string.Format in Data Annotation Validation Attributes

Question

Is there any way to set Data Annotation messages using formatted string instead of direct constant,

I need to set required field ErrorMessage like in following code, but it gives me an error.

[Required(ErrorMessage = string.Format(SystemMessages.Required, "First Name"))]
public string FirstName { get; set; }

SystemMessages is a constant file and it has following code,

public const string Required = "{0} is required.";

Is there any way to set attributes like this?

Answer 1

string.Format result is not a compile time constant and can't bee evaluated by the compiler. Attribute values are limited to

constant expression, typeof expression or array creation expression of an attribute parameter type.

More formally, you can find description of limitations at msdn . Formatting a string doesn't fit any of them, which are:

• Simple types (bool, byte, char, short, int, long, float, and double)
• string
• System.Type
• enums
• object (The argument to an attribute parameter of type object must be a constant value of -one of the above types.)
• One-dimensional arrays of any of the above types

At best you can try something similar to:

[Required(ErrorMessage = "First Name" + Required)]
public string FirstName { get; set; }


public const string Required = " is required.";

Answer 2

Just use DisplayAttribute , if you want to output display name instead of property name:

[Required]
[Display(Name = "First Name")]
public string FirstName { get; set; }

Answer 3

Please implement IValidatableObject which use Validate method. It will look like:

  public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
  {
       var memberNames = new List<string>();
       if (string.IsNullOrEmpty(this.FirstName )
        {
            memberNames.Add("FirstName");
            yield return new ValidationResult(string.Format(Resources.Required, "First Name"), memberNames);
        }
  }

If it is not clear please just read about IValidatableObject . Good luck!