Call a web API from ASP.NET Core Blazor
Note
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This article describes how to call a web API from a Blazor app.
Note
The code examples in this article adopt nullable reference types (NRTs) and .NET compiler null-state static analysis, which are supported in ASP.NET Core 6.0 or later. When targeting ASP.NET Core 5.0 or earlier, remove the null type designation (?
) from the string?
, TodoItem[]?
, WeatherForecast[]?
, and IEnumerable<GitHubBranch>?
types in the article's examples.
Note
This article has loaded Blazor WebAssembly coverage for calling web APIs. The Blazor Server coverage addresses the following subjects:
- Use of the
HttpClient
factory infrastructure to provide anHttpClient
to the app. - Cross-origin resource sharing (CORS) pertaining to Blazor Server apps.
- Blazor framework component examples for testing web API access.
- Additional resources for developing Blazor Server apps that call a web API.
Blazor WebAssembly apps 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.
Examples in this article
In this article's component examples, a hypothetical todo list web API is used to create, read, update, and delete (CRUD) todo items on a server. The 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.
Use the following TodoItem
class with this article's examples if you build the examples into a test app:
public class TodoItem
{
public long Id { get; set; }
public string? Name { get; set; }
public bool IsComplete { get; set; }
}
For guidance on how to create a server-side web API, see Tutorial: Create a web API with ASP.NET Core. For information on Cross-origin resource sharing (CORS), see the Cross-origin resource sharing (CORS) section later in this article.
The Blazor WebAssembly examples that demonstrate obtaining weather data from a server API are based on a hosted Blazor WebAssembly solution created from the Blazor WebAssembly project template.
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 System.Net.Http.Json
package is provided by the .NET shared framework and doesn't require adding a package reference to the app.
Add the HttpClient
service
In Program.cs
, 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)
});
HttpClient
and 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.
@inject HttpClient Http
@if (todoItems == null)
{
<p>No Todo Items found.</p>
}
else
{
<ul>
@foreach (var item in todoItems)
{
<li>@item.Name</li>
}
</ul>
}
@code {
private TodoItem[]? todoItems;
protected override async Task OnInitializedAsync() =>
todoItems = await Http.GetFromJsonAsync<TodoItem[]>("api/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.
@inject HttpClient Http
<input @bind="newItemName" placeholder="New Todo Item" />
<button @onclick="AddItem">Add</button>
@code {
private string? newItemName;
private async Task AddItem()
{
var addItem = new TodoItem { Name = newItemName, IsComplete = false };
await Http.PostAsJsonAsync("api/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.
@inject HttpClient Http
<input type="checkbox" @bind="editItem.IsComplete" />
<input @bind="editItem.Name" />
<button @onclick="SaveItem">Save</button>
@code {
private string? id;
private TodoItem editItem = new TodoItem();
private void EditItem(long id)
{
editItem = todoItems.Single(i => i.Id == id);
}
private async Task SaveItem() =>
await Http.PutAsJsonAsync($"api/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
)
PatchAsJsonAsync sends an HTTP PATCH request with JSON-encoded content.
In the following component code, incompleteTodoItems
is an array of TodoItem
(not shown). The UpdateItem
method is triggered by selecting the <button>
element. JsonSerializerOptions.DefaultIgnoreCondition is set to JsonIgnoreCondition.WhenWritingDefault to demonstrate that only the IsCompleted
property is serialized in the PATCH request body.
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.
The following example doesn't show loading incompleteTodoItems
for brevity. See the GET from JSON (GetFromJsonAsync
) section for an example of loading items.
@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(
$"api/TodoItems/{id}",
new TodoItem() { IsComplete = true },
new JsonSerializerOptions()
{
DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
});
}
PatchAsJsonAsync returns an HttpResponseMessage. To deserialize the JSON content from the response message, use the ReadFromJsonAsync extension method. The following example reads JSON weather data as an array:
var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ??
Array.Empty<WeatherForecast>();
In the preceding example, an empty array is created if no weather data is returned by the method, so content
isn't null after the statement executes.
Additional extension methods
System.Net.Http includes additional extension methods for sending HTTP requests and receiving HTTP responses. HttpClient.DeleteAsync is used to send an HTTP DELETE request to a web API.
In the following component code, the <button>
element calls the DeleteItem
method. The bound <input>
element supplies the id
of the item to delete.
Note
When targeting ASP.NET Core 5.0 or earlier, add @using
directives to the following component for System.Net.Http and System.Threading.Tasks.
