyException) when (!TodoItemExists(id))
    {
        return NotFound();
    }

    return NoContent();
}

Warning

As with the other examples in the JsonPatch in ASP.NET Core web API article, the preceding PATCH controller action doesn't protect the web API from over-posting attacks. For more information, see Tutorial: Create a web API with ASP.NET Core.

Additional extension methods

System.Net.Http includes additional extension methods for sending HTTP requests and receiving HTTP responses. HttpClient.DeleteAsync is used to send an HTTP DELETE request to a web API.

In the following component code, the <button> element calls the DeleteItem method. The bound <input> element supplies the id of the item to delete.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http and System.Threading.Tasks.

await Http.DeleteAsync($"todoitems/{id}");

Named HttpClient with IHttpClientFactory

IHttpClientFactory services and the configuration of a named HttpClient are supported.

Note

An alternative to using a named HttpClient from an IHttpClientFactory is to use a typed HttpClient. For more information, see the Typed HttpClient section.

Add the Microsoft.Extensions.Http NuGet package to the app.

Note

For guidance on adding packages to .NET apps, see the articles under Install and manage packages at Package consumption workflow (NuGet documentation). Confirm correct package versions at NuGet.org.

In the Program file:

builder.Services.AddHttpClient("WebAPI", client => 
    client.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.

The most common use cases for using the client's own base address are:

  • The client project (.Client) of a Blazor Web App (.NET 8 or later) makes web API calls from WebAssembly components or code that runs on the client in WebAssembly to APIs in the server app.
  • The client project (Client) of a hosted Blazor WebAssembly app makes web API calls to the server project (Server).

If you're calling an external web API (not in the same URL space as the client app), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddHttpClient("WebAPI", client => 
    client.BaseAddress = new Uri(https://localhost:5001));

In the following component code:

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

FetchDataViaFactory.razor:

@page "/fetch-data-via-factory"
@inject IHttpClientFactory ClientFactory

<h1>Fetch data via <code>IHttpClientFactory</code></h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

@code {
    private WeatherForecast[]? forecasts;

    protected override async Task OnInitializedAsync()
    {
        var client = ClientFactory.CreateClient("WebAPI");

        forecasts = await client.GetFromJsonAsync<WeatherForecast[]>(
            "WeatherForecast");
    }
}

For an additional example based on calling Microsoft Graph with a named HttpClient, see Use Graph API with ASP.NET Core Blazor WebAssembly.

Typed HttpClient

Typed HttpClient uses one or more of the app's HttpClient instances, default or named, to return data from one or more web API endpoints.

Note

An alternative to using a typed HttpClient is to use a named HttpClient from an IHttpClientFactory. For more information, see the Named HttpClient with IHttpClientFactory section.

Add the Microsoft.Extensions.Http NuGet package to the app.

Note

For guidance on adding packages to .NET apps, see the articles under Install and manage packages at Package consumption workflow (NuGet documentation). Confirm correct package versions at NuGet.org.

WeatherForecastHttpClient.cs:

using System.Net.Http.Json;

namespace BlazorSample.Client;

public class WeatherForecastHttpClient(HttpClient http)
{
    private readonly HttpClient http = http;
    private WeatherForecast[]? forecasts;

    public async Task<WeatherForecast[]> GetForecastAsync()
    {
        forecasts = await http.GetFromJsonAsync<WeatherForecast[]>(
            "WeatherForecast");

        return forecasts ?? [];
    }
}
using System.Net.Http.Json;

public class WeatherForecastHttpClient
{
    private readonly HttpClient http;
    private WeatherForecast[]? forecasts;

    public WeatherForecastHttpClient(HttpClient http)
    {
        this.http = http;
    }

    public async Task<WeatherForecast[]> GetForecastAsync()
    {
        forecasts = await http.GetFromJsonAsync<WeatherForecast[]>(
            "WeatherForecast");

        return forecasts ?? Array.Empty<WeatherForecast>();
    }
}

In the Program file:

builder.Services.AddHttpClient<WeatherForecastHttpClient>(client => 
    client.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.

The most common use cases for using the client's own base address are:

  • The client project (.Client) of a Blazor Web App (.NET 8 or later) makes web API calls from WebAssembly/Auto components or code that runs on the client in WebAssembly to APIs in the server app.
  • The client project (Client) of a hosted Blazor WebAssembly app makes web API calls to the server project (Server).

If you're calling an external web API (not in the same URL space as the client app), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddHttpClient<WeatherForecastHttpClient>(client => 
    client.BaseAddress = new Uri(https://localhost:5001));

Components inject the typed HttpClient to call the web API.

In the following component code:

  • An instance of the preceding WeatherForecastHttpClient is injected, which creates a typed HttpClient.
  • The typed HttpClient is used to issue a GET request for JSON weather forecast data from the web API.

Note

When targeting ASP.NET Core 5.0 or earlier, add an @using directive to the following component for System.Threading.Tasks.

FetchDataViaTypedHttpClient.razor:

@page "/fetch-data-via-typed-httpclient"
@inject WeatherForecastHttpClient Http

<h1>Fetch data via typed <code>HttpClient</code></h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

@code {
    private WeatherForecast[]? forecasts;

    protected override async Task OnInitializedAsync()
    {
        forecasts = await Http.GetForecastAsync();
    }
}

HttpClient and HttpRequestMessage with Fetch API request options

The guidance in this section applies to client-side scenarios that rely upon bearer token authentication.

HttpClient (API documentation) and HttpRequestMessage can be used to customize requests. For example, you can specify the HTTP method and request headers. The following component makes a POST request to a web API endpoint and shows the response body.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http and System.Net.Http.Json.

