c# - Writing a CompareTo DataAnnotation Attribute

Question

I have a situation where I want to compare to fields (example, ensuring the start time is before the end time). I'm using the System.ComponentModel.DataAnnotations attributes for my validation.

My first thought was something like this:

public enum CompareToOperation
{
    EqualTo,
    LessThan,
    GreaterThan
}

public class CompareToAttribute : ValidationAttribute
{
    CompareToOperation _Operation;
    IComparable _Comparision;

    public CompareToAttribute(CompareToOperation operation, Func<IComparable> comparison)
    {
       _Operation = operation;
       _Comparision = comparison();
    }

    public override bool IsValid(object value)
    {
    if (!(value is IComparable))
        return false;

    switch (_Operation)
    {
        case CompareToOperation.EqualTo: return _Comparision.Equals(value);
        case CompareToOperation.GreaterThan: return _Comparision.CompareTo(value) == 1;
        case CompareToOperation.LessThan: return _Comparision.CompareTo(value) == -1;
    }

    return false;
    }
}

public class SimpleClass
{
   public DateTime Start {get;set;}
   [CompareTo(CompareToOperation.GreaterThan, () => this.Start)] // error here
   public DateTime End {get;set;}
}

This doesn't work however, there's a compiler error where the attribute is marked:

Expression cannot contain anonymous methods or lambda expressions

Does anyone have a solution for this? Or a different approach for validating one field compared to the value of another?

Answer 1

Check The AccountMOdel in the default project of MVC2, There is an attribute PropertiesMustMatchAttribute applied to the ChangePasswordModel to validate that the NewPassword and ConfirmPassword Match

   [AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public sealed class PropertiesMustMatchAttribute : ValidationAttribute
{
    private const string _defaultErrorMessage = "'{0}' and '{1}' do not match.";

    private readonly object _typeId = new object();

    public PropertiesMustMatchAttribute(string originalProperty, string confirmProperty)
        : base(_defaultErrorMessage)
    {
        OriginalProperty = originalProperty;
        ConfirmProperty = confirmProperty;
    }

    public string ConfirmProperty
    {
        get;
        private set;
    }

    public string OriginalProperty
    {
        get;
        private set;
    }

    public override object TypeId
    {
        get
        {
            return _typeId;
        }
    }

    public override string FormatErrorMessage(string name)
    {
        return String.Format(CultureInfo.CurrentUICulture, ErrorMessageString,
            OriginalProperty, ConfirmProperty);
    }

    public override bool IsValid(object value)
    {
        PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(value);
        object originalValue = properties.Find(OriginalProperty, true /* ignoreCase */).GetValue(value);
        object confirmValue = properties.Find(ConfirmProperty, true /* ignoreCase */).GetValue(value);
        return Object.Equals(originalValue, confirmValue);
    }
}