C# Language C# 6.0 Features Null propagation
Example
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.
Basics
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
Use with the Null-Coalescing Operator (??)
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
Use with Indexers
The null-conditional operator can be used with indexers:
var firstStudentName = classroom?.Students?[0]?.Name;
In the above example:
- The first
?.ensures thatclassroomis notnull. - The second
?ensures that the entireStudentscollection is notnull. - The third
?.after the indexer ensures that the[0]indexer did not return anullobject. It should be noted that this operation can still throw anIndexOutOfRangeException.
Use with void Functions
Null-conditional operator can also be used with void functions. However in this case, the statement will not evaluate to null. It will just prevent a NullReferenceException.
List<string> list = null;
list?.Add("hi"); // Does not evaluate to null
Use with Event Invocation
Assuming the following event definition:
private event EventArgs OnCompleted;
When invoking an event, traditionally, it is best practice to check if the event is null in case no subscribers are present:
var handler = OnCompleted;
if (handler != null)
{
handler(EventArgs.Empty);
}
Since the null-conditional operator has been introduced, the invocation can be reduced to a single line:
OnCompleted?.Invoke(EventArgs.Empty);
Limitations
Null-conditional operator produces rvalue, not lvalue, that is, it cannot be used for property assignment, event subscription etc. For example, the following code will not work:
// Error: The left-hand side of an assignment must be a variable, property or indexer
Process.GetProcessById(1337)?.EnableRaisingEvents = true;
// Error: The event can only appear on the left hand side of += or -=
Process.GetProcessById(1337)?.Exited += OnProcessExited;
Gotchas
Note that:
int? nameLength = person?.Name.Length; // safe if 'person' is null
is not the same as:
int? nameLength = (person?.Name).Length; // avoid this
because the former corresponds to:
int? nameLength = person != null ? (int?)person.Name.Length : null;
and the latter corresponds to:
int? nameLength = (person != null ? person.Name : null).Length;
Despite ternary operator ?: is used here for explaining the difference between two cases, these operators are not equivalent. This can be easily demonstrated with the following example:
void Main()
{
var foo = new Foo();
Console.WriteLine("Null propagation");
Console.WriteLine(foo.Bar?.Length);
Console.WriteLine("Ternary");
Console.WriteLine(foo.Bar != null ? foo.Bar.Length : (int?)null);
}
class Foo
{
public string Bar
{
get
{
Console.WriteLine("I was read");
return string.Empty;
}
}
}
Which outputs:
Null propagation
I was read
0
Ternary
I was read
I was read
0
To avoid multiple invocations equivalent would be:
var interimResult = foo.Bar;
Console.WriteLine(interimResult != null ? interimResult.Length : (int?)null);
And this difference somewhat explains why null propagation operator is not yet supported in expression trees.
