Custom Error Responses in a .NET Core Web API with RFC Specification

To implement an error response model based on RFC specification in a .NET Core Web API using C#, you can follow these steps:

Define the error response model according to the RFC specification. The model should include the following properties

  • “status”: HTTP status code (integer)
  • “title”: brief error message (string)
  • “detail”: detailed error message (string)
  • “type”: error type identifier (URI)
  • “instance”: URI of the occurrence that cause the error

Here is an example of how the model can be defined:

public class ErrorResponse
{
    public int Status { get; set; }
    public string Title { get; set; }
    public string Detail { get; set; }
    public string Type { get; set; }
    public string Instance { get; set; }
}

Create an exception handler in the Startup.cs file. This handler should catch any unhandled exceptions that occur during the processing of a request and return an error response using the error response model defined above

Here is an example of how the exception handler can be implemented

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler(errorApp =>
        {
            errorApp.Run(async context =>
            {
                context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
                context.Response.ContentType = "application/problem+json";

                var error = new ErrorResponse
                {
                    Status = context.Response.StatusCode,
                    Title = "An error occurred while processing your request.",
                    Detail = "Please try again later.",
                    Type = "https://www.example.com/errors/internal-server-error",
                    Instance = context.Request.Path
                };

                await context.Response.WriteAsync(JsonConvert.SerializeObject(error));
            });
        });
    }
}

In this example, the exception handler returns an error response with an HTTP status code of 500 (Internal Server Error) and a JSON payload containing the error response model defined above

Throw exceptions in your controller methods when an error occurs. These exceptions will be caught by the exception handler created in above step and returned as an error response to the client.

Here is an example how to throw an exception in a controller method:

[HttpGet("{id}")]
public async Task<ActionResult<Item>> GetItem(int id)
{
    var item = await _context.Items.FindAsync(id);

    if (item == null)
    {
        throw new ItemNotFoundException(id);
    }

    return item;
}

In this example, an ItemNotFoundException is thrown if the item with the specified ID cannot be found in the database. This exception will be caught by the exception handler and returned to the client as an error response.

That’s it! With these steps, you have implemented an error response model based on RFC specification in your .NET Core Web API using C#.

To provide extensions to the error response model based on different errors, you can create custom exception classes that inherit from the base Exception class, and add properties to those classes to represent the additional information that you want to include in the error response.

For example, suppose that you want to add an error code and a stack trace to the error response for a specific type of error. You can create a custom exception class like this:

public class MyCustomException : Exception
{
    public string ErrorCode { get; private set; }
    public string StackTrace { get; private set; }

    public MyCustomException(string message, string errorCode, string stackTrace) 
        : base(message)
    {
        ErrorCode = errorCode;
        StackTrace = stackTrace;
    }
}

When this exception is thrown, you can catch it in the exception handler that you created in the previous step, and include the additional information in the error response model.

Here’s an example of how to catch and handle the MyCustomException in the exception handler:

app.UseExceptionHandler(errorApp =>
{
    errorApp.Run(async context =>
    {
        context.Response.StatusCode = (int)HttpStatusCode.InternalServerError;
        context.Response.ContentType = "application/problem+json";

        var error = new ErrorResponse
        {
            Status = context.Response.StatusCode,
            Title = "An error occurred while processing your request.",
            Detail = "Please try again later.",
            Type = "https://www.example.com/errors/internal-server-error",
            Instance = context.Request.Path
        };

        var exception = context.Features.Get<IExceptionHandlerFeature>().Error;
        if (exception is MyCustomException customException)
        {
            error.Detail = customException.Message;
            error.ErrorCode = customException.ErrorCode;
            error.StackTrace = customException.StackTrace;
        }

        await context.Response.WriteAsync(JsonConvert.SerializeObject(error));
    });
});

In this example, if the exception that was caught is a MyCustomException then the error response will include the error code and stack trace properties from the exception.

You can create as many custom exception classes as you need, each with its own set of properties to represent the additional information that you want to include in the error response. Then, in the exception handler, you can check the type of the exception that was caught and include the appropriate properties in the error response model.

Conclusion

Custom error responses in a .NET Core Web API can provide better information to users and developers. By following RFC specifications and adding extension properties to exception classes, clients can better understand what went wrong and how to fix it, leading to a more efficient application.

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