null-conditional-operators.md

title	Null-conditional Operators (C# and Visual Basic)
ms.date	07/20/2015
ms.prod	.net
ms.technology	devlang-csharp
ms.topic	article
ms.assetid	9c7b2c8f-a785-44ca-836c-407bfb6d27f5
caps.latest.revision	3
author	BillWagner
ms.author	wiwagn

Null-conditional Operators (C# and Visual Basic)

Used to test for null before performing a member access (?.) or index (?[) operation. These operators help you write less code to handle null checks, especially for descending into data structures.

int? length = customers?.Length; // null if customers is null   
Customer first = customers?[0];  // null if customers is null  
int? count = customers?[0]?.Orders?.Count();  // null if customers, the first customer, or Orders is null  

Dim length = customers?.Length  ' null if customers is null  
Dim first as Customer = customers?(0)  ' null if customers is null  
Dim count as Integer? = customers?(0)?.Orders?.Count()  ' null if customers, the first customer, or Orders is null  

The last example demonstrates that the null-condition operators are short-circuiting. If one operation in a chain of conditional member access and index operation returns null, then the rest of the chain’s execution stops. Other operations with lower precedence in the expression continue. For example, E in the following executes in the second line, and the ?? and == operations execute. In the first line, the ?? short circuits and E does not execute when the left side evaluates to non-null.

A?.B?.C?[0] ?? E  
A?.B?.C?[0] == E  

A?.B?.C?(0) ?? E  
A?.B?.C?(0) == E  

Another use for the null-condition member access is invoking delegates in a thread-safe way with much less code. The old way requires code like the following:

var handler = this.PropertyChanged;  
if (handler != null)  
    handler(…);

Dim handler = AddressOf(Me.PropertyChanged)  
If handler IsNot Nothing  
    Call handler(…)  

The new way is much simpler:

PropertyChanged?.Invoke(e)  

PropertyChanged?.Invoke(e)

The new way is thread-safe because the compiler generates code to evaluate PropertyChanged one time only, keeping the result in a temporary variable.

You need to explicitly call the Invoke method because there is no null-conditional delegate invocation syntax PropertyChanged?(e). There were too many ambiguous parsing situations to allow it.

Language Specifications

[!INCLUDECSharplangspec]

For more information, see the Visual Basic Language Reference.

See Also

?? (null-coalescing operator)
C# Reference
C# Programming Guide
Visual Basic Language Reference
Visual Basic Programming Guide