Question

So in C#, you might have the following code:

void DoSomething()
{
    //some code.
    int x = 5;
    //some more code.
}

As soon as you enter DoSomething, the CLR sets up space for int x. Why does it not wait until it reaches the line with int x =5 on it? Especially since even though x is bound, it doesn’t let you actually use it until that line is reached anyway?

Answer 1

As soon as you enter DoSomething, the CLR sets up space for int x. Why does it not wait until it reaches the line with int x = 5 on it?

The question is not answerable because the entire question is founded on an incorrect premise. The storage space for the local variable may be:

• allocated when the method is first entered
• allocated when control reaches the declaration
• allocated when control reaches the initialization (assuming initialization and declaration are different)
• allocated under special circumstances — if for example the local is a closed-over local of a lambda, or in an iterator block, or in an async block, how and when the local storage is allocated can get complicated
• elided entirely; if the local is never used then it might not be allocated in the first place.

The C# compiler and the jit compiler definitely ensure that the local storage is allocated in a way that is correct, and attempt to ensure that it is efficient. How they choose to do so depends on the exact situation. It might be more efficient to allocate the space up front, and it might be more efficient to allocate it for only as long as the variable is in use; the jitter is permitted broad lattitude in choosing the lifetime of a local variable. Local variables are permitted to live both longer and shorter than their scopes would imply if the jitter can do so without violating program correctness.

Since the premise of the question is incorrect, there is no answer to the question. Ask a better question.