C#•3mo ago

How to understand pining static field, const and instance field

I am learning unsafe C#, and trying to understand when to use fixed statement. Here's the example:

class K { public int k; }
struct C
{
    public int instanceMember;
    public readonly static int shared;
    public const int con = 0;
    unsafe void M(int j, K k)
    {
        fixed (int* p = &j) { } // CS0213
        fixed (int* p = &k.k) { }
        int local = 0;
        fixed (int* p = &local) { } // CS0213
        int[] localArray = [];
        fixed (int* p = localArray) fixed (int* pp = p) { } // CS8385
        string localString = "abc";
        fixed (char* p = localString) fixed (char* pp = p) { } // CS8385
        K kk = new();
        fixed (int* p = &kk.k) { }
        fixed (int* p = &this.instanceMember) { }
        fixed (int* p = &C.shared) { }
        fixed (int* p = &C.con) { } // CS0211
    }
}

class K { public int k; }
struct C
{
    public int instanceMember;
    public readonly static int shared;
    public const int con = 0;
    unsafe void M(int j, K k)
    {
        fixed (int* p = &j) { } // CS0213
        fixed (int* p = &k.k) { }
        int local = 0;
        fixed (int* p = &local) { } // CS0213
        int[] localArray = [];
        fixed (int* p = localArray) fixed (int* pp = p) { } // CS8385
        string localString = "abc";
        fixed (char* p = localString) fixed (char* pp = p) { } // CS8385
        K kk = new();
        fixed (int* p = &kk.k) { }
        fixed (int* p = &this.instanceMember) { }
        fixed (int* p = &C.shared) { }
        fixed (int* p = &C.con) { } // CS0211
    }
}

The question is, why can I pin a static field? Is it because static field live during the whole cycle, so it's managed? And what about const, is it because const isn't a variable or field so simply can't get the address? And for instance field, since it's fixed inside the instance method, so ...? I don't really know how to explain this. Please let me know if I understand them wrong, thank you!

4 Replies

Unknown User•3mo ago

Message Not Public

Aaron•3mo ago

static fields have no requirement to live in pinned memory, so you have to pin them, and consts don't have a memory location consts are copied into all the places you use them, they aren't "fields"

wtq40cy3n•3mo ago

I found a description from documentation

Note: As stated in §12.8.7, outside an instance constructor or static constructor for a struct or class that defines a readonly field, that field is considered a value, not a variable. As such, its address cannot be taken. Similarly, the address of a constant cannot be taken.

wtq40cy3n•3mo ago

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/unsafe-code#2365-the-address-of-operator

Unsafe code - C# language specification

This chapter describes elements of C# that are allowed in unsafe code blocks. These elements include pointers, fixed buffers and dynamic memory allocation.

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How to understand pining static field, const and instance field