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?; // null if customers is null int? count = customers??.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
== 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? ?? E A?.B?.C? == 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:
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.
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.
For more information, see the Visual Basic Language Reference.