Modify default ErrorMessage for StringLength validation

You can specify the StringLength attribute as follows on numerous properties

[StringLength(20, ErrorMessageResourceName = "StringLengthMessage", ErrorMessageResourceType = typeof(Resource))]
public string OfficePhone { get; set; }
[StringLength(20, ErrorMessageResourceName = "StringLengthMessage", ErrorMessageResourceType = typeof(Resource))]
public string CellPhone { get; set; }

and add the string resource (named StringLengthMessage) in your resource file

"Maximum length is {1}"

Message is defined once and has a variable place holder should you change your mind regarding the length to test against.

You can specify the following:

1. {0} – Name
2. {1} – Maximum Length
3. {2} – Minimum Length

Update

To minimize duplication even further you can subclass StringLengthAttribute:

public class MyStringLengthAttribute : StringLengthAttribute
{
    public MyStringLengthAttribute() : this(20)
    {
    }

    public MyStringLengthAttribute(int maximumLength) : base(maximumLength)
    {
        base.ErrorMessageResourceName = "StringLengthMessage";
        base.ErrorMessageResourceType = typeof (Resource);
    }
}

Or you can override FormatErrorMessage if you want to add additional parameters. Now the properties look as follows:

[MyStringLength]
public string OfficePhone { get; set; }
[MyStringLength]
public string CellPhone { get; set; }