Marshal Union(C ) with a Struct which contains an Array in C#

Question

I try to Marshal a unmanaged c++ dll in c#, but the marshaller fails when creating my union.

Why does this code fail?

    [StructLayout(LayoutKind.Sequential)]
    public struct StructWithArray
    {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
        public int[] MySimpleArray;
        //More Stuff
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct Union
    {
        [FieldOffset(0)]
        public int Int; //Or anything else
        [FieldOffset(0), MarshalAs(UnmanagedType.Struct)]
        public StructWithArray MyStructWithArray;
        //More Structs
    }

And then constructing the Union:

Union MyUnion = new Union();

It fails if I run the code with the following Message: (Translated)

{"The Type "Union" of the Assembly [...] could not be loaded because it contained an Objectfield at Offset 0, which is not aligned correctly or got overlapped by a field which isnt a ObjectField":"Union"}

Any Suggestions?

Ps: The original code is heavily simplified to show only the Problem. There are much more Structs, and the Union is also contained by another Struct.

Possibly an Pack issue. Try first adding attribute Pack=4 to first structure. Do you have #pragma pack in your header files for those struct. — user629926, Jun 28 '12 at 11:14
I dont have a #pragma in the header file. And in this code i havent even loaded the dll. this example is enough to get the error. Pack=1, Pack=4 tested, Fixed Size Tested, copying the Array directly to the union tested, all with the same result. — user1431279, Jun 28 '12 at 11:26
See this http://stackoverflow.com/questions/1190079/incorrectly-aligned-or-overlapped-by-a-non-object-field-error It seems that you solution is to replace first structure with [StructLayout(LayoutKind.Sequential)] public struct StructWithArray {int a;int b; //More Stuff } — user629926, Jun 28 '12 at 12:58
Yes, I think this is the only workaround. Or for native structures, i can use "unsafe" and "fixed array", but as i said, this only works for native structures like int, uint etc. — user1431279, Jun 28 '12 at 13:05

score 0 · Answer 1 · answered May 29 '13 at 13:25

0

Declare MySimpleArray as a fixed unsafe array:

[StructLayout(LayoutKind.Sequential)]
public unsafe struct StructWithArray
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
    public fixed int MySimpleArray[2];
    //More Stuff
}

[StructLayout(LayoutKind.Explicit)]
public struct Union
{
    [FieldOffset(0)]
    public int Int; //Or anything else
    [FieldOffset(0), MarshalAs(UnmanagedType.Struct)]
    public StructWithArray MyStructWithArray;
    //More Structs
}

answered May 29 '13 at 13:25

Kaveh Shahbazian

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Sadly the keyword "fixed" only works with ValueTypes like ints, bytes etc. Assuming I have an array of other structures, the "fixed" keyword doesn't work anymore. – user1431279 Jan 30 '14 at 21:30

Marshal Union(C ) with a Struct which contains an Array in C#

1 Answers1