{x:Null} markup extension
In XAML markup, specifies a null value for a property.
XAML attribute usage
<object property="{x:Null}" .../>
Remarks
null is the null reference keyword for C# and C++. The Microsoft Visual Basic keyword for a null reference is Nothing.
The initial default value can vary between dependency properties, and it is not necessarily null. Further, many dependency properties will not accept null as a value (whether through markup or code) due to their internal implementation. In such cases, setting a XAML attribute value with {x:Null} can result in a parser exception.
Some Windows Runtime types are nullable. In cases where a nullable type does not already have null as the default, you could use {x:Null} in XAML to set to the null value. If using Visual C++ component extensions (C++/CX), nullable types are represented as Platform::IBox<T>. If using Microsoft .NET languages, nullable types are represented as Nullable<T>.