How to marshal this nested, and Pointer Used C structure in C# - pinvoke

Question

typedef struct pt_input_bir
{
PT_BYTE byForm;
union {
PT_BIR *pBIR; ///< Used when byForm = PT_FULLBIR_INPUT */
PT_LONG lSlotNr; ///< Used when byForm = PT_SLOT_INPUT */
PT_BYTE abyReserved[20]; /** For future use */
} InputBIR;
} PT_INPUT_BIR
typedef struct pt_bir {
PT_BIR_HEADER Header;
PT_BYTE Data[1];
} PT_BIR
typedef struct pt_bir_header {
PT_DWORD Length;
PT_BYTE HeaderVersion;
PT_BYTE Type;
PT_WORD FormatOwner;
PT_WORD FormatID;
PT_CHAR Quality;
PT_BYTE Purpose;
PT_DWORD FactorsMask;
} PT_BIR_HEADER
and the C function is
PT_STATUS StoreFinger (
IN PT_CONNECTION hConnection,
IN PT_INPUT_BIR *pTemplate,
OUT PT_LONG *plSlotNr
)
Now I need to do the wrapper for the above C function in C#.
How should I marshal the PT_INPUT_BIR* structure and how should I unmarshal it after return of this function?
Please help me to solve this.
/********************** FOR MORE DETAIL ABOUT THIS QUESTION**************************/
C struct and function are defined in above. pls refer there.
C# Struct :
For C# struct declaration i have maintatined two struct for the one C struct. bcz one is for setting the values and another one id for passing to c function.
C# app struct:
[StructLayout(LayoutKind.Sequential)]//for app
public struct FPINPUTBIR
{
public byte byForm;
public InputBIRType InputBIR;
}
[StructLayout(LayoutKind.Sequential)] // here when i use explicit it throws exception so i removed it.
public struct InputBIRType
{
// [FieldOffset(0)]
public FPBIR pBIR;
//[FieldOffset(0)]
public int lSlotNr;
//[FieldOffset(0)]
[MarshalAs(UnmanagedType.ByValArray, SizeConst = 20)]
public byte[] abyReserved;
}
C# wrapper struct:
[StructLayout(LayoutKind.Sequential)]
public struct FP_INPUTBIR
{
public byte byForm;
public IntPtr mIPBIR;
}
[StructLayout(LayoutKind.Explicit, Size = 20, CharSet = CharSet.Ansi)]
public struct Input_BIRType
{
[FieldOffset(0)]
public IntPtr mBIR;
[FieldOffset(0)]
public int lSlotNr;
//[FieldOffset(8)]
//[MarshalAs(UnmanagedType.ByValArray, SizeConst = 20)]
//public byte[] abyReserved;
}
finally i will copy the value from the C# app struct to wrapper struct before the call the C fun()
2a) C# App Side Code is :
//here mAppMemory is already known value
FPINPUTBIR lfipdata = new FPINPUTBIR();
FPDATA lfpdata = new FPDATA();
lfipdata.byForm = (byte)eFPVerifyBy.FULLBIR_INPUT;
lfipdata.InputBIR = new InputBIRType();
lfipdata.InputBIR.abyReserved = new byte[20];
lfipdata.InputBIR.pBIR.Data = new byte[mAppMemory[listBox2.SelectedIndex].Header.Length];
Array.Copy(mAppMemory[listBox2.SelectedIndex].Data, lfipdata.InputBIR.pBIR.Data, mAppMemory[listBox2.SelectedIndex].Header.Length);
lfipdata.InputBIR.pBIR.Header = mAppMemory[listBox2.SelectedIndex].Header;
Verify(ref lfipdata); //calling from C# APP side to C# wrapper
C# wrapper side:
public int Verify(ref FPINPUTBIR apStoredTemplate )
{
// i passed the args (apStoredTemplate ) but throws exception struct mismatch with C struct.
//here i don't know what should i do.
CDLL.StoreFinger(..,ref apStoredTemplate,.. ); //pls refer the C function above
}
Questions:
Do i really need two C# structures for this.
what should i do inside the C# wrapper function. please remeber i have two C# struct with diff members.
Thanks.

Answer 1

You just need a little extension on what you used in the previous question for PT_BIR. There we marshalled that variable length struct as byte[]. You can use the same code to generate the byte array, and I won't revisit that.
Next you need the union. That is:
[StructLayout(LayoutKind.Explicit, Size = 20)]
public struct PT_INPUT_BIR_UNION
{
[FieldOffset(0)]
public IntPtr pBIR;
[FieldOffset(0)]
public int lSlotNr; // I'm guessing what PT_LONG is
}
No need to declare the reserved part of the union. The size takes care of that.
Then PT_INPUT_BIR is
[StructLayout(LayoutKind.Sequential)]
public struct PT_INPUT_BIR
{
Byte byForm;
PT_INPUT_BIR_UNION InputBirUnion;
}
Then you need to use GCHandle to pin the PT_BIR byte array. Let's keep to the same naming as used at that question, and assume that the PT_BIR is held in a byte[] variable named data.
GCHandle handle = GCHandle.Alloc(data, GCHandleType.Pinned);
try
{
PT_INPUT_BIR inputBir;
inputBir.byForm := ...;
inputBir.InputBirUnion.pBIR = handle.AddrOfPinnedObject();
// now call StoreFinger passing ref inputBir
}
finally
{
handle.Free();
}
When you declare StoreFinger the PT_BIR* parameter should be declared as ref PT_BIR.