在 C# 中返回固定指针

发布于 2024-12-24 22:08:42 字数 768 浏览 1 评论 0原文

我想知道从一种方法返回固定指针到另一种方法是否安全 - 固定对象是否仍然保持固定？例如，

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; 
    }
}

我问的原因是使用这种风格的东西，我一直收到 AccessViolation 异常。但自从我将其更改为例如直接从 Class1 设置值以来，我还没有看到问题发生。

编辑：我认为它可能被修复的原因是，如果从“TestData * ptr = x.GetDataPtr（）”之外尝试放置fixed（），你会得到“你无法获取已经固定的表达式的地址”。 ..我现在明白了，虽然它说的是我的“Test()”中的 var“ptr”已经被修复了。

原文

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

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 e.g. 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..

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笑看君怀她人 2024-12-31 22:08:42

从方法返回结束了固定的范围，因此一旦返回，指针就不再固定。在链上传递固定指针是安全的，例如

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

，但返回会使指针再次变得非固定。

从逻辑上讲，这是有道理的：CLR 没有其他方法可以决定指针从调用返回后何时应变为非固定。

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, e.g.

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.

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