This will be a very simple blog post but it might come in handy when you want to add (complex) logic to validate property list items with a custom class. Let’s presume we have a page with a property list of type CustomData.

[ContentType(GUID = "{EA443137-A4FD-402A-A9AB-BD0045DA700C}")]
public class ExamplePage : PageData
{
    [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor<CustomData>))]
    public virtual  IList<CustomData> MyCustomData { get; set; }
}
public class CustomData
{
    public int FirstValue { get; set; }
    public int SecondValue { get; set; } // SecondValue should be > FirstValue
}
[PropertyDefinitionTypePlugIn]
public class CustomDataPropertyList : PropertyList<CustomData>
{
}

As I want to add some validation to CustomData, the first thing I tried was adding IValidatableObject to the CustomData class, however the Validate method was never called. Episerver has a couple of built-in validation attributes for validating the items of a PropertyList (with primitive types).

Existing validation attributes

I decided to go the same route and write a custom validation attribute and add my validation logic using an attribute.

[AttributeUsage(AttributeTargets.Property)]
public class CustomDataValidationAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (!(value is CustomData customData))
        {
            return false;
        }

        return customData.FirstValue < customData.SecondValue;
    }
}

[AttributeUsage(AttributeTargets.Property)]
public class ItemCustomDataValidationAttribute : CustomDataValidationAttribute
{
    public override bool IsValid(object value)
    {
        return ListItemValidator.IsValid(value, base.IsValid);
    }
}

/// <summary>
/// Helper method. <see cref="EPiServer.DataAnnotations.Internal.ListItemValidator"/>
/// </summary>
public static class ListItemValidator
{
    public static bool IsValid(object value, Func<object, bool> itemValidator)
    {
        if (value == null)
        {
            return true;
        }

        return value is IEnumerable enumerable && enumerable.Cast<object>().All(itemValidator);
    }
}

CustomDataValidationAttribute contains the actual validation logic. ItemCustomDataValidationAttribute is the attribute that will be added to the PropertyList property on the ExamplePage in order to validate the items in the list. Lastly ListItemValidator is a helper method to iterate the list and execute the validation for each of the items in the PropertyList. Add the ItemCustomDataValidationAttribute to the property and you’ll receive an error message when the validation fails.

[ContentType(GUID = "{EA443137-A4FD-402A-A9AB-BD0045DA700C}")]
public class MyPage : PageData
{
    [ItemCustomDataValidation]
    [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor<CustomData>))]
    public virtual  IList<CustomData> MyCustomData { get; set; }
}

This way you can add custom logic to validate a single item, as we did in the example, or validate a list of items as a whole (just add logic to ItemCustomDataValidationAttribute).

NOTE: this API is still internal, use at own risk. Only lists using primitive types are supported by Episerver