attribute dependent on another field

Take a look at this: http://blogs.msdn.com/b/simonince/archive/2010/06/04/conditional-validation-in-mvc.aspx

I’ve modded the code somewhat to suit my needs. Perhaps you benefit from those changes as well.

public class RequiredIfAttribute : ValidationAttribute
{
    private RequiredAttribute innerAttribute = new RequiredAttribute();
    public string DependentUpon { get; set; }
    public object Value { get; set; }

    public RequiredIfAttribute(string dependentUpon, object value)
    {
        this.DependentUpon = dependentUpon;
        this.Value = value;
    }

    public RequiredIfAttribute(string dependentUpon)
    {
        this.DependentUpon = dependentUpon;
        this.Value = null;
    }

    public override bool IsValid(object value)
    {
        return innerAttribute.IsValid(value);
    }
}

public class RequiredIfValidator : DataAnnotationsModelValidator<RequiredIfAttribute>
{
    public RequiredIfValidator(ModelMetadata metadata, ControllerContext context, RequiredIfAttribute attribute)
        : base(metadata, context, attribute)
    { }

    public override IEnumerable<ModelClientValidationRule> GetClientValidationRules()
    {
        // no client validation - I might well blog about this soon!
        return base.GetClientValidationRules();
    }

    public override IEnumerable<ModelValidationResult> Validate(object container)
    {
        // get a reference to the property this validation depends upon
        var field = Metadata.ContainerType.GetProperty(Attribute.DependentUpon);

        if (field != null)
        {
            // get the value of the dependent property
            var value = field.GetValue(container, null);

            // compare the value against the target value
            if ((value != null && Attribute.Value == null) || (value != null && value.Equals(Attribute.Value)))
            {
                // match => means we should try validating this field
                if (!Attribute.IsValid(Metadata.Model))
                    // validation failed - return an error
                    yield return new ModelValidationResult { Message = ErrorMessage };
            }
        }
    }
}

Then use it:

public DateTime? DeptDateTime { get; set; }
[RequiredIf("DeptDateTime")]
public string DeptAirline { get; set; }