Return allocated value from Unmanaged code to managed

Question

I would like to do the following: Call Unamanaged method that returns the array of type MyStruct[] that it allocated.

Example of c code:

MyStruct[] foo(int size)
{
   Mystruct* st = (MyStruct*)malloc(size * sizeof(MyStruct));
   return st;
}

How should Implement c# calling method?

Thank you!

I am trying to use existing algorithm written in unmanaged.dll and want to get the result :) — Sergey Kucher, Aug 19 '11 at 14:19
That code doesn't do anything useful. Might as well use Marshal.AllocCoTaskMem(). — Hans Passant, Aug 19 '11 at 14:42

Martin Liversage · Accepted Answer · 2011-08-19T14:38:33.683

The Marshal class in the System.Runtime.InteropServices namespace has a lot of methods to help you marshal data to and from unmanaged code.

You need to declare your native method:

[DllImport("myclib.dll")]
public static extern IntPtr Foo(Int32 size);

And also your C struct as a managed value type (you can use attributes on the fields to control exactly how they are mapped to native memory when they are marshalled):

[StructLayout(LayoutKind.Sequential)]
struct MyStruct {
  public Char Character;
  public Int32 Number;
}

Then you can use Marshal.PtrToStructure to marshal each element of the array into a managed value:

var n = 12;
var pointer = Foo(n);
var array = new MyStruct[n];
var structSize = Marshal.SizeOf(typeof(MyStruct));
for (var i = 0; i < n; ++i) {
  array[i] = (MyStruct) Marshal.PtrToStructure(pointer, typeof(MyStruct));
  pointer += structSize;
}

Note that you are using malloc to allocate the memory in the C code. C# us unable to free that memory and you will have to provide another method to free the allocated memory.

score 1 · Answer 2 · answered Aug 19 '11 at 14:15

1

This should help you http://msdn.microsoft.com/en-us/library/aa288468%28v=vs.71%29.aspx

answered Aug 19 '11 at 14:15

Zenwalker

1,883
1
14
27

Read that I haven't found there how to return array type of meclared struct – Sergey Kucher Aug 19 '11 at 14:17
Its no direct solution,work around could be to pass the array and let the library fill it and send it back. As explained here http://answers.unity3d.com/questions/132083/returning-a-byte-array-from-objc-to-c-script-on-io.html – Zenwalker Aug 19 '11 at 14:29

score 1 · Answer 3 · edited May 23 '17 at 10:28

1

Refer to this thread, it has some information about how to return a dynamically allocated struct from C++ to C# as an array.

edited May 23 '17 at 10:28

Community

1
1

answered Aug 19 '11 at 14:19

dcp

54,410
22
144
164

Return allocated value from Unmanaged code to managed

3 Answers3