Handle errors in ASP.NET Core web APIs

This article describes how to handle errors and customize error handling with ASP.NET Core web APIs.

Developer Exception Page

The Developer Exception Page shows detailed stack traces for server errors. It uses DeveloperExceptionPageMiddleware to capture synchronous and asynchronous exceptions from the HTTP pipeline and to generate error responses. For example, consider the following controller action, which throws an exception:

[HttpGet("Throw")]
public IActionResult Throw() =>
    throw new Exception("Sample exception.");

When the Developer Exception Page detects an unhandled exception, it generates a default plain-text response similar to the following example:

HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
Server: Kestrel
Transfer-Encoding: chunked

System.Exception: Sample exception.
   at HandleErrorsSample.Controllers.ErrorsController.Get() in ...
   at lambda_method1(Closure , Object , Object[] )
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()

...

If the client requests an HTML-formatted response, the Developer Exception Page generates a response similar to the following example:

HTTP/1.1 500 Internal Server Error
Content-Type: text/html; charset=utf-8
Server: Kestrel
Transfer-Encoding: chunked

<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <meta charset="utf-8" />
        <title>Internal Server Error</title>
        <style>
            body {
    font-family: 'Segoe UI', Tahoma, Arial, Helvetica, sans-serif;
    font-size: .813em;
    color: #222;
    background-color: #fff;
}

h1 {
    color: #44525e;
    margin: 15px 0 15px 0;
}

...

To request an HTML-formatted response, set the Accept HTTP request header to text/html.

Warning

Don't enable the Developer Exception Page unless the app is running in the Development environment. Don't share detailed exception information publicly when the app runs in production. For more information on configuring environments, see Use multiple environments in ASP.NET Core.

Exception handler

In non-development environments, use Exception Handling Middleware to produce an error payload:

  1. In Program.cs, call UseExceptionHandler to add the Exception Handling Middleware:

    var app = builder.Build();
    
    app.UseHttpsRedirection();
    
    if (!app.Environment.IsDevelopment())
    {
        app.UseExceptionHandler("/error");
    }
    
    app.UseAuthorization();
    
    app.MapControllers();
    
    app.Run();
    
  2. Configure a controller action to respond to the /error route:

    [Route("/error")]
    public IActionResult HandleError() =>
        Problem();
    

The preceding HandleError action sends an RFC 7807-compliant payload to the client.

Warning

Don't mark the error handler action method with HTTP method attributes, such as HttpGet. Explicit verbs prevent some requests from reaching the action method.

For web APIs that use Swagger / OpenAPI, mark the error handler action with the [ApiExplorerSettings] attribute and set its IgnoreApi property to true. This attribute configuration excludes the error handler action from the app's OpenAPI specification:

[ApiExplorerSettings(IgnoreApi = true)]

Allow anonymous access to the method if unauthenticated users should see the error.

Exception Handling Middleware can also be used in the Development environment to produce a consistent payload format across all environments:

  1. In Program.cs, register environment-specific Exception Handling Middleware instances:

    if (app.Environment.IsDevelopment())
    {
        app.UseExceptionHandler("/error-development");
    }
    else
    {
        app.UseExceptionHandler("/error");
    }
    

    In the preceding code, the middleware is registered with:

    • A route of /error-development in the Development environment.
    • A route of /error in non-Development environments.

  2. Add controller actions for both the Development and non-Development routes:

    [Route("/error-development")]
    public IActionResult HandleErrorDevelopment(
        [FromServices] IHostEnvironment hostEnvironment)
    {
        if (!hostEnvironment.IsDevelopment())
        {
            return NotFound();
        }
    
        var exceptionHandlerFeature =
            HttpContext.Features.Get<IExceptionHandlerFeature>()!;
    
        return Problem(
            detail: exceptionHandlerFeature.Error.StackTrace,
            title: exceptionHandlerFeature.Error.Message);
    }
    
    [Route("/error")]
    public IActionResult HandleError() =>
        Problem();
    

Use exceptions to modify the response

The contents of the response can be modified from outside of the controller using a custom exception and an action filter:

  1. Create a well-known exception type named HttpResponseException:

    public class HttpResponseException : Exception
    {
        public HttpResponseException(int statusCode, object? value = null) =>
            (StatusCode, Value) = (statusCode, value);
    
        public int StatusCode { get; }
    
        public object? Value { get; }
    }
    
  2. Create an action filter named HttpResponseExceptionFilter:

    public class HttpResponseExceptionFilter : IActionFilter, IOrderedFilter
    {
        public int Order => int.MaxValue - 10;
    
        public void OnActionExecuting(ActionExecutingContext context) { }
    
        public void OnActionExecuted(ActionExecutedContext context)
        {
            if (context.Exception is HttpResponseException httpResponseException)
            {
                context.Result = new ObjectResult(httpResponseException.Value)
                {
                    StatusCode = httpResponseException.StatusCode
                };
    
                context.ExceptionHandled = true;
            }
        }
    }
    

    The preceding filter specifies an Order of the maximum integer value minus 10. This Order allows other filters to run at the end of the pipeline.

  3. In Program.cs, add the action filter to the filters collection:

    builder.Services.AddControllers(options =>
    {
        options.Filters.Add<HttpResponseExceptionFilter>();
    });
    

Validation failure error response

For web API controllers, MVC responds with a ValidationProblemDetails response type when model validation fails. MVC uses the results of InvalidModelStateResponseFactory to construct the error response for a validation failure. The following example replaces the default factory with an implementation that also supports formatting responses as XML, in Program.cs:

builder.Services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.InvalidModelStateResponseFactory = context =>
            new BadRequestObjectResult(context.ModelState)
            {
                ContentTypes =
                {
                    // using static System.Net.Mime.MediaTypeNames;
                    Application.Json,
                    Application.Xml
                }
            };
    })
    .AddXmlSerializerFormatters();

Client error response

An error result is defined as a result with an HTTP status code of 400 or higher. For web API controllers, MVC transforms an error result to produce a ProblemDetails.

The automatic creation of a ProblemDetails for error status codes is enabled by default, but error responses can be configured in one of the following ways:

  1. Use the problem details service
  2. Implement ProblemDetailsFactory
  3. Use ApiBehaviorOptions.ClientErrorMapping

Default problem details response

The following Program.cs file was generated by the web application templates for API controllers:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();

var app = builder.Build();

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

Consider the following controller, which returns BadRequest when the input is invalid:

[Route("api/[controller]/[action]")]
[ApiController]
public class Values2Controller : ControllerBase
{
    // /api/values2/divide/1/2
    [HttpGet("{Numerator}/{Denominator}")]
    public IActionResult Divide(double Numerator, double Denominator)
    {
        if (Denominator == 0)
        {
            return BadRequest();
        }

        return Ok(Numerator / Denominator);
    }

    // /api/values2 /squareroot/4
    [HttpGet("{radicand}")]
    public IActionResult Squareroot(double radicand)
    {
        if (radicand < 0)
        {
            return BadRequest();
        }

        return Ok(Math.Sqrt(radicand));
    }
}

A problem details response is generated with the previous code when any of the following conditions apply:

  • The /api/values2/divide endpoint is called with a zero denominator.
  • The /api/values2/squareroot endpoint is called with a radicand less than zero.

The default problem details response body has the following type, title, and status values:

{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "Bad Request",
  "status": 400,
  "traceId": "00-84c1fd4063c38d9f3900d06e56542d48-85d1d4-00"
}

Problem details service

ASP.NET Core supports creating Problem Details for HTTP APIs using the IProblemDetailsService. For more information, see the Problem details.

The following code configures the app to generate a problem details response for all HTTP client and server error responses that don't have a body content yet:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddProblemDetails();

var app = builder.Build();

app.UseExceptionHandler();
app.UseStatusCodePages();

if (app.Environment.IsDevelopment())
{
    app.UseDeveloperExceptionPage();
}

app.MapControllers();
app.Run();

Consider the API controller from the previous section, which returns BadRequest when the input is invalid:

[Route("api/[controller]/[action]")]
[ApiController]
public class Values2Controller : ControllerBase
{
    // /api/values2/divide/1/2
    [HttpGet("{Numerator}/{Denominator}")]
    public IActionResult Divide(double Numerator, double Denominator)
    {
        if (Denominator == 0)
        {
            return BadRequest();
        }

        return Ok(Numerator / Denominator);
    }

    // /api/values2 /squareroot/4
    [HttpGet("{radicand}")]
    public IActionResult Squareroot(double radicand)
    {
        if (radicand < 0)
        {
            return BadRequest();
        }

        return Ok(Math.Sqrt(radicand));
    }
}

A problem details response is generated with the previous code when any of the following conditions apply:

  • An invalid input is supplied.
  • The URI has no matching endpoint.
  • An unhandled exception occurs.