@inject HttpClient Http
<input @bind="id" />
<button @onclick="DeleteItem">Delete</button>
@code {
private long id;
private async Task DeleteItem() =>
await Http.DeleteAsync($"api/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 Program.cs
:
builder.Services.AddHttpClient("WebAPI", client =>
client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));
In the following component code:
- An instance of IHttpClientFactory creates a named HttpClient.
- The named 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 @using
directives to the following component for System.Net.Http, System.Net.Http.Json, and System.Threading.Tasks.
Pages/FetchDataViaFactory.razor
:
@page "/fetch-data-via-factory"
@using {PROJECT NAME}.Shared
@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 ℃
@forecast.TemperatureF ℉
</li>
}
</ul>
}
@code {
private WeatherForecast[]? forecasts;
protected override async Task OnInitializedAsync()
{
var client = ClientFactory.CreateClient("WebAPI");
forecasts = await client.GetFromJsonAsync<WeatherForecast[]>(
"WeatherForecast");
}
}
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;
using {PROJECT NAME}.Shared;
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 Program.cs
:
builder.Services.AddHttpClient<WeatherForecastHttpClient>(client =>
client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));
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.
Pages/FetchDataViaTypedHttpClient.razor
:
@page "/fetch-data-via-typed-httpclient"
@using {PROJECT NAME}.Shared
@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 ℃
@forecast.TemperatureF ℉
</li>
}
</ul>
}
@code {
private WeatherForecast[]? forecasts;
protected override async Task OnInitializedAsync()
{
forecasts = await Http.GetForecastAsync();
}
}
HttpClient
and HttpRequestMessage
with Fetch API request options
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.
Pages/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>
<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/api/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 WebAssembly's implementation of HttpClient uses Fetch API. Fetch API allows the configuration of several request-specific options. Options can be configured with HttpRequestMessage extension methods shown in the following table.
Extension method | Fetch API request property |
---|---|
SetBrowserRequestCache | cache |
SetBrowserRequestCredentials | credentials |
SetBrowserRequestIntegrity | integrity |
SetBrowserRequestMode | mode |
Set additional options using the generic SetBrowserRequestOption extension method.
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.
Call web API example
The following example calls a web API. The example requires a running web API based on the sample app described by the Tutorial: Create a web API with ASP.NET Core article. This example makes requests to the web API at https://localhost:10000/api/TodoItems
. If a different web API address is used, update the ServiceEndpoint
constant value in the component's @code
block.
The following example makes a cross-origin resource sharing (CORS) request from http://localhost:5000
or https://localhost:5001
to the web API. Add the following CORS Middleware configuration to the web API's service's Program.cs
file:
app.UseCors(policy =>
policy.WithOrigins("http://localhost:5000", "https://localhost:5001")
.AllowAnyMethod()
.WithHeaders(HeaderNames.ContentType));
Adjust the domains and ports of WithOrigins
as needed for the Blazor app. For more information, see Enable Cross-Origin Requests (CORS) in ASP.NET Core.
By default, ASP.NET Core apps use ports 5000 (HTTP) and 5001 (HTTPS). To run both apps on the same machine at the same time for testing, use a different port for the web API app (for example, port 10000). For more information on setting the port, see Configure endpoints for the ASP.NET Core Kestrel web server.
