Using IDataErrorInfo with Nested Objects

Question

I am using MVVM and I want to use IDataErrorInfo to validate my View.

My current implementation includes Nested objects and different ViewModels. e.g. Business Entity 'Customer' contains Business Entity 'Address'. I am accessing Address directly in my view, like "Customer.Address". To validate changes in Address I would have to implement IDataErrorInfo in Address.

I use Customer or Address in different Views/ViewModels. Usage in different Views/ViewModels lead to different Validation Behavior. Thus, implementing the validation in the Entity itself is insufficient.

Exposing the properties I want to change directly in the ViewModel (creating new Properties that directly set/get the entity) seems to make the ViewModel way too rigid. and quite too large.

I cannot inherit from Base Classes, as some Business Entities already derive from other objects (A fact I cannot change). The only option I see at the moment is adding an interface to the ViewModel to the Business Entities, and forwarding this[] calls in the Business Entities to that ViewModel Interface.

Is there a best practice on how to validate these nested objects in the ViewModel?

EDIT: One more reason Validation I don't see Validation in the Business Objects as a usable idea is that I need different Business Objects in my ViewModel to validate the View and the data entry.

Answer 1

One way I have done this in the past is to expose a ValidationDelegate on the Model, which allows the ViewModel to attach its own validation code to the model.

Typically I do this because I use the Model layer as plain data objects, so my Models only validate basic things such as max-length or not-nulls, while any advanced validation not specific to the data model gets done in the ViewModel. This typically includes things such as ensuring an item is unique, or that a user has permission to set a value to a specific range, or even something like your case where the validation only exists for a specific action.

public class CustomerViewModel
{
    // Keeping these generic to reduce code here, but it
    // should include PropertyChange notification
    public AddressModel Address { get; set; }

    public CustomerViewModel()
    {
        Address = new AddressModel();
        Address.AddValidationDelegate(ValidateAddress);
    }

    // Validation Delegate to validate Adderess
    private string ValidateAddress(object sender, string propertyName)
    {
        // Do your ViewModel-specific validation here.
        // sender is your AddressModel and propertyName 
        // is the property on the address getting validated

        // For example:
        if (propertyName == "Street1" && string.IsNullOrEmpty(Address.Street1))
            return "Street1 cannot be empty";

        return null;
    }
}

Here's the code I usually use for the validation delegate:

#region IDataErrorInfo & Validation Members

#region Validation Delegate

public delegate string ValidationDelegate(
    object sender, string propertyName);

private List<ValidationDelegate> _validationDelegates = 
    new List<ValidationDelegate>();

public void AddValidationDelegate(ValidationDelegate func)
{
    _validationDelegates.Add(func);
}

public void RemoveValidationDelegate(ValidationDelegate func)
{
    if (_validationDelegates.Contains(func))
        _validationDelegates.Remove(func);
}

#endregion // Validation Delegate

#region IDataErrorInfo for binding errors

string IDataErrorInfo.Error { get { return null; } }

string IDataErrorInfo.this[string propertyName]
{
    get { return this.GetValidationError(propertyName); }
}

public string GetValidationError(string propertyName)
{
    string s = null;

    foreach (var func in _validationDelegates)
    {
        s = func(this, propertyName);
        if (s != null)
            return s;
    }

    return s;
}

#endregion // IDataErrorInfo for binding errors

#endregion // IDataErrorInfo & Validation Members