The automatic creation of a ProblemDetails for error status codes is disabled when the SuppressMapClientErrors property is set to true:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.SuppressMapClientErrors = true;
    });

var app = builder.Build();

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

Using the preceding code, when an API controller returns BadRequest, an HTTP 400 response status is returned with no response body. SuppressMapClientErrors prevents a ProblemDetails response from being created, even when calling WriteAsync for an API Controller endpoint. WriteAsync is explained later in this article.

The next section shows how to customize the problem details response body, using CustomizeProblemDetails, to return a more helpful response. For more customization options, see Customizing problem details.

Customize problem details with CustomizeProblemDetails

The following code uses ProblemDetailsOptions to set CustomizeProblemDetails:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();

builder.Services.AddProblemDetails(options =>
        options.CustomizeProblemDetails = (context) =>
        {

            var mathErrorFeature = context.HttpContext.Features
                                                       .Get<MathErrorFeature>();
            if (mathErrorFeature is not null)
            {
                (string Detail, string Type) details = mathErrorFeature.MathError switch
                {
                    MathErrorType.DivisionByZeroError =>
                    ("Divison by zero is not defined.",
                                          "https://wikipedia.org/wiki/Division_by_zero"),
                    _ => ("Negative or complex numbers are not valid input.",
                                          "https://wikipedia.org/wiki/Square_root")
                };

                context.ProblemDetails.Type = details.Type;
                context.ProblemDetails.Title = "Bad Input";
                context.ProblemDetails.Detail = details.Detail;
            }
        }
    );

var app = builder.Build();

app.UseHttpsRedirection();

app.UseStatusCodePages();

app.UseAuthorization();

app.MapControllers();

app.Run();

The updated API controller:

[Route("api/[controller]/[action]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // /api/values/divide/1/2
    [HttpGet("{Numerator}/{Denominator}")]
    public IActionResult Divide(double Numerator, double Denominator)
    {
        if (Denominator == 0)
        {
            var errorType = new MathErrorFeature
            {
                MathError = MathErrorType.DivisionByZeroError
            };
            HttpContext.Features.Set(errorType);
            return BadRequest();
        }

        return Ok(Numerator / Denominator);
    }

    // /api/values/squareroot/4
    [HttpGet("{radicand}")]
    public IActionResult Squareroot(double radicand)
    {
        if (radicand < 0)
        {
            var errorType = new MathErrorFeature
            {
                MathError = MathErrorType.NegativeRadicandError
            };
            HttpContext.Features.Set(errorType);
            return BadRequest();
        }

        return Ok(Math.Sqrt(radicand));
    }

}

The following code contains the MathErrorFeature and MathErrorType, which are used with the preceding sample:

// Custom Http Request Feature
class MathErrorFeature
{
    public MathErrorType MathError { get; set; }
}

// Custom math errors
enum MathErrorType
{
    DivisionByZeroError,
    NegativeRadicandError
}

A problem details response is generated with the previous code when any of the following conditions apply:

  • The /divide endpoint is called with a zero denominator.
  • The /squareroot endpoint is called with a radicand less than zero.
  • The URI has no matching endpoint.

The problem details response body contains the following when either squareroot endpoint is called with a radicand less than zero:

{
  "type": "https://en.wikipedia.org/wiki/Square_root",
  "title": "Bad Input",
  "status": 400,
  "detail": "Negative or complex numbers are not allowed."
}

View or download sample code

Implement ProblemDetailsFactory

MVC uses Microsoft.AspNetCore.Mvc.Infrastructure.ProblemDetailsFactory to produce all instances of ProblemDetails and ValidationProblemDetails. This factory is used for:

To customize the problem details response, register a custom implementation of ProblemDetailsFactory in Program.cs:

builder.Services.AddControllers();
builder.Services.AddTransient<ProblemDetailsFactory, SampleProblemDetailsFactory>();

Use ApiBehaviorOptions.ClientErrorMapping

Use the ClientErrorMapping property to configure the contents of the ProblemDetails response. For example, the following code in Program.cs updates the Link property for 404 responses:

builder.Services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.ClientErrorMapping[StatusCodes.Status404NotFound].Link =
            "https://httpstatuses.com/404";
    });

Additional resources

This article describes how to handle errors and customize error handling with ASP.NET Core web APIs.

