Question

In C# 4.0, say I have

List<HSZPAIR> myList

with three elements where the HSZPAIR struct is defined by:

[StructLayout(LayoutKind.Sequential)]
public struct HSZPAIR
{
    public IntPtr hszSvc;
    public IntPtr hszTopic;
}

How do I create a byte array for the entire myList? In C++, you could just cast as array of structs down to a byte array. I’m not sure how to do that in C#.

I’m using an old Windows API function in the DDEML library that requires a byte array and the number of elements in the array as arguments. If you are interested in more background, the API function is:

[DllImport("user32.dll", EntryPoint="DdeCreateDataHandle", CharSet=CharSet.Ansi)]
    public static extern IntPtr DdeCreateDataHandle(int idInst, byte[] pSrc, int cb, int cbOff, IntPtr hszItem, int wFmt, int afCmd);

Here is it’s documentation on MSDN. The pSrc argument is the byte array of HSZPAIR structs. The size of the array is the cb argument.

Answer 1

Converting a struct to an array of bytes is kind of painful. You have to serialize it yourself. But it might not be necessary.

Given your list:

List<HSZPAIR> myList;

You can get an array by calling ToArray:

HSZPAIR[] myArray = myList.ToArray();

Now, change your managed prototype so that it takes an HSZPAIR[] rather than a byte[]:

public static extern IntPtr DdeCreateDataHandle(
    int idInst, HSZPAIR[] pSrc, int cb, int cbOff, IntPtr hszItem, int wFmt, int afCmd);

That should work. After all, as you pointed out, an array of HSZPAIR really is just an array of bytes.