UseCorsForPaths.cs

public static void UseCorsForPaths(
	this IApplicationBuilder builder,
	string policyName,
	PathString[] paths
)
{
	builder.Use(async (ctx, next) =>
	{
		if (paths.Any((p) => ctx.Request.Path.StartsWithSegments(p)))
		{
			var corsOptions = ctx.RequestServices.GetRequiredService<IOptions<CorsOptions>>();

			var policy = corsOptions.Value.GetPolicy(policyName)
				?? throw new InvalidOperationException($"CORS policy '{policyName}' not found.");

			var corsService = ctx.RequestServices.GetRequiredService<ICorsService>();

			var corsResult = corsService.EvaluatePolicy(ctx, policy);

			if (corsResult.IsPreflightRequest)
			{
				corsService.ApplyResult(corsResult, ctx.Response);
				ctx.Response.StatusCode = StatusCodes.Status204NoContent;
			}
			else
			{
				ctx.Response.OnStarting(() =>
				{
					try
					{
						corsService.ApplyResult(corsResult, ctx.Response);
					}
					catch { /* Ignored */ }

					return Task.CompletedTask;
				});

				await next(ctx);
			}
		}
		else
		{
			await next(ctx);
		}
	});
}