app.UseCustomErrorPages();

public class CustomErrorPagesMiddleware
{private readonly RequestDelegate _next;private readonly ILogger _logger;public CustomErrorPagesMiddleware(RequestDelegate next,ILoggerFactory loggerFactory){_next = next;_logger = loggerFactory.CreateLogger<CustomErrorPagesMiddleware>();}public async Task Invoke(HttpContext context){try{await _next(context);}catch (Exception ex){_logger.LogError(0, ex, "An unhandled exception has occurred while executing the request");if (context.Response.HasStarted){_logger.LogWarning("The response has already started, the error page middleware will not be executed.");throw;}try{context.Response.Clear();context.Response.StatusCode = 500;return;}catch (Exception ex2){_logger.LogError(0, ex2, "An exception was thrown attempting to display the error page.");}throw;}finally{var statusCode = context.Response.StatusCode;if (statusCode == 404 || statusCode == 500){await ErrorPage.ResponseAsync(context.Response, statusCode);}}}
}

public static class ErrorPage
{public static async Task ResponseAsync(HttpResponse response, int statusCode){if (statusCode == 404){await response.WriteAsync(Page404);}else if (statusCode == 500){await response.WriteAsync(Page500);}}private static string Page404 => @"html...";private static string Page500 => @"html...";
}

public static class CustomErrorPagesExtensions
{public static IApplicationBuilder UseCustomErrorPages(this IApplicationBuilder app){if (app == null){throw new ArgumentNullException(nameof(app));}return app.UseMiddleware<CustomErrorPagesMiddleware>();}
}