How to fix "The CORS protocol does not allow specifying a wildcard (any) origin and credentials at the same time" error

I had the same issue and I removed AllowCredentials() that fixed the issue for me.

You should have provided the rest of your code... Is this a Blazor client application or Razor Components application (formally known as Server-Side Blazor) ? I guess this is a Blazor client application, right ? Why do you instantiate an HttpClient ? You should use DI (Perhaps Constructor Injection) instead, injecting an HttpClient instance provided by Blazor itself.

The problem is probably server side, though it surfaces as a client one... Try the following:

Get https://www.nuget.org/packages/Microsoft.AspNetCore.Cors/

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(options =>
    {
        options.AddPolicy("CorsPolicy",
            builder => builder.AllowAnyOrigin()
                .AllowAnyMethod()
                .AllowAnyHeader());
    });
     .....
}

And this:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)    
{
      app.UseCors("CorsPolicy");
}

Note, once again: CORS needs to be enabled on the server side, not in blazor. See https://docs.microsoft.com/en-us/aspnet/core/security/cors for details on how to enable CORS in ASP.NET Core.

Blazor:

 @page "/<template>"
 @inject HttpClient Http


@functions {

    protected override async Task OnInitAsync()
    {
        var response= await Http.GetJsonAsync<string>    
                      ("https://example.com?prm=2");

    }

}  

Hope this helps...

I also faced same issue, and I found solution here:

Setup Any Origin And Any Credentials

Change your CORS setup in startup.cs file like this

public void ConfigureServices(IServiceCollection services)
{
    // ...
    services.AddCors(options =>
    {
        options.AddDefaultPolicy(builder => 
            builder.SetIsOriginAllowed(_ => true)
            .AllowAnyMethod()
            .AllowAnyHeader()
            .AllowCredentials());
    });
}

It works for me.