Passing structures between C# and C/C++

Data structures can be passed between managed C# code and unmanaged C/C++ dynamic link libraries but it can be a little complicated. The trick is to define structures that match between the two different languages; and perhaps write wrapper functions that hide away the complexity of the memory marshaling code. I will show a couple of simple examples of passing a system time data structure from C/C++ to C# and vice versa here.

The C/C++ definition of the system time structure is as follows:

typedef struct _SYSTEMTIME {

    WORD wYear;

    WORD wMonth;

    WORD wDayOfWeek;

    WORD wDay;

    WORD wHour;

    WORD wMinute;

    WORD wSecond;

    WORD wMilliseconds;

} SYSTEMTIME, *PSYSTEMTIME, *LPSYSTEMTIME;

The matching C# equivalent of the system time structure can be defined as shown below.

private struct SYSTEMTIME

{

    public UInt16 wYear;

    public UInt16 wMonth;

    public UInt16 wDayOfWeek;

    public UInt16 wDay;

    public UInt16 wHour;

    public UInt16 wMinute;

    public UInt16 wSecond;

    public UInt16 wMilliseconds;

}

Example 1: Passing a structure from C/C++ to C#
If the C/C++ function has the following signature:

HRESULT GetDateTime( HANDLE hClient, SYSTEMTIME* pSysTime );

Then the following wrapper functions can be used to call the C/C++ function.

[DllImport("MyDynamicLinkLib.dll")]

private static extern int GetDateTime(IntPtr hClient, IntPtr pSysTime);

public static int GetDateTime(IntPtr hClient, out DateTime sysTime)

{

    int hr = 0;

    IntPtr pSysTime = IntPtr.Zero;

    SYSTEMTIME theTime = new SYSTEMTIME();

    sysTime = new DateTime();

    pSysTime = Marshal.AllocHGlobal( Marshal.SizeOf(theTime));

    hr = GetDateTime(hClient, pSysTime);

    theTime = (SYSTEMTIME) Marshal.PtrToStructure(pSysTime, typeof(SYSTEMTIME));

    Marshal.FreeHGlobal(pSysTime);

    sysTime = new DateTime(theTime.wYear, theTime.wMonth, theTime.wDay, theTime.wHour, theTime.wMinute, theTime.wSecond, theTime.wMilliseconds);

    return hr;

}

To use the wrapper functions, simply do something like the following:

IntPtr hClient = IntPtr.Zero;

DateTime dt = new DateTime();

int ret = GetDateTime( hClient, out dt);

Example 2: Passing a structure from C# to C/C++
If the C/C++ function has the following signature,

HRESULT SetDateTime( HANDLE hClient, SYSTEMTIME* pSysTime );

Then the following wrapper functions can be used to call the C/C++ function.

[DllImport("MyDynamicLinkLib.dll")]

private static extern int SetDateTime(IntPtr hClient, IntPtr pSysTime);

public static int SetDateTime(IntPtr hClient, DateTime sysTime)

{

    int hr = 0;

    IntPtr pSysTime = IntPtr.Zero;

    SYSTEMTIME theTime = new SYSTEMTIME();

    theTime.wYear = (ushort) sysTime.Year;

    theTime.wMonth = (ushort) sysTime.Month;

    theTime.wDay = (ushort) sysTime.Day;

    theTime.wDayOfWeek = (ushort) sysTime.DayOfWeek;

    theTime.wHour = (ushort) sysTime.Hour;

    theTime.wMinute = (ushort) sysTime.Minute;

    theTime.wSecond = (ushort) sysTime.Second;

    theTime.wMilliseconds = (ushort) sysTime.Millisecond;

    pSysTime = Marshal.AllocHGlobal(Marshal.SizeOf(theTime));

    hr = SetDateTime(hClient, pSysTime);

    Marshal.FreeHGlobal(pSysTime);

    return hr;

}

Do the following to call the wrapper functions.

DateTime dt = new DateTime();

int hr = SetDateTime(hClient, dt);