Marshal a C++ structure with union to C#

Question

0.00/5 (No votes)

See more:

Reading up on marshaling, I thought the following would be valid but the marshaling fails. The C++ definition is as follows:

typedef struct
{
    char* t;
    size_t t_length;
    double d;
} A;

typedef struct
{
    char* t;
    size_t t_length;
    char* k;
    size_t k_length;
} B;

typedef struct
{   
    char* id;
    size_t id_length;
    int enumOfType;
    union
    {
        A typeA;
        B typeB;
    }
} NofAB

I've defined the C# classes as follows:

[StructLayout(LayoutKind.Sequential)]
public class A
{
    public string t;
    public IntPtr tLength;  // IntPtr is the only thing that maps correctly in x86/x64
    public double d;
};

[StructLayout(LayoutKind.Sequential)]
public class B
{
    public string t;
    public IntPtr tLength;
    public string k;
    public IntPtr kLength;
};

[StructLayout(LayoutKind.Explicit)]
public class ABUnion
{
    [FieldOffset(0)]
    public A TypeA;
    [FieldOffset(0)]
    public B TypeB;
}

[StructLayout(LayoutKind.Sequential]
public class NofAB
{
    public string id;
    public IntPtr idLength;
    public int enumOfType;
    public ABUnion AOrB;  
}

When I attempt to marshal PtrToStructure using NofAB, I get a LoadTypeException on ABUnion. This is a simplified example where I really have a union of 5 possible types but I didn't want to put all that code in place.
Can anyone explain to me why this doesn't work? What can I do to make it work?
Thanks.

Posted 5-Aug-13 8:13am

MarjKK

Updated 5-Aug-13 8:15am

v2

Add a Solution

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Answer 1 · 2013-08-05T08:23:00

C++ unions can be matched in .NET by the layouts created using [FieldOffset] member attribute, System.Runtime.InteropServices.FieldOffsetAttribute. Please see:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.fieldoffsetattribute.aspx[^].

The code sample in the MDSN article referenced above clearly shows how to use the attribute, but perhaps you need an idea on how to mimic the union. Here is how: in case of union, members participating in different cases of union take identical addresses relative to the starting of the structure. Hence, with [FieldOffset], you will have to specify the same offset to two different members representing members of the union. In your case, the members A and B should be at the same offset, which you can calculate taking into consideration member layout of the structure (aligned by 32-bit, 16-bit or 8-bit boundary) and memory taken by the previous members in sequential order. I would advice to use this attribute on all previous members of the structure, for certainty.

—SA