asked on

Pointers to C++ unions/structs in C#

Does anyone know how to map the following C++ union/structs into a C# struct? I am unable to force the String member to align with the numeric and bool members. The C# compiler forces an offset of 8 bytes for the String member. If anyone has an idea of how I can delclare these unions in C# please give an example.

union bfdValueUnion {
double Float;
double Double;
char* String;
int Integer;
unsigned char Boolean;
};

typedef struct bfdValue {
unsigned int Type;
union bfdValueUnion Value;
} bfdValue;
typedef union bfdValueUnion bfdValueUnion;

dkloeck

An union is made in c# like this:

[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)]
struct TestExplicit
{
[System.Runtime.InteropServices.FieldOffset(0)]
public long lg;

[System.Runtime.InteropServices.FieldOffset(0)]
public int i1;

[System.Runtime.InteropServices.FieldOffset(4)]
public int i2;

[System.Runtime.InteropServices.FieldOffset(8)]
public double d;

[System.Runtime.InteropServices.FieldOffset(12)]
public char c;

[System.Runtime.InteropServices.FieldOffset(14)]
public byte b;
}

I have to go..I finish later if nobody has answered :)

EJPanyard

ASKER

That's close to what I have but the C# compiler will not allow char* to be aligned with an int or a double. Is there anyway to force it. Here is what have but I'm forced to set the offset for the char* to eight bytes which does not match the c++ structures so the calls fails

[StructLayout(LayoutKind.Explicit)]
public struct bfdValueUnion
{
[FieldOffset(0)]
public double Float;
[FieldOffset(0)]
public double Double;
[FieldOffset(8),MarshalAs(UnmanagedType.LPStr)]
public string String;
[FieldOffset(0)]
public int Integer;
[FieldOffset(0)]
public bool Boolean;
}

[StructLayout(LayoutKind.Explicit)]
public struct bfdValue
{
[FieldOffset(0)]
public int Type;
[FieldOffset(0)]
public bfdValueUnion Value;
}

ASKER CERTIFIED SOLUTION

dkloeck

membership

This solution is only available to members.

To access this solution, you must be a member of Experts Exchange.

Start Free Trial

itsmeandnobodyelse

I don't know why you need the union in C# but if you only need it for passing it to a C function you might consider using it as an xml string, e. g. like

<?xml version="1.0"?>
<.DOCTYPE bfd >
<bfd type="Double">12.345</bfd>

and pass that string as a char array to the dll. There you easily could parse it and fill the appropriate members of the bfdValue struct..

Regards, Alex

EJPanyard

ASKER

Hi,

Here is what I finally did that worked thanks for the input. I'm splitting the points here but this is the ONLY way the I could get C# and the CLR to align the float, int, bool, with char* in the same struct. I'm giving the points because the suggestion helped me solve the problem. I could not change the C++ code and the whole reason I did this is so that I can call these dlls from C#. Thanks for the input.

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct bfdValueUnion
{

// union bfdValueUnion {
// double Float;
// double Double;
// bfdStringPtr String;
// int Integer;
// unsigned char Boolean;
//};

[FieldOffset(0)]
public double Float;
[FieldOffset(0)]
public double Double;
[FieldOffset(0), MarshalAs(UnmanagedType.LPStr)] //
public IntPtr String;
[FieldOffset(0)]
public Int32 Integer;
[FieldOffset(0)]
public bool Boolean;

}

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct bfdValue
{

// typedef struct bfdValue {
// unsigned int Type;
// union bfdValueUnion Value;
// } bfdValue;

[FieldOffset(0)]
public UInt32 Type;
[FieldOffset(8)]
public bfdValueUnion Value;

}

To use string in C# do the following:

string str = Marshal.PtrToStringAnsi(aValLst[nIndex].Value.String);

EJPanyard

ASKER

Here is what I finally did that worked thanks for the input. I'm splitting the points here but this is the ONLY way the I could get C# and the CLR to align the float, int, bool, with char* in the same struct. I'm giving the points because the suggestion helped me solve the problem. I could not change the C++ code and the whole reason I did this is so that I can call these dlls from C#. Thanks for the input.

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct bfdValueUnion
{

// union bfdValueUnion {
// double Float;
// double Double;
// bfdStringPtr String;
// int Integer;
// unsigned char Boolean;
//};

[FieldOffset(0)]
public double Float;
[FieldOffset(0)]
public double Double;
[FieldOffset(0), MarshalAs(UnmanagedType.LPStr)] //
public IntPtr String;
[FieldOffset(0)]
public Int32 Integer;
[FieldOffset(0)]
public bool Boolean;

}

[StructLayout(LayoutKind.Explicit, CharSet = CharSet.Ansi)]
public struct bfdValue
{

// typedef struct bfdValue {
// unsigned int Type;
// union bfdValueUnion Value;
// } bfdValue;

[FieldOffset(0)]
public UInt32 Type;
[FieldOffset(8)]
public bfdValueUnion Value;

}

To use string in C# do the following:

string str = Marshal.PtrToStringAnsi(aValLst[nIndex].Value.String);