MediatR fluent validation response from pipeline behavior

Question

I have a MediatR Pipeline behavior for validating commands with the FluentValidation library. I've seen many examples where you throw a ValidationException from the behavior, and that works fine for me. However in my scenario I want to update my response object with the validation errors.

I am able to build and run the following code. When I set a break point within the if statement the CommandResponse is constructed with the validation errors as expected - but when the response is received by the original caller it is null:

public class RequestValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse> where TRequest : IRequest<TResponse>
{
    private readonly IEnumerable<IValidator<TRequest>> _validators;

    public RequestValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
    {
         _validators = validators;
    }

    public Task<TResponse> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<TResponse> next)
    {
        var context = new ValidationContext(request);

        // Run the associated validator against the request
        var failures = _validators
            .Select(v => v.Validate(context))
            .SelectMany(result => result.Errors)
            .Where(f => f != null)
            .ToList();

        if(failures.Count != 0)
        {
            var commandResponse = new CommandResponse(failures) { isSuccess = false };
            return commandResponse as Task<TResponse>;
        }
        else
        {   
            return next();
        }
    }
}

I think it has to do with my attempt to cast it as Task - but without this I get compiler errors. I'm returning the same type that my command handler would if validation passes so I am at a loss as to why it returns a null instance of the expected response. I feel like there is a better way to handle this, but I've tried a number of variations to no avail. Any suggestions? Is there a better pattern to use? I'd prefer to keep this in the pipeline as it will be reused a lot.

Answer 1

I ended up adding exception handling middleware to the MVC project. Instead of trying to pass back the validation errors as an object I throw a ValidationException inside of the pipeline behavior and the middleware handles any and all exceptions across the entire project. This actually worked out better as I handle all exceptions in one place higher up in the processing chain.

Here is the updated portion of the code I posted:

if(failures.Count != 0)
{
    // If any failures are found, throw a custom ValidationException object
    throw new ValidationException(failures);
}
else
{   
    // If validation passed, allow the command or query to continue:
    return next();
}

Here is the exception handling middleware:

public class ErrorHandlingMiddleware
{
    private readonly RequestDelegate next;

    public ErrorHandlingMiddleware(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context /* other dependencies */)
    {
        try
        {
            await next(context);
        }
        catch (Exception ex)
        {
            await HandleExceptionAsync(context, ex);
        }
    }


    private static Task HandleExceptionAsync(HttpContext context, Exception exception)
    {
        // Log issues and handle exception response

        if (exception.GetType() == typeof(ValidationException))
        {
            var code = HttpStatusCode.BadRequest;
            var result = JsonConvert.SerializeObject(((ValidationException)exception).Failures);
            context.Response.ContentType = "application/json";
            context.Response.StatusCode = (int)code;
            return context.Response.WriteAsync(result);

        }
        else
        {
            var code = HttpStatusCode.InternalServerError;
            var result = JsonConvert.SerializeObject(new { isSuccess = false, error = exception.Message });
            context.Response.ContentType = "application/json";
            context.Response.StatusCode = (int)code;
            return context.Response.WriteAsync(result);
        }
    }
}

You then register the middleware in your Startup before MVC is added:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseMiddleware(typeof(ErrorHandlingMiddleware));
    app.UseMvc();
}

Note: You can also create an extension method for your middleware:

public static class ErrorHandlingMiddlewareExtension
{
    public static IApplicationBuilder UseErrorHandlingMiddleware(
        this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<ErrorHandlingMiddleware>();
    }
}

Which allows you to register it like this:

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseErrorHandlingMiddleware();
    app.UseMvc();
}