Developer Exception Page

The Developer Exception Page shows detailed stack traces for server errors. It uses DeveloperExceptionPageMiddleware to capture synchronous and asynchronous exceptions from the HTTP pipeline and to generate error responses. For example, consider the following controller action, which throws an exception:

[HttpGet("Throw")]
public IActionResult Throw() =>
    throw new Exception("Sample exception.");

When the Developer Exception Page detects an unhandled exception, it generates a default plain-text response similar to the following example:

HTTP/1.1 500 Internal Server Error
Content-Type: text/plain; charset=utf-8
Server: Kestrel
Transfer-Encoding: chunked

System.Exception: Sample exception.
   at HandleErrorsSample.Controllers.ErrorsController.Get() in ...
   at lambda_method1(Closure , Object , Object[] )
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncActionResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeNextActionFilterAsync()

...

If the client requests an HTML-formatted response, the Developer Exception Page generates a response similar to the following example:

HTTP/1.1 500 Internal Server Error
Content-Type: text/html; charset=utf-8
Server: Kestrel
Transfer-Encoding: chunked

<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <meta charset="utf-8" />
        <title>Internal Server Error</title>
        <style>
            body {
    font-family: 'Segoe UI', Tahoma, Arial, Helvetica, sans-serif;
    font-size: .813em;
    color: #222;
    background-color: #fff;
}

h1 {
    color: #44525e;
    margin: 15px 0 15px 0;
}

...

To request an HTML-formatted response, set the Accept HTTP request header to text/html.

Warning

Don't enable the Developer Exception Page unless the app is running in the Development environment. Don't share detailed exception information publicly when the app runs in production. For more information on configuring environments, see Use multiple environments in ASP.NET Core.

Exception handler

In non-development environments, use Exception Handling Middleware to produce an error payload:

  1. In Program.cs, call UseExceptionHandler to add the Exception Handling Middleware:

    var app = builder.Build();
    
    app.UseHttpsRedirection();
    
    if (!app.Environment.IsDevelopment())
    {
        app.UseExceptionHandler("/error");
    }
    
    app.UseAuthorization();
    
    app.MapControllers();
    
    app.Run();
    
  2. Configure a controller action to respond to the /error route:

    [Route("/error")]
    public IActionResult HandleError() =>
        Problem();
    

The preceding HandleError action sends an RFC 7807-compliant payload to the client.

Warning

Don't mark the error handler action method with HTTP method attributes, such as HttpGet. Explicit verbs prevent some requests from reaching the action method.

For web APIs that use Swagger / OpenAPI, mark the error handler action with the [ApiExplorerSettings] attribute and set its IgnoreApi property to true. This attribute configuration excludes the error handler action from the app's OpenAPI specification:

[ApiExplorerSettings(IgnoreApi = true)]

Allow anonymous access to the method if unauthenticated users should see the error.

Exception Handling Middleware can also be used in the Development environment to produce a consistent payload format across all environments:

  1. In Program.cs, register environment-specific Exception Handling Middleware instances:

    if (app.Environment.IsDevelopment())
    {
        app.UseExceptionHandler("/error-development");
    }
    else
    {
        app.UseExceptionHandler("/error");
    }
    

    In the preceding code, the middleware is registered with:

    • A route of /error-development in the Development environment.
    • A route of /error in non-Development environments.

