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

Question

In my C # code, I am trying to get an array of structures from an inherited C ++ DLL (code that I cannot change).

In this C ++ code, the structure is defined as follows:

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

The method I call (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 structures, so I know how many structures are returned.

In my C # code, I defined MyStruct as follows:

[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 is as follows:

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

Finally, the code that retrieves 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));
    ...
}

, ( ) . _id _description . , , , : . .

, ++ get_my_structures() . .

++ :

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

[ 18/11/2009]

++ ( , ):

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;

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Answer 1

, , ++. .

dll.cpp - 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 - 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);
        }
    }
}

3 .

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Answer 2

. .. IntPtr:

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

# Marshal.PtrToString ..

Answer 3

UnmanagedType.LPTStr char *. StringBuilder const char *: CharSet:

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

DllImport,

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

?

, , IntPtr Mashal :

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

Answer 4

, , . , CharSet, StructLayout, UnmanagedType.LPTStr, , char *, , .

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

Answer 5

, , .. [MarshalAsAttribute (UnmanagedType.LPTStr), SizeConst =, ArraySubType = System.Runtime.InteropServices.UnmanagedType.AnsiBStr)]

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