Pages/CallWebAPI.razor
:
@page "/call-web-api"
@inject HttpClient Http
<h1>Todo Items</h1>
@if (todoItems == null)
{
<p>No Todo Items found.</p>
}
else
{
<table class="table">
<thead>
<tr>
<th class="text-center">Complete</th>
<th>Name</th>
<th></th>
</tr>
</thead>
<tbody>
<tr id="editRow" style="display:@editRowStyle">
<td class="text-center">
<input type="checkbox" @bind="editItem.IsComplete" />
</td>
<td>
<input @bind="editItem.Name" />
</td>
<td class="text-center">
<button class="btn btn-success" @onclick="SaveItem">
Save
</button>
<button class="btn btn-danger"
@onclick="@(() => editRowStyle = "none")">
Cancel
</button>
</td>
</tr>
@foreach (var item in todoItems)
{
<tr>
<td class="text-center">
@if (item.IsComplete)
{
<span>✔</span>
}
</td>
<td>@item.Name</td>
<td class="text-center">
<button class="btn btn-warning"
@onclick="@(() => EditItem(item.Id))">
Edit
</button>
<button class="btn btn-danger"
@onclick="@(async () => await DeleteItem(item.Id))">
Delete
</button>
</td>
</tr>
}
<tr id="addRow">
<td></td>
<td>
<input @bind="newItemName" placeholder="New Todo Item" />
</td>
<td class="text-center">
<button class="btn btn-success" @onclick="AddItem">Add</button>
</td>
</tr>
</tbody>
</table>
}
@code {
private const string ServiceEndpoint = "https://localhost:10000/api/TodoItems";
private TodoItem[]? todoItems;
private TodoItem editItem = new();
private string editRowStyle = "none";
private string? newItemName;
protected override async Task OnInitializedAsync() => await GetTodoItems();
private async Task GetTodoItems() =>
todoItems = await Http.GetFromJsonAsync<TodoItem[]>(ServiceEndpoint);
private void EditItem(long id)
{
if (todoItems is not null)
{
editItem = todoItems.Single(i => i.Id == id);
editRowStyle = "table-row";
}
}
private async Task AddItem()
{
var addItem = new TodoItem { Name = newItemName, IsComplete = false };
await Http.PostAsJsonAsync(ServiceEndpoint, addItem);
newItemName = string.Empty;
await GetTodoItems();
editRowStyle = "none";
}
private async Task SaveItem()
{
if (editItem is not null)
{
await Http.PutAsJsonAsync($"{ServiceEndpoint}/{editItem.Id}",
editItem);
}
await GetTodoItems();
editRowStyle = "none";
}
private async Task DeleteItem(long id)
{
await Http.DeleteAsync($"{ServiceEndpoint}/{id}");
await GetTodoItems();
editRowStyle = "none";
}
private class TodoItem
{
public long Id { get; set; }
public string? Name { get; set; }
public bool IsComplete { get; set; }
}
}
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.
Pages/FetchDataReturnsHTMLOnException.razor
:
@page "/fetch-data-returns-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 ℃
@forecast.TemperatureF ℉
</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.
Note
This article has loaded Blazor Server coverage for calling web APIs. The Blazor WebAssembly coverage addresses the following subjects:
- Blazor WebAssembly examples based on an client-side WebAssembly app that calls a web API to create, read, update, and delete todo list items.
System.Net.Http.Json
package.HttpClient
service configuration.HttpClient
and JSON helpers (GetFromJsonAsync
,PostAsJsonAsync
,PutAsJsonAsync
,DeleteAsync
).IHttpClientFactory
services and the configuration of a namedHttpClient
.- Typed
HttpClient
. HttpClient
andHttpRequestMessage
to customize requests.- Call web API example with cross-origin resource sharing (CORS) and how CORS pertains to Blazor WebAssembly apps.
- How to handle web API response errors in developer code.
- Blazor framework component examples for testing web API access.
- Additional resources for developing Blazor WebAssembly apps that call a web API.
Blazor Server apps call web APIs using HttpClient instances, typically created using IHttpClientFactory. For guidance that applies to Blazor Server, see Make HTTP requests using IHttpClientFactory in ASP.NET Core.
A Blazor Server app doesn't include an HttpClient service by default. Provide an HttpClient to the app using the HttpClient
factory infrastructure.
In Program.cs
:
builder.Services.AddHttpClient();
The following Blazor Server 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.
Pages/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 Blazor Server file upload example that uploads files to a web API controller in the ASP.NET Core Blazor file uploads article.
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 information on CORS requests in Blazor WebAssembly apps, see ASP.NET Core Blazor WebAssembly additional security scenarios.
For information on CORS, see Enable Cross-Origin Requests (CORS) in ASP.NET Core. The article's examples don't pertain directly to Blazor WebAssembly apps, but the article is useful for learning general CORS concepts.
For more information, see Enable Cross-Origin Requests (CORS) in ASP.NET Core.
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
- ASP.NET Core Blazor WebAssembly additional security scenarios: Includes coverage on using HttpClient to make secure web API requests.
- Enable Cross-Origin Requests (CORS) in ASP.NET Core: Although the content applies to ASP.NET Core apps, not Blazor WebAssembly apps, the article covers general CORS concepts.
- Cross-Origin resource sharing (CORS) at W3C
- Fetch API
- ASP.NET Core Blazor Server additional security scenarios: Includes coverage on using HttpClient to make secure web API requests.
- Make HTTP requests using IHttpClientFactory in ASP.NET Core
- Enforce HTTPS in ASP.NET Core
- Enable Cross-Origin Requests (CORS) in ASP.NET Core
- Kestrel HTTPS endpoint configuration
- Cross-Origin resource sharing (CORS) at W3C
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