使用IValidatableObject在MVC 3中验证上传的图像

Question

I'm running into some trouble validating uploaded images using the new IValidatabeObject interface in asp.net MVC 3.

I need to do two things:

• Make sure that the uploaded bytes are really an image
• Make sure that this image is 200px wide

I thought I could accomplish this using a try/catch in the validation block:

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        try
        {
            System.Drawing.Image img = System.Drawing.Image.FromStream(this.ImageFile.InputStream);
            if (img.Width != 200)
                yield return new ValidationResult("This picture isn't the right size!!!!");
        }
        catch (Exception ex)
        {
            yield return new ValidationResult("This isn't a real image!") ;
        }
    }

Of course, in a real world production app I would put some lightweight validation in front of this to check the file extension to make sure it is a .jpg to find easy problems without invoking the expensive try/catch mechanism. But ultimately I need to be sure it's a good image, so I do need to make sure that Image.FromStream will parse it successfully.

The compiler doesn't like this approach and tells me that I can't yield return from a try/catch block. If I dispense with the yield return and return an IEnumerable directly I get another compiler error:

        catch (Exception ex)
        {
            return new ValidationResult[] {new ValidationResult("This isn't a real image!") };
        }

This gives new error: "Cannot return a value from an iterator. Use yield return statement..."

I like having the validation logic as part of the model, so I would hate to abandon the IValidatableObject approach, but I don't see a way to make it work.

Answer 1

Move the result to outside the try catch block

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext) 
{
  ValidationResult result = null;
  try
    {
        var img = System.Drawing.Image.FromStream(this.ImageFile.InputStream);
        if (img.Width != 200) 
            result = new ValidationResult("This picture isn't the right size!!!!");
    }
    catch (Exception ex)
    {
        result = new ValidationResult("This isn't a real image!") ;
    }

  if (result != null) 
    yield return result;
}