Blazor WebAssembly exception while calling REST API calls - WebAssembly.JSException: TypeError: Failed to fetch at WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler.doFetch

I am trying to call REST API call from web Blazor WebAssembly using the following code

var userInfo = new ValidateUserRequest { UserName = UserName, Password = password };
var response = await Http.PostJsonAsync<string>($"{APIBaseUrl}/Security/ValidateUser", userInfo);

The CORS is enabled in the API.

But when running the application, I got the following error.

WebAssembly.JSException: TypeError: Failed to fetch at WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler.doFetch (System.Threading.Tasks.TaskCompletionSource`1[TResult] tcs, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) <0x22fe098 + 0x00988> in <c4a9717a3f954719acec8a5e4e87c87d>:0 at WebAssembly.Net.Http.HttpClient.WasmHttpMessageHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) <0x22d6978 + 0x00180> in <c4a9717a3f954719acec8a5e4e87c87d>:0 at System.Net.Http.HttpClient.FinishSendAsyncBuffered (System.Threading.Tasks.Task`1[TResult] sendTask, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationTokenSource cts, System.Boolean disposeCts) <0x235f0f8 + 0x00278> in <f29c040f027c48c299bba2e6843ac2e0>:0 at Microsoft.AspNetCore.Components.HttpClientJsonExtensions.SendJsonAsync[T] (System.Net.Http.HttpClient httpClient, System.Net.Http.HttpMethod method, System.String requestUri, System.Object content) <0x22a1c88 + 0x00118> in <393f0d2066e842d38b0e8ac688095cf7>:0 at BlazorTestApp2.Shared.Login.ValidateUser (System.String userName, System.String password) [0x0006d] in C:\Repo\BlazorTestApp2\BlazorTestApp2\Shared\Login.razor:83 at BlazorTestApp2.Shared.Login.LoginWithPassword () [0x0003f] in C:\Repo\BlazorTestApp2\BlazorTestApp2\Shared\Login.razor:116

Is this can be fixed anyway?

net version used : 3.1.0-preview4.19579.2

[sukeshchand]

I have upgraded the version to 3.2.0-preview2.20160.5 and the issue got fixed.

For more info about new upgrade https://devblogs.microsoft.com/aspnet/blazor-webassembly-3-2-0-preview-2-release-now-available/

[WoodyJ007]

I have the same problem using the latest builds. Internet has been telling me it’s a cors issue and I’ve been going around in circles. The above basic code works fine on console and other apps. Tried it on a blazor server side and works fine so something is up with WebAssembly? Tried it on two from scratch projects on two different PCs and same result. Any ideas? Known issue still?

FYI. it’s when connecting to a webapi on HTTP and project was set to configure for HTTPS.

` public async Task<string> GetSomeDataAsync() { var _client = new HttpClient();

    var response = await _client.GetStringAsync(@"http://dummy.restapiexample.com/api/v1/employees");

    return response;
}

`

Create Blazor server project with “Configure HTTPS” NOT ticked. Works Create Blazor server project with “Configure HTTPS” ticked. Works

Create the client web assembly project with “Configure HTTPS” NOT ticked. Works Create the client web assembly project with “Configure HTTPS” ticked. Fail