How to enable CORS in ASP.NET Core

I am trying to enable cross origin resources sharing on my ASP.NET Core Web API, but I am stuck.

The EnableCors attribute accepts policyName of type string as parameter:

// Summary:
//     Creates a new instance of the Microsoft.AspNetCore.Cors.Core.EnableCorsAttribute.
//
// Parameters:
//   policyName:
//     The name of the policy to be applied.
public EnableCorsAttribute(string policyName);

What does the policyName mean and how can I configure CORS on an ASP.NET Core Web API?

Solution

For ASP.NET Core 6:

var  MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
                      builder =>
                      {
                          builder.WithOrigins("http://example.com",
                                              "http://www.contoso.com");
                      });
});

// services.AddResponseCaching();

builder.Services.AddControllers();

var app = builder.Build();
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();

app.UseCors(MyAllowSpecificOrigins);

app.UseAuthorization();

app.MapControllers();

app.Run();

See the official docs for more samples.

For ASP.NET Core 3.1 and 5.0:

You have to configure a CORS policy at application startup in the ConfigureServices method:

public void ConfigureServices(IServiceCollection services)
{
    services.AddCors(o => o.AddPolicy("MyPolicy", builder =>
    {
        builder.WithOrigins("http://example.com")
               .AllowAnyMethod()
               .AllowAnyHeader();
    }));

    // ...
}

The CorsPolicyBuilder in builder allows you to configure the policy to your needs. You can now use this name to apply the policy to controllers and actions:

[EnableCors("MyPolicy")]

Or apply it to every request:

public void Configure(IApplicationBuilder app)
{
    app.UseCors("MyPolicy");

    // ...

    // This should always be called last to ensure that
    // middleware is registered in the correct order.
    app.UseMvc();
}