How to remove prefixes from ModelState keys?

Question

For example, there is a Web Api action method:

public HttpMessageResponse Post(UserDto userDto)
{
    if (!this.ModelState.IsValid)
    {
        return this.Req         


        

Answer 1

For the first part:

Also error messages returned to a client shouldn't contain those prefixes

I agree having the parameter name as a prefix on all model state errors isn't great behavior. Fortunately, the service that has this behavior is replaceable. You just need to have a custom IBodyModelValidator. Here's what it would look like (using the Decorator pattern to let the default service do most of the work):

public class PrefixlessBodyModelValidator : IBodyModelValidator
{
    private readonly IBodyModelValidator _innerValidator;

    public PrefixlessBodyModelValidator(IBodyModelValidator innerValidator)
    {
        if (innerValidator == null)
        {
            throw new ArgumentNullException("innerValidator");
        }

        _innerValidator = innerValidator;
    }

    public bool Validate(object model, Type type, ModelMetadataProvider metadataProvider, HttpActionContext actionContext, string keyPrefix)
    {
        // Remove the keyPrefix but otherwise let innerValidator do what it normally does.
        return _innerValidator.Validate(model, type, metadataProvider, actionContext, String.Empty);
    }
}

Then, wrap the default service with yours:

config.Services.Replace(typeof(IBodyModelValidator), new PrefixlessBodyModelValidator(config.Services.GetBodyModelValidator()));

For the second part:

elso replace "modelState" with "errors"

The reason it currently says "modelState" is your current code:

return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);

Is effectively doing the following:

HttpError error = new HttpError(ModelState, false);
return Request.CreateResponse(HttpStatusCode.BadRequest, error);

Since HttpError is being serialized, and it has a property named "ModelState", that's what you see in the response.

If you want a different property name, you can use a custom error class:

public class PrettyHttpError
{
    public PrettyHttpError(ModelStateDictionary modelState)
    {
        Message = "Your request is invalid.";
        Errors = new Dictionary<string, IEnumerable<string>>();

        foreach (var item in modelState)
        {
            var itemErrors = new List<string>();
            foreach (var childItem in item.Value.Errors)
            {
                itemErrors.Add(childItem.ErrorMessage);
            }
            Errors.Add(item.Key, itemErrors);
        }
    }

    public string Message { get; set; }

    public IDictionary<string, IEnumerable<string>> Errors { get; set; }
}

And then create your response with this error type instead of HttpError:

PrettyHttpError error = new PrettyHttpError(ModelState);
return Request.CreateResponse(HttpStatusCode.BadRequest, error);

The combination of PrettyHttpError and PrefixlessBodyModelValidator gives the output you requested.