Pinning an updateble struct before passing to unmanaged code?

Question

I using some old API and need to pass the a pointer of a struct to unmanaged code that runs asynchronous.

In other words, after i passing the struct pointer to the unman

Answer 1

To answer your edit:

Just thought... is there a way to pin the parent object that contains the struct, and then get the pointer of the struct rather than the container object?

I think so. If anything, you should be able to with a managed array of structures (possibly an array of one).

Here is an example code:

    [StructLayout(LayoutKind.Sequential)]
    struct SomeStructure
    {
        public int first;
        public int second;
        public SomeStructure(int first, int second) { this.first=first; this.second=second; }
    }
    
    /// 

    /// For this question on Stack Overflow:
    /// https://stackoverflow.com/questions/1850488/pinning-an-updateble-struct-before-passing-to-unmanaged-code
    /// 
    private static void TestModifiableStructure()
    {
        SomeStructure[] objArray = new SomeStructure[1];
        objArray[0] = new SomeStructure(10, 10);
        
        GCHandle hPinned = GCHandle.Alloc(objArray, GCHandleType.Pinned);

        //Modify the pinned structure, just to show we can
        objArray[0].second = 42;

        Console.WriteLine("Before unmanaged incrementing: {0}", objArray[0].second);
        PlaceholderForUnmanagedFunction(hPinned.AddrOfPinnedObject());
        Console.WriteLine("Before unmanaged incrementing: {0}", objArray[0].second);
        
        //Cleanup
        hPinned.Free();
    }
    
    //Simulates an unmanaged function that accesses ptr->second
    private static void PlaceholderForUnmanagedFunction(IntPtr ptr)
    {
        int secondInteger = Marshal.ReadInt32(ptr, 4);
        secondInteger++;
        Marshal.WriteInt32(ptr, 4, secondInteger);
    }

And its output:

Before unmanaged incrementing: 42
Before unmanaged incrementing: 43