The ?.
operator and ?[...]
operator are called the null-conditional operator. It is also sometimes referred to by other names such as the safe navigation operator.
This is useful, because if the .
(member accessor) operator is applied to an expression that evaluates to null
, the program will throw a NullReferenceException
. If the developer instead uses the ?.
(null-conditional) operator, the expression will evaluate to null instead of throwing an exception.
Note that if the ?.
operator is used and the expression is non-null, ?.
and .
are equivalent.
var teacherName = classroom.GetTeacher().Name;
// throws NullReferenceException if GetTeacher() returns null
If the classroom
does not have a teacher, GetTeacher()
may return null
. When it is null
and the Name
property is accessed, a NullReferenceException
will be thrown.
If we modify this statement to use the ?.
syntax, the result of the entire expression will be null
:
var teacherName = classroom.GetTeacher()?.Name;
// teacherName is null if GetTeacher() returns null
Subsequently, if classroom
could also be null
, we could also write this statement as:
var teacherName = classroom?.GetTeacher()?.Name;
// teacherName is null if GetTeacher() returns null OR classroom is null
This is an example of short-circuiting: When any conditional access operation using the null-conditional operator evaluates to null, the entire expression evaluates to null immediately, without processing the rest of the chain.
When the terminal member of an expression containing the null-conditional operator is of a value type, the expression evaluates to a Nullable<T>
of that type and so cannot be used as a direct replacement for the expression without ?.
.
bool hasCertification = classroom.GetTeacher().HasCertification;
// compiles without error but may throw a NullReferenceException at runtime
bool hasCertification = classroom?.GetTeacher()?.HasCertification;
// compile time error: implicit conversion from bool? to bool not allowed
bool? hasCertification = classroom?.GetTeacher()?.HasCertification;
// works just fine, hasCertification will be null if any part of the chain is null
bool hasCertification = classroom?.GetTeacher()?.HasCertification.GetValueOrDefault();
// must extract value from nullable to assign to a value type variable
You can combine the null-conditional operator with the Null-coalescing Operator (??
) to return a default value if the expression resolves to null
. Using our example above:
var teacherName = classroom?.GetTeacher()?.Name ?? "No Name";
// teacherName will be "No Name" when GetTeacher()
// returns null OR classroom is null OR Name is null