VarCharLengthValidatorProvider.cs

public class VarCharLengthValidatorProvider : AssociatedValidatorProvider
{
    protected override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
    {
        var columnAttribute = attributes.OfType<System.Data.Linq.Mapping.ColumnAttribute>().FirstOrDefault();

        if (columnAttribute == null || !columnAttribute.DbType.ToLower().Contains("varchar"))
            yield break;
        var match = Regex.Match(columnAttribute.DbType, @"(\d+)", RegexOptions.IgnoreCase);
        if (match.Success)
        {
            var maxLength = int.Parse(match.Value);
            yield return new StringLengthAttributeAdapter(metadata, context, new StringLengthAttribute(maxLength));
        }
    }
}