  2. Add controller actions for both the Development and non-Development routes:

    [Route("/error-development")]
    public IActionResult HandleErrorDevelopment(
        [FromServices] IHostEnvironment hostEnvironment)
    {
        if (!hostEnvironment.IsDevelopment())
        {
            return NotFound();
        }
    
        var exceptionHandlerFeature =
            HttpContext.Features.Get<IExceptionHandlerFeature>()!;
    
        return Problem(
            detail: exceptionHandlerFeature.Error.StackTrace,
            title: exceptionHandlerFeature.Error.Message);
    }
    
    [Route("/error")]
    public IActionResult HandleError() =>
        Problem();
    

Use exceptions to modify the response

The contents of the response can be modified from outside of the controller using a custom exception and an action filter:

  1. Create a well-known exception type named HttpResponseException:

    public class HttpResponseException : Exception
    {
        public HttpResponseException(int statusCode, object? value = null) =>
            (StatusCode, Value) = (statusCode, value);
    
        public int StatusCode { get; }
    
        public object? Value { get; }
    }
    
  2. Create an action filter named HttpResponseExceptionFilter:

    public class HttpResponseExceptionFilter : IActionFilter, IOrderedFilter
    {
        public int Order => int.MaxValue - 10;
    
        public void OnActionExecuting(ActionExecutingContext context) { }
    
        public void OnActionExecuted(ActionExecutedContext context)
        {
            if (context.Exception is HttpResponseException httpResponseException)
            {
                context.Result = new ObjectResult(httpResponseException.Value)
                {
                    StatusCode = httpResponseException.StatusCode
                };
    
                context.ExceptionHandled = true;
            }
        }
    }
    

    The preceding filter specifies an Order of the maximum integer value minus 10. This Order allows other filters to run at the end of the pipeline.

  3. In Program.cs, add the action filter to the filters collection:

    builder.Services.AddControllers(options =>
    {
        options.Filters.Add<HttpResponseExceptionFilter>();
    });
    

Validation failure error response

For web API controllers, MVC responds with a ValidationProblemDetails response type when model validation fails. MVC uses the results of InvalidModelStateResponseFactory to construct the error response for a validation failure. The following example replaces the default factory with an implementation that also supports formatting responses as XML, in Program.cs:

builder.Services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.InvalidModelStateResponseFactory = context =>
            new BadRequestObjectResult(context.ModelState)
            {
                ContentTypes =
                {
                    // using static System.Net.Mime.MediaTypeNames;
                    Application.Json,
                    Application.Xml
                }
            };
    })
    .AddXmlSerializerFormatters();

Client error response

An error result is defined as a result with an HTTP status code of 400 or higher. For web API controllers, MVC transforms an error result to produce a ProblemDetails.

The error response can be configured in one of the following ways:

  1. Implement ProblemDetailsFactory
  2. Use ApiBehaviorOptions.ClientErrorMapping

Implement ProblemDetailsFactory

MVC uses Microsoft.AspNetCore.Mvc.Infrastructure.ProblemDetailsFactory to produce all instances of ProblemDetails and ValidationProblemDetails. This factory is used for:

To customize the problem details response, register a custom implementation of ProblemDetailsFactory in Program.cs:

builder.Services.AddControllers();
builder.Services.AddTransient<ProblemDetailsFactory, SampleProblemDetailsFactory>();

Use ApiBehaviorOptions.ClientErrorMapping

Use the ClientErrorMapping property to configure the contents of the ProblemDetails response. For example, the following code in Program.cs updates the Link property for 404 responses:

builder.Services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.ClientErrorMapping[StatusCodes.Status404NotFound].Link =
            "https://httpstatuses.com/404";
    });

Custom Middleware to handle exceptions

The defaults in the exception handling middleware work well for most apps. For apps that require specialized exception handling, consider customizing the exception handling middleware.

Produce a ProblemDetails payload for exceptions

ASP.NET Core doesn't produce a standardized error payload when an unhandled exception occurs. For scenarios where it's desirable to return a standardized ProblemDetails response to the client, the ProblemDetails middleware can be used to map exceptions and 404 responses to a ProblemDetails payload. The exception handling middleware can also be used to return a ProblemDetails payload for unhandled exceptions.

Additional resources

This article describes how to handle and customize error handling with ASP.NET Core web APIs.

