在C＃中返回一个固定的指针

Question

I was wondering is it safe to return a fixed pointer from one method to another method - does the pinned object still stay fixed? eg

struct TestData
{
    public int value;
}

public class Class1
{
    private TestData data; 

    public unsafe TestData* GetDataPtr()
    {
        fixed (TestData* ptr = &data)
        {
            // IS THIS SAFE?
            return ptr; 
        }
    }
}

public class Class2
{
    public unsafe void Test()
    {
        Class1 x = new Class1();

        TestData* ptr = x.GetDataPtr(); // is this still fixed?
        ptr->value = 2; 
    }
}

The reason i'm asking is using something in this style I've been getting AccessViolation exceptions. But since I changed it to eg set value direct from Class1 i haven't seen the issue occur.

EDIT: the reason i thought it may be fixed is if from outside the "TestData* ptr = x.GetDataPtr()" you try to put fixed( ) you get "you cannot take the address of an already fixed expression". .. i get it now though it's speaking about my var "ptr" in "Test()" already being fixed..

Answer 1

Returning from the method ends the scope of fixed , hence the pointer is no longer fixed once you return. It is safe to pass fixed pointers up the chain, eg

fixed(TestData* ptr = &data) {
    MyFunction1(ptr);
    MyFunction2(ptr);
}

But returning makes the pointer non-fixed again.

Logically, this makes sense: there is no alternative way for the CLR to decide when the pointer should become non-fixed after you have returned it from the call.