Marshalling an array of structures from C++ to C#

Question

In my C# code I'm trying to fetch an array of structures from a legacy C++ DLL (the code I cannot change).

In that C++ code, the structure is defined like this:

struct MyStruct
{
    char* id;
    char* description;
};

The method that I'm calling (get_my_structures) returns a pointer to an array of MyStruct structures:

MyStruct* get_my_structures()
{
    ...
}

There is another method that returns the number of stuctures so I do know how many structures get returned.

In my C# code, I have defined MyStruct like this:

[StructLayout(LayoutKind.Sequential)]  
public class MyStruct
{
  [MarshalAsAttribute(UnmanagedType.LPStr)]    // <-- also tried without this
  private string _id;
  [MarshalAsAttribute(UnmanagedType.LPStr)]
  private string _description;
}

The interop signature looks like this:

[DllImport("legacy.dll", EntryPoint="get_my_structures")]
public static extern IntPtr GetMyStructures();

Finally, the code that fetches the array of MyStruct structures looks like this:

int structuresCount = ...;
IntPtr myStructs = GetMyStructures();
int structSize = Marshal.SizeOf(typeof(MyStruct));    // <- returns 8 in my case
for (int i = 0; i < structuresCount; i++)
{
    IntPtr data = new IntPtr(myStructs.ToInt64() + structSize * i);
    MyStruct ms = (MyStruct) Marshal.PtrToStructure(data, typeof(MyStruct));
    ...
}

The trouble is, only the very first structure (one at the offset zero) gets marshaled correctly. Subsequent ones have bogus values in _id and _description members. The values are not completely trashed, or so it seems: they are strings from some other memory locations. The code itself does not crash.

I have verified that the C++ code in get_my_structures() does return correct data. The data is not accidentally deleted or modified during or after the call.

Viewed in a debugger, C++ memory layout of the returned data looks like this:

0: id (char*)           <---- [MyStruct 1]
4: description (char*)
8: id (char*)           <---- [MyStruct 2]
12: description (char*)
16: id (char*)          <---- [MyStruct 3]
...

[Update 18/11/2009]

Here is how the C++ code prepares these structures (the actual code is much uglier, but this is a close enough approximation):

static char buffer[12345] = {0};
MyStruct* myStructs = (MyStruct*) &buffer;
for (int i = 0; i < structuresCount; i++)
{
    MyStruct* ms = <some other permanent address where the struct is>;
    myStructs[i].id = (char*) ms->id;
    myStructs[i].description = (char*) ms->description;
}
return myStructs;

Admittedly, the code above does some ugly casting and copies raw pointers around, but it still does seem to do that correctly. At least that's what I see in the debugger: the above (static) buffer does contain all these naked char* pointers stored one after another, and they point to valid (non-local) locations in memory.

Pavel's example shows that this is really the only place where things can go wrong. I will try to analyze what happens with those 'end' locations where the strings really are, not the locations where the pointers get stored.

Answer 1

You have to use UnmanagedType.LPTStr for char*. Also a StringBuilder is recommended for a non const char*: And a CharSet specification:

[StructLayout(LayoutKind.Sequential, Charset = CharSet.Auto)]  
public class MyStruct
{
  [MarshalAsAttribute(UnmanagedType.LPTStr)]
  private StringBuilder _id;
  [MarshalAsAttribute(UnmanagedType.LPTStr)]
  private StringBuilder _description;
}

As for the DllImport declaration, have you tried

[DllImport("legacy.dll", EntryPoint="get_my_structures")]
public static extern MarshalAs(UnmanagedType.LPArray) MyStruct[] GetMyStructures();

?

Also, if the previous doesn't work, leave it at IntPtr and try to Mashal the returned structs like this:

for (int i = 0; i < structuresCount; i++)
{
    MyStruct ms = (MyStruct) Marshal.PtrToStructure(myStructs, typeof(MyStruct));
    ...
    myStructs += Marshal.SizeOf(ms);
}

Answer 2

I usually end up working these things out by trial and error. Make sure you have the CharSet property set on your StructLayout, and I would try UnmanagedType.LPTStr, seems to work better for char *, even though I am not sure why.

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]  
public class MyStruct
{
    [MarshalAsAttribute(UnmanagedType.LPTStr)]
    private string _id;
    [MarshalAsAttribute(UnmanagedType.LPTStr)]
    private string _description;
}

Answer 3

I think, also, in addition to the answers given, that you need to supply the length as well, ie [MarshalAsAttribute(UnmanagedType.LPTStr), SizeConst = , ArraySubType = System.Runtime.InteropServices.UnmanagedType.AnsiBStr)]

This is a trial and error to get this right, also, another thing to consider, in some WinAPI calls that expect a string parameter, usually a ref parameter, it might be worth your while to try the StringBuilder class also...Nothing else comes to mind on this other than the points I have mentioned here... Hope this helps, Tom

Answer 4

I would change the structure. Instead of strings etc. , use IntPtr:

[StructLayout(LayoutKind.Sequential)]  
public class MyStruct
{
  private IntPtr _id;
  private IntPtr _description;
}

Then each value of the C# array could be manually marshalled to string using Marshal.PtrToString taking into account charset etc.

Answer 5

I cannot reproduce your problem, which leads me to suspect that it's really on C++ side of things. Here's the complete source code for my attempt.

dll.cpp - compile with cl.exe /LD:

extern "C" {

struct MyStruct
{
    char* id;
    char* description;
};

__declspec(dllexport)
MyStruct* __stdcall get_my_structures()
{
    static MyStruct a[] =
    {
        { "id1", "desc1" },
        { "id2", "desc2" },
        { "id3", "desc3" }
    };
    return a;

}

}

test.cs - compile with csc.exe /platform:x86:

using System;
using System.Runtime.InteropServices;


[StructLayout(LayoutKind.Sequential)]  
public class MyStruct
{
  [MarshalAsAttribute(UnmanagedType.LPStr)]
  public string _id;
  [MarshalAsAttribute(UnmanagedType.LPStr)]
  public string _description;
}


class Program
{
    [DllImport("dll")]
    static extern IntPtr get_my_structures();

    static void Main()
    {
        int structSize = Marshal.SizeOf(typeof(MyStruct));
        Console.WriteLine(structSize);

        IntPtr myStructs = get_my_structures();
        for (int i = 0; i < 3; ++i)
        {
            IntPtr data = new IntPtr(myStructs.ToInt64() + structSize * i);
            MyStruct ms = (MyStruct) Marshal.PtrToStructure(data, typeof(MyStruct));

            Console.WriteLine();
            Console.WriteLine(ms._id);
            Console.WriteLine(ms._description);
        }
    }
}

This correctly prints out all 3 structs.

Can you show your C++ code that fills the structs? The fact that you can call it from C++ directly and get correct results does not necessarily mean it's correct. For example, you could be returning a pointer to a stack-allocated struct. When doing a direct call, then, you'd get a technically invalid pointer, but the data would likely remain preserved. When doing P/Invoke marshalling, the stack could be overwritten by P/Invoke data structures by the point it tries to read values from there.