View or download sample code (How to download)

Developer Exception Page

The Developer Exception Page is a useful tool to get detailed stack traces for server errors. It uses DeveloperExceptionPageMiddleware to capture synchronous and asynchronous exceptions from the HTTP pipeline and to generate error responses. To illustrate, consider the following controller action:

[HttpGet("{city}")]
public WeatherForecast Get(string city)
{
    if (!string.Equals(city?.TrimEnd(), "Redmond", StringComparison.OrdinalIgnoreCase))
    {
        throw new ArgumentException(
            $"We don't offer a weather forecast for {city}.", nameof(city));
    }
    
    return GetWeather().First();
}

Run the following curl command to test the preceding action:

curl -i https://localhost:5001/weatherforecast/chicago

The Developer Exception Page displays a plain-text response if the client doesn't request HTML-formatted output. The following output appears:

HTTP/1.1 500 Internal Server Error
Transfer-Encoding: chunked
Content-Type: text/plain
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Date: Fri, 27 Sep 2019 16:13:16 GMT

System.ArgumentException: We don't offer a weather forecast for chicago. (Parameter 'city')
   at WebApiSample.Controllers.WeatherForecastController.Get(String city) in C:\working_folder\aspnet\AspNetCore.Docs\aspnetcore\web-api\handle-errors\samples\3.x\Controllers\WeatherForecastController.cs:line 34
   at lambda_method(Closure , Object , Object[] )
   at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ActionMethodExecutor.SyncObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeActionMethodAsync>g__Logged|12_1(ControllerActionInvoker invoker)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.<InvokeNextActionFilterAsync>g__Awaited|10_0(ControllerActionInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Rethrow(ActionExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|19_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Logged|17_1(ResourceInvoker invoker)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)

HEADERS
=======
Accept: */*
Host: localhost:44312
User-Agent: curl/7.55.1

To display an HTML-formatted response instead, set the Accept HTTP request header to the text/html media type. For example:

curl -i -H "Accept: text/html" https://localhost:5001/weatherforecast/chicago

Consider the following excerpt from the HTTP response:

HTTP/1.1 500 Internal Server Error
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Date: Fri, 27 Sep 2019 16:55:37 GMT

<!DOCTYPE html>
<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <meta charset="utf-8" />
        <title>Internal Server Error</title>
        <style>
            body {
    font-family: 'Segoe UI', Tahoma, Arial, Helvetica, sans-serif;
    font-size: .813em;
    color: #222;
    background-color: #fff;
}

The HTML-formatted response becomes useful when testing via tools like Postman. The following screen capture shows both the plain-text and the HTML-formatted responses in Postman:

Test the Developer Exception Page in Postman.

Warning

Enable the Developer Exception Page only when the app is running in the Development environment. Don't share detailed exception information publicly when the app runs in production. For more information on configuring environments, see Use multiple environments in ASP.NET Core.

Don't mark the error handler action method with HTTP method attributes, such as HttpGet. Explicit verbs prevent some requests from reaching the action method. Allow anonymous access to the method if unauthenticated users should see the error.

Exception handler

In non-development environments, Exception Handling Middleware can be used to produce an error payload:

  1. In Startup.Configure, invoke UseExceptionHandler to use the middleware:

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseExceptionHandler("/error");
        }
    
        app.UseHttpsRedirection();
        app.UseRouting();
        app.UseAuthorization();
        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
    
  2. Configure a controller action to respond to the /error route:

    [ApiController]
    public class ErrorController : ControllerBase
    {
        [Route("/error")]
        public IActionResult Error() => Problem();
    }
    

The preceding Error action sends an RFC 7807-compliant payload to the client.

Exception Handling Middleware can also provide more detailed content-negotiated output in the local development environment. Use the following steps to produce a consistent payload format across development and production environments:

  1. In Startup.Configure, register environment-specific Exception Handling Middleware instances:

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseExceptionHandler("/error-local-development");
        }
        else
        {
            app.UseExceptionHandler("/error");
        }
    }
    

    In the preceding code, the middleware is registered with:

    • A route of /error-local-development in the Development environment.
    • A route of /error in environments that aren't Development.

  2. Apply attribute routing to controller actions:

    [ApiController]
    public class ErrorController : ControllerBase
    {
        [Route("/error-local-development")]
        public IActionResult ErrorLocalDevelopment(
            [FromServices] IWebHostEnvironment webHostEnvironment)
        {
            if (webHostEnvironment.EnvironmentName != "Development")
            {
                throw new InvalidOperationException(
                    "This shouldn't be invoked in non-development environments.");
            }
    
            var context = HttpContext.Features.Get<IExceptionHandlerFeature>();
    
            return Problem(
                detail: context.Error.StackTrace,
                title: context.Error.Message);
        }
    
        [Route("/error")]
        public IActionResult Error() => Problem();
    }
    

    The preceding code calls ControllerBase.Problem to create a ProblemDetails response.

