How can I pass a pointer to an array using p/invoke in C#?

Question

Example C API signature:

void Func(unsigned char* bytes);

In C, when I want to pass a pointer to an array, I can do:

unsigned char* bytes = new unsigned char[1000];
Func(bytes); // call

How do I translate the above API to P/Invoke such that I can pass a pointer to C# byte array?

Answer 1

The easiest way to pass an array of bytes is to declare the parameter in your import statement as a byte array.

[DllImport EntryPoint="func" CharSet=CharSet.Auto, SetLastError=true]
public extern static void Func(byte[]);

byte[] ar = new byte[1000];
Func(ar);

You should also be able to declare the parameter as an IntPtr and Marshal the data manually.

[DllImport EntryPoint="func" CharSet=CharSet.Auto, SetLastError=true]
public extern static void Func(IntPtr p);

byte[] ar = new byte[1000];
IntPtr p = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(byte)) * ar.Length);
Marshal.Copy(ar, 0, p, ar.Length);
Func(p);
Marshal.FreeHGlobal(p);

Answer 2

You can use unsafe code:

unsafe 
{
     fixed(byte* pByte = byteArray)
     IntPtr intPtr = new IntPtr((void *) pByte);
     Func(intPtr);
}

If you need to use safe code, you can use a few tricks:

IntPtr intPtr = Marshal.AllocHGlobal(Marshal.SizeOf(byteArray));
Marshal.Copy(byteArray, 0, intPtr, Marshal.SizeOf(byteArray));

Func(intPtr);

Marshal.FreeHGlobal(intPtr);

However, the safe code is going to be slow IMHO.

Answer 3

Here is the appropriate signature for the native function.

[System.Runtime.InteropServices.DllImportAttribute("<Unknown>", EntryPoint="Func")]
public static extern  void Func(System.IntPtr bytes) ;