使用`？`不允许检查null和设置值

Question

Why is setting a value using the new ? operator for pre-checking of not null is not usable for assignments?

For example:

var list = // ... Some list
var entry = list.FirstOrDefault();
entry?.Value = 123;

This does not compile due to the error:

The left-hand side of an assignment must be a variable, property or indexer.

But calling a method or some thing else works.

What is the reason for this compiler behavior?

Answer 1

C# compiler could definitely make this statement work by declaring that once the chain of ? s evaluates to null , all further processing stops.

The problem, however, is what to do when there are side effects on the right:

entry?.Value = CallSomeFunctionWithSideEffects(123);

Semantics of this is drastically different from other assignments, when the right-hand side is always evaluated. Here, however, evaluation of the right-hand side would need to be postponed until the left side is known to produce a non- null value.

Function invocation does not suffer from the same problem, because invocation target is processed ahead of parameters:

entry?.SetValue(CallSomeFunctionWithSideEffects(123));

Answer 2

Because entry?.Value might evaluate to null .

Then your code is equivalent to

null = 123;

which is not valid.