IDataErrorInfo/WPF Binding - Validation over many ViewModels - wpf

Question

I'm trying to find an easy way to validate over a collection of ViewModels, using the IDataErrorInfo Interface.
I have a ListBox, which is bound to an ObservableCollection of ViewModels.
1 Class "DataView<VMUser>" with an ObservableCollection<VMUser>
1 ViewModel Class "VMUser"
If I implement the IDataErrorInfo into my ViewModel, i can validate for example if the Age > 21 and so on... But i cannot validate if there is no other user with the same email for example, because the ViewModels don't know anything from each other.
I didn't find a way to force the Bindings in my VMUser-DataTemplate to use the IDataErrorInfo of the DataView Class. (without clicking the OK-Button...)

Answer 1

For validation that is based on business rules, I usually expose a Validation Delegate that my ViewModel can set.
For example, the ViewModel containing your collection might look like this:
public ParentViewModel()
{
foreach(var user in UserCollection)
user.AddValidationErrorDelegate(ValidateUser);
}
private string ValidateUser(object sender, string propertyName)
{
if (propertyName == "Email")
{
var user = (UserVM)sender;
if (UserCollection.Count(p => p.Email== user.Email) > 1)
return "Another user already has this Email Address";
}
return null;
}
The idea is that your Model should only contain raw data, therefore it should only validate raw data. This can include validating things like maximum lengths, required fields, and allowed characters. Business Logic, which includes business rules, should be validated in the ViewModel, and this allows that to happen.
The actual implementation of my IDataErrorInfo on the UserVM class would look like this:
#region IDataErrorInfo & Validation Members
/// <summary>
/// List of Property Names that should be validated
/// </summary>
protected List<string> ValidatedProperties = new List<string>();
#region Validation Delegate
public delegate string ValidationErrorDelegate(object sender, string propertyName);
private List<ValidationErrorDelegate> _validationDelegates = new List<ValidationErrorDelegate>();
public void AddValidationErrorDelegate(ValidationErrorDelegate func)
{
_validationDelegates.Add(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)
{
// If user specified properties to validate, check to see if this one exists in the list
if (ValidatedProperties.IndexOf(propertyName) < 0)
{
//Debug.Fail("Unexpected property being validated on " + this.GetType().ToString() + ": " + propertyName);
return null;
}
string s = null;
// If user specified a Validation method to use, Validate property
if (_validationDelegates.Count > 0)
{
foreach (ValidationErrorDelegate func in _validationDelegates)
{
s = func(this, propertyName);
if (s != null)
{
return s;
}
}
}
return s;
}
#endregion // IDataErrorInfo for binding errors
#region IsValid Property
public bool IsValid
{
get
{
return (GetValidationError() == null);
}
}
public string GetValidationError()
{
string error = null;
if (ValidatedProperties != null)
{
foreach (string s in ValidatedProperties)
{
error = GetValidationError(s);
if (error != null)
{
return error;
}
}
}
return error;
}
#endregion // IsValid Property
#endregion // IDataErrorInfo & Validation Members