为什么Silverlight View不会引发null异常？

Question

Given a Silverlight View with the following binding:

<TextBox Width="200" Text="{Binding Customer.FirstName, Mode=TwoWay}"/>

and the code behind has the following:

CustomerClass Customer {get; set;}

This will not throw a NullReferenceException however the following

String FirstName 
{
    get { return Customer.FirstName; }
}

does when I attempt to bind to FirstName instead of Customer.FirstName , why is this and how could this be corrected? (Other than just binding directly to Customer.FirstName or initializing the CustomerClass object)

EDIT : To address the possible duplicate issue. I thought that the binding still tried to get a reference when the view is first initialized, is this not the case? If so then I can see the difference between getting a reference and just being bound at view time

Answer 1

I thought that the binding still tried to get a reference when the view is first initialized, is this not the case?

The binding process is designed to handle null values and will check the initial target reference which is given and not attempt any action if null. Remember binding is simply the process of reflection off of a named path/location and not the actual extraction of the value.

When the binding discovers that the location it is trying to reflect off of which is Customer.FirstName is null, it stops right there.

But when it is given a binding of FirstName , that location reference for the binding is quite valid. Bingo! Then when the ultimate operation after the binding goes to extract a value the getter is called which ultimately throws an exception because either Customer or FirstName is null.

how could this be corrected?

Look into adding to the binding with either TargetNullValue , which is an alternate binding to use when null. Or directly providing a FallbackValue to use while the binding is null.

Or design the GUI to not rely on sub properties of an object which may be null.

Answer 2

XAML bindings will not throw a NullReferenceException in and of themselves, but they also don't catch exceptions that are thrown by property getters. In some "flavors," like WPF, there are extended binding properties like FallbackValue and TargetNullValue . (I'm not sure how much of that is supported in Silverlight.)

When a binding fails, a trace warning message will be written, but an exception will not be thrown.

In the first case, the property getter is evaluated and returning null .

In the second case, the property getter is evaluated but cannot return a value to the binding system because of the exception.

To correct it, simply check for null in the getter and return a default value if so:

string FirstName 
{
    get { return Customer == null ? string.Empty : Customer.FirstName; }
}