Additional model validation

In regards to your question:

I feel as though there could be a way to refactor this code so that Request.CreateErrorResponse only occurs in the code once, but I cannot see how at the moment.

I would consider reducing the nesting (Arrow code) of the If statements and perform follow the return early mantra.

public HttpResponseMessage Post(LongUrlModel model)
{
    if (!ModelState.IsValid)
    {
        return BadRequest();
    }

    var baseAddress = Request.RequestUri.ToString().TrimEnd('/');
    if (model.LongUrl.StartsWith(baseAddress))
    {
        ModelState.AddModelError("", "You cant shorten a URL that belongs to this domain");
        return BadRequest();
    }

    // Perform the valid code logic here
}

private HttpResponseMessage BadRequest()
{
    return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);
}

In regards to your other question

I am not at all fond of the conditional logic here

There's a few things I can think of you could try.

1. Create a custom validation attribute and decorate the LongUrl property.

     public class UrlLengthAttribute: ValidationAttribute
     {
         public UrlLengthAttribute()
         {
         }
   
         protected override ValidationResult IsValid(object value, ValidationContext validationContext)
         {
           var httpContext = new HttpContextWrapper(HttpContext.Current);
           var url = value as string;
   
           if(string.isNullOrWhiteSpace())
           {
             return base.IsValid(value, validationContext);
           }
   
           // I'm not 100% on this code but you get the general idea
           var baseAddress = httpContext.RequestUri.ToString().TrimEnd('/');
   
           if(url.StartsWith(baseAddress))
           {
             return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
           }
   
           return null;
         }
     }
   

You can then use this to decorate your model:

    public class LongUrlModel
    {
        [UrlLength(ErrorMessage = "You cant shorten a URL that belongs to this   domain")]    
        public string LongUrl { get; set; }
    }

1. You could the extract the model validation out into a separate Validator class and use that in the controller instead. At the very least it might be moving towards the lines of creating a unit testable module.

     public class LongUrlModelValidator
     {
        public LongUrlModelValidator(HttpContextBase httpContext)
        {
         // store and use
        }
   
        public bool Validate(LongUrlModel model)
        {
         // perform validation logic here
        }
     }