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; }

Just use the Foolproof validation library that is available on Codeplex: https://foolproof.codeplex.com/

It supports, amongst others, the following "requiredif" validation attributes / decorations:

[RequiredIf]
[RequiredIfNot]
[RequiredIfTrue]
[RequiredIfFalse]
[RequiredIfEmpty]
[RequiredIfNotEmpty]
[RequiredIfRegExMatch]
[RequiredIfNotRegExMatch]

To get started is easy:

1. Download the package from the provided link
2. Add a reference to the included .dll file
3. Import the included javascript files
4. Ensure that your views references the included javascript files from within its HTML for unobtrusive javascript and jquery validation.

Using NuGet Package Manager I intstalled this: https://github.com/jwaliszko/ExpressiveAnnotations

And this is my Model:

using ExpressiveAnnotations.Attributes;

public bool HasReferenceToNotIncludedFile { get; set; }

[RequiredIf("HasReferenceToNotIncludedFile == true", ErrorMessage = "RelevantAuditOpinionNumbers are required.")]
public string RelevantAuditOpinionNumbers { get; set; }

I guarantee you this will work!