C# – Passing C# data type parameters to dll written in C++


Still working on a problem that started from here
Calling C++ dll function from C#: Of structs, strings and wchar_t arrays., but with a different approach.

Following the example Calling Managed Code from Unmanaged Code and vice-versa I wrote a managed wrapper in C++ to access the unmanages class in the unmanaged C++ dll.

It looks like this:

//in header file
public __gc class TSSLDllWrapper
     //this is the unmanaged class
CcnOCRsdk * _sdk;

bool convertHKID_Name(char *code, RECO_DATA *o_data);

//in .cpp file
  _sdk = new CcnOCRsdk();

bool TSSLDllWrapper::convertHKID_Name(char *code, RECO_DATA *o_data)
return _sdk->convertHKID_Name(code, o_data);

//C++ RECO_DATA structure definition:
struct RECO_DATA{
wchar_t FirstName[200];
wchar_t Surname[200];

Now I have a dll that I can import into my C# project.

Here is the problem however:
When I want to call the method from the dll file, like this:

TSSLDllWrapper wrapper = new TSSLDllWrapper();
bool res = wrapper.convertHKID_NameSimple( //need to pass parameters here );

It expects the C++ parameters – pointers to char and RECO_DATA.

How can I fix this and pass C++ types from C# code?

Best Solution

One way to convert most C data types is to use the PInvoke Interop Assitant. It will create proper C# / VB.Net types for most C structures. Here is the output for RECO_DATA

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential, CharSet=System.Runtime.InteropServices.CharSet.Unicode)]
public struct RECO_DATA {

    /// wchar_t[200]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst=200)]
    public string FirstName;

    /// wchar_t[200]
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.ByValTStr, SizeConst=200)]
    public string Surname;

For the char* parameter, you can pass IntPtr.Zero or use Marshal.StringToCoTaskMemAnsi to get the job done.