Marshaling a pointer to an array of types (managed C# -> unmanaged C++)

Question

I am having some trouble settling on a way to represent a structure that contains a pointer to an array of shorts in my managed code. The struct looks like this:

typedef struct
{
    short size;
    unsigned short** shortValues;
} UnmanagedStruct;

memory for ' shortValues ' is allocated inside unmanaged code -- therefore even though that field is simply a pointer to an array of short values, an additional level of indirection was added so that allocated memory is seen by the caller (managed code) too. The ' size ' field represents the number of elements in the array. How do I represent this in managed code?

I thought I'd pass it in just an IntPtr , then I couldn't figure out how to access the values once the unmanaged call returns.

Answer 1

Is unsafe code ok?

public unsafe struct UnmanagedStruct
{
    public short size;
    public ushort** shortValues;
}

[DllImport("example.dll")]
public static extern void GetUnmanagedStruct(out UnmanagedStruct s);

If you have a pointer to an array of ushort s:

public static unsafe void WriteValues()
{
    UnmanagedStruct s;
    GetUnmanagedStruct(out s);
    for (var i = 0; i < s.size; i++)
    {
        ushort x = (*s.shortValues)[i];
        Console.WriteLine(x);
    }
}

If you have an array of null-terminated arrays of ushort s:

public static unsafe void WriteValues()
{
    UnmanagedStruct s;
    GetUnmanagedStruct(out s);
    for (var i = 0; i < s.size; i++)
    {
        for (ushort* p = s.shortValues[i]; p != null; p++)
        {
            ushort x = *p;
            Console.WriteLine(x);
        }
    }
}