WinRT XAML Validation library: Implicit and Explicit Validation Execution

Now that you have defined all your validation attributes on your model classes, you want to perform validation to detect if your entity is valid or not. Here you can go two ways with the WinRT XAML Validation library: implicit validation validates your entity automatically when properties change, while with explicit validation you call a function to perform the validation.

Validatable model classes

A prerequisite to perform validation on a model entity with the WinRT XAML Validation library is that your model class derives from the ValidationBindableBase class. This class stores all validation messages for you and is able to perform validation on your entity:

public class AuctionBid : ValidationBindableBase { ... }

Implicit Validation Execution

Implicit validation is triggered automatically when an entity property changes, thus the setter is called. Implicit validation is an opt-in mechanism, meaning that you have to activate this functionality first before your entity gets validated. Just set the IsImplicitValidationEnabled property on your model entity that derives from ValidationBindableBase to true:

var bid = new AuctionBid();
bid.IsImplicitValidationEnabled = true;

If you want to disable implicit validation execution temporarily, you are able to set IsImplicitValidationEnabled=false at any time. Finally for implicit validation to fire, you have to implement your model properties explicitly with a backing field. In the property’s setter, you’ve to call the SetProperty() method from the base class ValidationBindableBase, which performs the validation of the matching attributes for the property:

[ValidateAgainstMaxBid(ShowMessageOnProperty = false)]
public class AuctionBid : ValidationBindableBase
{
    private double newBid;

    [Range(0, double.MaxValue, ErrorMessage = "Value must be greater 0.")]
    [ValidateGreaterCurrentBid]
    public double NewBid
    {
        get { return this.newBid; }
        set { base.SetProperty(ref this.newBid, value); }
    }

    ...
}

Explicit Validation Execution

In many situations you don’t want to execute implicit validation everytime a property changes. For this case the WinRT XAML Validation library gives you the ability to execute validation explicitly by calling the ValidationBindableBase.ValidateAsync() method. Because ValidationBindableBase will be the base class for your model entities, that means calling the method on the entity itself:

var bid = new AuctionBid();
...
bool isValid = await bid.ValidateAsync();

The method returns true if the entity is valid and false otherwise. It’s async because of the async validation ability of attributes from the WinRT XAML Validation library.