Question

I have a model:

public class Product
{
    public int Rating { get; set; }
    ...
}

and a View Model:

public class ProductViewModel: IDataErrorProvider
{
    public int Temperature { get; set; }
    public Product CurrentProduct { get; set; }

    public string this[string columnName]
    {
        get
        {
            if (columnName == "Rating")
            {
                if (CurrentProduct.Rating > Temperature)
                    return "Rating is too high for current temperature";
            }
            return null;
        }
    }
}

My view has an instance of ProductViewModel as the DataContext. The view has the field:

<TextBox Text={Binding Path=CurrentProduct.Rating, ValidatesOnDataErrors=True} .../>

By default, validation occurs on the IDataErrorProvider of the bound object (Product), not the DataContext (ProductViewModel). So in the above instance, ProductViewModel validation is never called. This is just a simple example but illustrates the problem. The model doesn’t (and shouldn’t) know about Temperature, so the design dictates that the VM should perform the validation on that field.

Yes, I could hack it and replicate the bound properties of the model directly in the ViewModel, but I would have thought there must be an easier way to redirect the call to the VM rather than the model?

Answer 1

I’ve encountered that problem before, and my solution is to expose a validation delegate from my Models which is checked when validating the class, and the ViewModel can use this to hook addition validation to the class that is unrelated to Model itself

For example, I would use code that looked something like this from the ViewModel to attach a validation delegate to the Model anytime its set

public class ProductViewModel
{
    public int Temperature { get; set; }

    private product _currentProduct;
    public Product CurrentProduct 
    { 
        get { return _currentProduct; }
        set
        {
            if (value != _currentProduct)
            {
                if (_currentProduct != null)
                    _currentProduct.RemoveValidationDelegate(ValidateProduct);

                _currentProduct = value;

                if (_currentProduct != null)
                    _currentProduct.AddValidationDelegate(ValidateProduct);

                RaisePropertyChanged("CurrentProduct");
            }
        }
    }

    // Product Validation Delegate to verify temperature
    private string ValidateProduct(object sender, string propertyName)
    {
        if (propertyName == "Rating")
        {
            if (CurrentProduct.Rating > Temperature)
                return "Rating is too high for current temperature";
        }

        return null;
    }
}

The actual code that adds the ValidationDelegate to the Model is pretty generic, so I typically have it in a BaseViewModel so all Models can have this functionality without me having to type it out for each one

#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

I also have this approach outlined in my blog post here if you want to see another example.