问题
While experimenting with ternary and null coalesce operators in C# I discovered that it is possible to use assignments on the right-hand side of expressions, for example this is a valid C# code:
int? a = null;
int? b = null;
int? c = a ?? (b = 12);
int? d = a == 12 ? a : (b = 15);
Strangely enough, not only the assignment on the right-hand side of the expression is evaluated to its own right-hand side (meaning that the third line here is evaluated to 12 and not to something like b = 12 => void), but this assignment also effectively works, so that two variables are assigned in one statement. One can also use any computable expression on the right-hand side of this assignment, with any available variable.
This behaviour seems to me to be very strange. I remember having troubles with if (a = 2) instead of if (a == 2) comparison in C++, which is always evaluated to true and this is a common mistake after switching from Basic/Haskell to C++.
Is it a documented feature? Is there any name for it?
回答1:
This happens as consequence of the assignment operator also returning the value:
The assignment operator (=) stores the value of its right-hand operand in the storage location, property, or indexer denoted by its left-hand operand and returns the value as its result.
The expression b = 12 not only assigns 12 to b, but also returns this value.
回答2:
Multiple assignment works in C#:
int a;
int b;
int c;
a = b = c = 5;
//all variables = 5;
if(6 == (c = 6)){
//this will be true;
}
If you put a variable on the right side of an equation, even if it has just been assigned a value itself on the same line, it returns its value/reference.
来源：https://stackoverflow.com/questions/8385931/ternary-null-coalescing-operator-and-assignment-expression-on-the-right-hand-sid