How do I implement custom target bindings to avoid null reference exceptions

1. Define the Custom Target Binding

First, you need to create a custom target binding class. This class should inherit from `MvxTargetBinding` or one of its subclasses, depending on your requirements. For example, if you're working with a custom view (`MyView`) and want to bind to its `MyProperty`, you might create a binding like this:

csharp
public class MyViewMyPropertyTargetBinding : MvxPropertyInfoTargetBinding
{
    private bool _subscribed;

    public override MvxBindingMode DefaultMode => MvxBindingMode.TwoWay;

    public MyViewMyPropertyTargetBinding(object target, PropertyInfo targetPropertyInfo)
        : base(target, targetPropertyInfo)
    {
    }

    protected override void SetValueImpl(object target, object value)
    {
        var view = target as MyView;
        if (view == null) return; // Check for null to avoid NullReferenceException

        view.MyProperty = (string)value;
    }

    public override void SubscribeToEvents()
    {
        var myView = View;
        if (myView == null)
        {
            MvxBindingTrace.Trace(MvxTraceLevel.Error, "Error - MyView is null in MyViewMyPropertyTargetBinding");
            return;
        }

        _subscribed = true;
        myView.MyPropertyChanged += HandleMyPropertyChanged;
    }

    private void HandleMyPropertyChanged(object sender, EventArgs e)
    {
        var myView = View;
        if (myView == null) return; // Check for null

        FireValueChanged(myView.MyProperty);
    }

    protected override void Dispose(bool isDisposing)
    {
        base.Dispose(isDisposing);
        if (isDisposing)
        {
            var myView = View;
            if (myView != null && _subscribed)
            {
                myView.MyPropertyChanged -= HandleMyPropertyChanged;
                _subscribed = false;
            }
        }
    }
}

2. Register the Custom Target Binding

After defining the custom target binding, you need to register it in your application's setup. This is typically done in the `Setup.cs` file by overriding the `FillTargetFactories` method:

csharp
protected override void FillTargetFactories(IMvxTargetBindingFactoryRegistry registry)
{
    base.FillTargetFactories(registry);
    registry.RegisterPropertyInfoBindingFactory(
        typeof(MyViewMyPropertyTargetBinding),
        typeof(MyView), "MyProperty");
}

3. Handling Null Values

To avoid null reference exceptions, ensure that you check for null before accessing any object. In the example above, checks are performed in `SetValueImpl`, `SubscribeToEvents`, and `HandleMyPropertyChanged` methods.

4. Defensive Programming

Adopt defensive programming practices to handle potential null references. This includes checking preconditions at the surface of your components and throwing specific exceptions if any precondition is not fulfilled. For example, you can use `ArgumentNullException` to handle unexpected null parameters:

csharp
if (customer == null)
{
    throw new ArgumentNullException(nameof(customer));
}

Alternatively, you can use the `ThrowIfNull` method for a more concise approach:

csharp
ArgumentNullException.ThrowIfNull(customer);

5. Using Null-Conditional Operator

In C#, you can use the null-conditional operator (`?.`) to safely navigate through nested objects without causing a null reference exception:

csharp
var authorName = nights?.Author?.Name;

This ensures that if `nights` or `Author` is null, the expression will return null instead of throwing an exception.

Conclusion

Implementing custom target bindings while avoiding null reference exceptions requires careful handling of null values, proper event subscription and cleanup, and adherence to defensive programming practices. By following these guidelines, you can create robust and reliable bindings for your custom views.