Use exceptions to modify the response

The contents of the response can be modified from outside of the controller. In ASP.NET 4.x Web API, one way to do this was using the HttpResponseException type. ASP.NET Core doesn't include an equivalent type. Support for HttpResponseException can be added with the following steps:

  1. Create a well-known exception type named HttpResponseException:

    public class HttpResponseException : Exception
    {
        public int Status { get; set; } = 500;
    
        public object Value { get; set; }
    }
    
  2. Create an action filter named HttpResponseExceptionFilter:

    public class HttpResponseExceptionFilter : IActionFilter, IOrderedFilter
    {
        public int Order { get; } = int.MaxValue - 10;
    
        public void OnActionExecuting(ActionExecutingContext context) { }
    
        public void OnActionExecuted(ActionExecutedContext context)
        {
            if (context.Exception is HttpResponseException exception)
            {
                context.Result = new ObjectResult(exception.Value)
                {
                    StatusCode = exception.Status,
                };
                context.ExceptionHandled = true;
            }
        }
    }
    

    The preceding filter specifies an Order of the maximum integer value minus 10. This Order allows other filters to run at the end of the pipeline.

  3. In Startup.ConfigureServices, add the action filter to the filters collection:

    services.AddControllers(options =>
        options.Filters.Add(new HttpResponseExceptionFilter()));
    

Validation failure error response

For web API controllers, MVC responds with a ValidationProblemDetails response type when model validation fails. MVC uses the results of InvalidModelStateResponseFactory to construct the error response for a validation failure. The following example uses the factory to change the default response type to SerializableError in Startup.ConfigureServices:

services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.InvalidModelStateResponseFactory = context =>
        {
            var result = new BadRequestObjectResult(context.ModelState);

            // TODO: add `using System.Net.Mime;` to resolve MediaTypeNames
            result.ContentTypes.Add(MediaTypeNames.Application.Json);
            result.ContentTypes.Add(MediaTypeNames.Application.Xml);

            return result;
        };
    });

Client error response

An error result is defined as a result with an HTTP status code of 400 or higher. For web API controllers, MVC transforms an error result to a result with ProblemDetails.

The error response can be configured in one of the following ways:

  1. Implement ProblemDetailsFactory
  2. Use ApiBehaviorOptions.ClientErrorMapping

Implement ProblemDetailsFactory

MVC uses Microsoft.AspNetCore.Mvc.Infrastructure.ProblemDetailsFactory to produce all instances of ProblemDetails and ValidationProblemDetails. This factory is used for:

To customize the problem details response, register a custom implementation of ProblemDetailsFactory in Startup.ConfigureServices:

public void ConfigureServices(IServiceCollection serviceCollection)
{
    services.AddControllers();
    services.AddTransient<ProblemDetailsFactory, CustomProblemDetailsFactory>();
}

Use ApiBehaviorOptions.ClientErrorMapping

Use the ClientErrorMapping property to configure the contents of the ProblemDetails response. For example, the following code in Startup.ConfigureServices updates the type property for 404 responses:

services.AddControllers()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.SuppressConsumesConstraintForFormFileParameters = true;
        options.SuppressInferBindingSourcesForParameters = true;
        options.SuppressModelStateInvalidFilter = true;
        options.SuppressMapClientErrors = true;
        options.ClientErrorMapping[StatusCodes.Status404NotFound].Link =
            "https://httpstatuses.com/404";
        options.DisableImplicitFromServicesParameters = true;
    });

Custom Middleware to handle exceptions

The defaults in the exception handling middleware work well for most apps. For apps that require specialized exception handling, consider customizing the exception handling middleware.

Producing a ProblemDetails payload for exceptions

ASP.NET Core doesn't produce a standardized error payload when an unhandled exception occurs. For scenarios where it's desirable to return a standardized ProblemDetails response to the client, the ProblemDetails middleware can be used to map exceptions and 404 responses to a ProblemDetails payload. The exception handling middleware can also be used to return a ProblemDetails payload for unhandled exceptions.