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By default, Problemize maps common .NET exceptions to appropriate HTTP status codes. You can customize this behavior by implementing your own status code mapper.

​
Understanding the default mapper

Problemize includes a StatusCodeMapper that handles common .NET exceptions: 
// 400 Bad Request
ArgumentNullException
ArgumentOutOfRangeException
InvalidOperationException
ValidationException
FormatException
OverflowException
NullReferenceException

// 401 Unauthorized
UnauthorizedAccessException

// 404 Not Found
KeyNotFoundException
FileNotFoundException
DirectoryNotFoundException

// 405 Method Not Allowed
NotSupportedException

// 408 Request Timeout
TimeoutException

// 501 Not Implemented
NotImplementedException

// 500 Internal Server Error (default)
All other exceptions

​
Implementing a custom mapper

To create your own status code mapper, implement the IStatusCodeMapper interface:
1

Create your mapper class

Implement the IStatusCodeMapper interface with your custom logic:
MyCustomStatusCodeMapper.cs
using FloppyShelf.Problemize.Interfaces;

namespace MyApi.Exceptions;

public class MyCustomStatusCodeMapper : IStatusCodeMapper
{
    public int GetStatusCode(Exception exception)
    {
        return exception switch
        {
            // Your custom exceptions
            ResourceNotFoundException => StatusCodes.Status404NotFound,
            BusinessRuleViolationException => StatusCodes.Status422UnprocessableEntity,
            DuplicateResourceException => StatusCodes.Status409Conflict,
            RateLimitExceededException => StatusCodes.Status429TooManyRequests,
            
            // Fallback to common .NET exceptions
            ArgumentNullException => StatusCodes.Status400BadRequest,
            UnauthorizedAccessException => StatusCodes.Status401Unauthorized,
            KeyNotFoundException => StatusCodes.Status404NotFound,
            TimeoutException => StatusCodes.Status408RequestTimeout,
            
            // Default for unknown exceptions
            _ => StatusCodes.Status500InternalServerError
        };
    }
}
You can chain exception types or add complex logic inside the switch expression based on exception properties.
2

Register your custom mapper

Pass your custom mapper to the UseExceptionHandling method in Program.cs:
Program.cs
using MyApi.Exceptions;
using FloppyShelf.Problemize;

var builder = WebApplication.CreateBuilder(args);

// Register services with custom status code mapper
builder.Services.UseExceptionHandling(
    statusCodeMapper: new MyCustomStatusCodeMapper()
);

var app = builder.Build();

// Enable exception handling middleware
app.UseExceptionHandling();

app.MapControllers();
app.Run();
3

Test your mapper

Throw your custom exceptions in your controllers and verify the correct status codes:
UserController.cs
[ApiController]
[Route("api/[controller]")]
public class UserController : ControllerBase
{
    [HttpGet("{id}")]
    public IActionResult GetUser(int id)
    {
        if (id <= 0)
            throw new ArgumentOutOfRangeException(nameof(id), "User ID must be positive");

        var user = _userService.FindById(id);
        
        if (user == null)
            throw new ResourceNotFoundException($"User with ID {id} not found");

        return Ok(user);
    }

    [HttpPost]
    public IActionResult CreateUser(CreateUserRequest request)
    {
        if (_userService.EmailExists(request.Email))
            throw new DuplicateResourceException($"User with email {request.Email} already exists");

        var user = _userService.Create(request);
        return CreatedAtAction(nameof(GetUser), new { id = user.Id }, user);
    }
}

​
Advanced scenarios

​
Conditional status code mapping

You can implement complex logic based on exception properties: 
public class AdvancedStatusCodeMapper : IStatusCodeMapper
{
    public int GetStatusCode(Exception exception)
    {
        return exception switch
        {
            // Map based on exception properties
            InvalidOperationException { Message: var msg } when msg.Contains("not found") 
                => StatusCodes.Status404NotFound,
            
            InvalidOperationException { Message: var msg } when msg.Contains("unauthorized") 
                => StatusCodes.Status401Unauthorized,
            
            // Check for specific error codes in custom exceptions
            BusinessException { ErrorCode: "PAYMENT_REQUIRED" } 
                => StatusCodes.Status402PaymentRequired,
            
            BusinessException { ErrorCode: "FORBIDDEN" } 
                => StatusCodes.Status403Forbidden,
            
            // Handle aggregate exceptions
            AggregateException agg when agg.InnerExceptions.Any(e => e is UnauthorizedAccessException) 
                => StatusCodes.Status401Unauthorized,
            
            _ => StatusCodes.Status500InternalServerError
        };
    }
}

​
Composing with the default mapper

You can delegate to the default mapper for common exceptions: 
public class ComposedStatusCodeMapper : IStatusCodeMapper
{
    private readonly StatusCodeMapper _defaultMapper = new();

    public int GetStatusCode(Exception exception)
    {
        // Handle custom exceptions first
        var statusCode = exception switch
        {
            ResourceNotFoundException => StatusCodes.Status404NotFound,
            BusinessRuleViolationException => StatusCodes.Status422UnprocessableEntity,
            DuplicateResourceException => StatusCodes.Status409Conflict,
            _ => 0 // Indicate no match
        };

        // Fall back to default mapper if no custom match
        return statusCode != 0 ? statusCode : _defaultMapper.GetStatusCode(exception);
    }
}
Always include a default case in your mapper to handle unexpected exception types. Returning 500 Internal Server Error is recommended for safety.

​
Best practices

Be specific

Map specific exceptions before generic ones in your switch expression.

Use standard codes

Stick to standard HTTP status codes that clients understand.

Include defaults

Always provide a default case to handle unknown exceptions.

Test thoroughly

Verify each exception type returns the expected status code.

​
Next steps

Custom exceptions

Learn how to create custom exception types for your domain.

Validation errors

Handle validation errors with ValidationProblemDetails.

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