Some P/Invoke C# to C marshalling questions working with booleans in structs

Question

I have some problems working with boolean types and marshalling this in a struct back and forth between C# and C. I am very rusty in C but hopefully there's nothing crucially wrong in that part.

As far as I've read/seen, .NET Boolean and C# bool type is 4 bytes long whilst the C type bool is only 1 byte. For memory footprint reasons, I do not which to use the defined BOOL 4 bytes version in the C code.

Here is some simple test code that hopefully will make my questions clear:

C code:

typedef struct
{
        double SomeDouble1;
        double SomeDouble2;
        int SomeInteger;
        bool SomeBool1;
        bool SomeBool2;
} TestStruct;

extern "C" __declspec(dllexport) TestStruct* __stdcall TestGetBackStruct(TestStruct* structs);

__declspec(dllexport) TestStruct* __stdcall TestGetBackStruct(TestStruct* structs)
{
    return structs;
}

I call this code in C# using the following definitions:

    [StructLayout(LayoutKind.Explicit)]
    public struct TestStruct
    {
        [FieldOffset(0)]
        public double SomeDouble1;
        [FieldOffset(8)]
        public double SomeDouble2;

        [FieldOffset(16)]
        public int SomeInteger;

        [FieldOffset(17), MarshalAs(UnmanagedType.I1)]
        public bool SomeBool1;
        [FieldOffset(18), MarshalAs(UnmanagedType.I1)]
        public bool SomeBool2;
    };

    [DllImport("Front.dll", CharSet = CharSet.Auto, SetLastError = true, CallingConvention = CallingConvention.StdCall)]
    public static extern IntPtr TestGetBackStruct([MarshalAs(UnmanagedType.LPArray, SizeConst = 1)] TestStruct[] structs);

and here is the actual test function in C#:

    [Test]
    public void Test_CheckStructParsing()
    {
        var theStruct = new TestStruct();
        theStruct.SomeDouble1 = 1.1;
        theStruct.SomeDouble2 = 1.2;
        theStruct.SomeInteger = 1;
        theStruct.SomeBool1 = true;
        theStruct.SomeBool2 = false;

        var structs = new TestStruct[] { theStruct };

        IntPtr ptr = TestGetBackStruct(structs);

        var resultStruct = (TestStruct)Marshal.PtrToStructure(ptr, typeof(TestStruct));
    }

This works in the sense that I do get a struct back (using the debugger to inspect it), but with totally wrong values. I.e. the marshalling does not work at all. I've tried different version of the C# struct without success. So here are my questions (1 & 2 most important):

Is the C function correct for this purpose?
How is the struct to be written correctly in order to get me the correct values in the struct back to C#? (Is it even necessary to define the struct with the StructLayout(LayoutKind.Explicit) attribute using the FieldOffset values or can I use StructLayout(LayoutKind.Sequential) instead)?
Since I am returning a pointer to the TestStruct in C, I guess it should be possible to get back an array of TestStructs in C#. But this does not seem to be possible using the Marshal.PtrToStructure function. Would it be possible in some other way?
Apparantly it is possible to use something called unions in C by having multiple struct fields point to the same memory allocation using the same FieldOffset attribute value. I understand this, but I still don't get yet when such scenario would be useful. Please enlighten me.
Can someone recommend a good book on C# P/Invoke to C/C++? I am getting a bit tired of getting pieces of information here and there on the web.

Much obliged for help with these questions. I hope they were not too many.

David Heffernan David Heffernan · Accepted Answer · 2011-07-07T14:22:02

Stop using LayoutKind.Explicit and get rid of the FieldOffset attributes and your code will work. Your offsets were not correctly aligning the fields.

public struct TestStruct
{
    public double SomeDouble1;
    public double SomeDouble2;
    public int SomeInteger;
    [MarshalAs(UnmanagedType.I1)]
    public bool SomeBool1;
    [MarshalAs(UnmanagedType.I1)]
    public bool SomeBool2;
};

Declare the function in C# like this:

public static extern void TestGetBackStruct(TestStruct[] structs);

The default marshalling will match your C++ declaration (your code is C++ rather than C in fact) but you must make sure that your allocate the TestStruct[] parameter in the C# code before calling the function. Normally you would also pass the length of the array as a parameter so that the C++ code knows how many structs there are.

Please don't try to return the array of structures from the function. Use the structs parameter as an in/out parameter.

I know of no book with an emphasis on P/Invoke. It appears to be something of a black art!

Some P/Invoke C# to C marshalling questions working with booleans in structs

1 Answers