How to customize validation attribute error message?

Question

At the moment I have a custom validation attribute called ExistingFileName (below) but i have given it error messages to display

    protected override System.ComponentModel.DataAnnotations.ValidationResult IsValid(object value, System.ComponentModel.DataAnnotations.ValidationContext validationContext)
    {
        if (value!=null)
        {
            string fileName = value.ToString();
            if (FileExists(fileName))
            {
                return new ValidationResult("Sorry but there is already an image with this name please rename your image");
            }
            else
            {
                return ValidationResult.Success;
            }  
        }
        else
        {
            return new ValidationResult("Please enter a name for your image");
        }
    }

I have implemented it like so:

[ExistingFileName]
public string NameOfImage { get; set; }

Im sure theres a way to define the error message when setting the attribute like below:

[ExistingFileName(errormessage="Blah blah blah")]
public string NameOfImage { get; set; }

But I'm not sure how? Any help is greatly appreciated

Answer 1

Instead of returning ValidationResult with a predefined string, try using the ErrorMessage property, or any other custom properties. For example:

private const string DefaultFileNotFoundMessage = 
    "Sorry but there is already an image with this name please rename your image";

private const string DefaultErrorMessage = 
    "Please enter a name for your image";

public string FileNotFoundMessage { get; set; }

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    if (value!=null)
    {
        string fileName = value.ToString();
        if (FileExists(fileName))
        {
            return new ValidationResult(FileNotFoundMessage ??
                                        DefaultFileNotFoundMessage);
        }
        else
        {
            return ValidationResult.Success;
        }  
    }
    else
    {
        return new ValidationResult(ErrorMessage ?? 
                                    DefaultErrorMessage);
    }
}

And in your annotation:

[ExistingFileName(FileNotFoundMessage = "Uh oh! Not Found!")]
public string NameOfImage { get; set; }

If you don't explicitely set a custom message, it will fallback to the predefined constant in your custom attribute.

Answer 2

Have you inherited from ValidationAttribute?

then you don't need to keep it in a separate variable. All error message code is available when you inherit from ValidationAttribute class.

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class ExistingFileNameAttribute : ValidationAttribute
{
    public string FileFoundMessage = "Sorry but there is already an image with this name please rename your image";
    public ExistingFileNameAttribute()
        : base("Please enter a name for your image")
    {            
    }

    public override ValidationResult IsValid(object value)
    {
        if (value!=null)
        {
            string fileName = value.ToString();
            if (FileExists(fileName))
            {
                return new ValidationResult(FileFoundMessage);
            }
            else
            {
                return ValidationResult.Success;
            }  
        }
        else
        {
            return new ValidationResult(ErrorMessage);
        }
    }
}

Now you can use this to validate your fields/properties

[ExistingFileName(ErrorMessage="Blah blah blah", FileFoundMessage = "Blah Bla")]
public string NameOfImage { get; set; }

and if you use it like below.

[ExistingFileName]
public string NameOfImage { get; set; }

then, it will use the default error message set in the constructor of the ExistingFileName attribute

Hope that helps.