TodoRequest.razor:

@page "/todo-request"
@using System.Net.Http.Headers
@using Microsoft.AspNetCore.Components.WebAssembly.Authentication
@inject HttpClient Http
@inject IAccessTokenProvider TokenProvider

<h1>ToDo Request</h1>

<h1>ToDo Request Example</h1>

<button @onclick="PostRequest">Submit POST request</button>

<p>Response body returned by the server:</p>

<p>@responseBody</p>

@code {
    private string? responseBody;

    private async Task PostRequest()
    {
        var requestMessage = new HttpRequestMessage()
        {
            Method = new HttpMethod("POST"),
            RequestUri = new Uri("https://localhost:10000/todoitems"),
            Content =
                JsonContent.Create(new TodoItem
                {
                    Name = "My New Todo Item",
                    IsComplete = false
                })
        };

        var tokenResult = await TokenProvider.RequestAccessToken();

        if (tokenResult.TryGetToken(out var token))
        {
            requestMessage.Headers.Authorization =
                new AuthenticationHeaderValue("Bearer", token.Value);

            requestMessage.Content.Headers.TryAddWithoutValidation(
                "x-custom-header", "value");

            var response = await Http.SendAsync(requestMessage);
            var responseStatusCode = response.StatusCode;

            responseBody = await response.Content.ReadAsStringAsync();
        }
    }

    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

Blazor's client-side implementation of HttpClient uses Fetch API and configures the underlying request-specific Fetch API options via HttpRequestMessage extension methods and WebAssemblyHttpRequestMessageExtensions. Set additional options using the generic SetBrowserRequestOption extension method. Blazor and the underlying Fetch API don't directly add or modify request headers. For more information on how user agents, such as browsers, interact with headers, consult external user agent documentation sets and other web resources.

The HTTP response is typically buffered to enable support for synchronous reads on the response content. To enable support for response streaming, use the SetBrowserResponseStreamingEnabled extension method on the request.

To include credentials in a cross-origin request, use the SetBrowserRequestCredentials extension method:

requestMessage.SetBrowserRequestCredentials(BrowserRequestCredentials.Include);

For more information on Fetch API options, see MDN web docs: WindowOrWorkerGlobalScope.fetch(): Parameters.

Handle errors

Handle web API response errors in developer code when they occur. For example, GetFromJsonAsync expects a JSON response from the web API with a Content-Type of application/json. If the response isn't in JSON format, content validation throws a NotSupportedException.

In the following example, the URI endpoint for the weather forecast data request is misspelled. The URI should be to WeatherForecast but appears in the call as WeatherForcast, which is missing the letter e in Forecast.

The GetFromJsonAsync call expects JSON to be returned, but the web API returns HTML for an unhandled exception with a Content-Type of text/html. The unhandled exception occurs because the path to /WeatherForcast isn't found and middleware can't serve a page or view for the request.

In OnInitializedAsync on the client, NotSupportedException is thrown when the response content is validated as non-JSON. The exception is caught in the catch block, where custom logic could log the error or present a friendly error message to the user.

Note

When targeting ASP.NET Core 5.0 or earlier, add @using directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.

ReturnHTMLOnException.razor:

@page "/return-html-on-exception"
@using {PROJECT NAME}.Shared
@inject HttpClient Http

<h1>Fetch data but receive HTML on unhandled exception</h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

<p>
    @exceptionMessage
</p>

@code {
    private WeatherForecast[]? forecasts;
    private string? exceptionMessage;

    protected override async Task OnInitializedAsync()
    {
        try
        {
            // The URI endpoint "WeatherForecast" is misspelled on purpose on the 
            // next line. See the preceding text for more information.
            forecasts = await Http.GetFromJsonAsync<WeatherForecast[]>("WeatherForcast");
        }
        catch (NotSupportedException exception)
        {
            exceptionMessage = exception.Message;
        }
    }
}

Note

The preceding example is for demonstration purposes. A web API can be configured to return JSON even when an endpoint doesn't exist or an unhandled exception occurs on the server.

For more information, see Handle errors in ASP.NET Core Blazor apps.

Cross-Origin Resource Sharing (CORS)

Browser security restricts a webpage from making requests to a different domain than the one that served the webpage. This restriction is called the same-origin policy. The same-origin policy restricts (but doesn't prevent) a malicious site from reading sensitive data from another site. To make requests from the browser to an endpoint with a different origin, the endpoint must enable Cross-Origin Resource Sharing (CORS).

For more information on server-side CORS, see Enable Cross-Origin Requests (CORS) in ASP.NET Core. The article's examples don't pertain directly to Razor component scenarios, but the article is useful for learning general CORS concepts.

For information on client-side CORS requests, see ASP.NET Core Blazor WebAssembly additional security scenarios.

Antiforgery support

To add antiforgery support to an HTTP request, inject the AntiforgeryStateProvider and add a RequestToken to the headers collection as a RequestVerificationToken:

@inject AntiforgeryStateProvider Antiforgery
private async Task OnSubmit()
{
    var antiforgery = Antiforgery.GetAntiforgeryToken();
    var request = new HttpRequestMessage(HttpMethod.Post, "action");
    request.Headers.Add("RequestVerificationToken", antiforgery.RequestToken);
    var response = await client.SendAsync(request);
    ...
}

For more information, see ASP.NET Core Blazor authentication and authorization.

Blazor framework component examples for testing web API access

Various network tools are publicly available for testing web API backend apps directly, such as Firefox Browser Developer and Postman. Blazor framework's reference source includes HttpClient test assets that are useful for testing:

HttpClientTest assets in the dotnet/aspnetcore GitHub repository

Note

Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).

Additional resources

General

Server-side

Client-side