Are function parameters evaluated in a C# null-conditional function call?

Question

Are function arguments always evaluated in a C# null-conditional function call?

i.e. in the following code:

obj?.foo(bar());

Is bar evaluated if obj is null?

Answer 1

The spec specifies that

A null_conditional_member_access expression E is of the form P?.A. Let T be the type of the expression P.A. The meaning of E is determined as follows:

• [...]

• If T is a non-nullable value type, then the type of E is T?, and the meaning of E is the same as the meaning of:

  ((object)P == null) ? (T?)null : P.A
  

  Except that P is evaluated only once.

• Otherwise the type of E is T, and the meaning of E is the same as the meaning of:

  ((object)P == null) ? null : P.A
  

  Except that P is evaluated only once.

In your case, P is obj. A is foo(bar()). If we expand both cases:

((object)obj == null) ? (T?)null : obj.foo(bar())

((object)obj == null) ? null : obj.foo(bar())

By the semantics of the ternary operator, when obj is null, the third operand, obj.foo(bar()) will not be evaluated.

Answer 2

Running test code indicates that in the Microsoft compiler at least the arguments are not evaluated, however the C# specification doesn't seem to specify this as required behaviour.

Answer 3

No.

There is no reason to evaluate bar() if obj is null.

Create the example, in dotnetfiddle or elswhere, and make bar output something. Nothing will be outputed.