How to convert c# string to c++ string when we pass string as parameter using winapis

Question

Using SendMessageW function I am passing c# string as a parameter to c++ function. I am typecasting to CString in c++ but it's value is empty. Please check below code and provide solution

-----------------------c# code ------------------

public unsafe IntPtr Testing()
{

  string string_aux = "Stringtochange";
  void* pt = Marshal.StringToBSTR(string_aux).ToPointer();
  IntPtr ab = new IntPtr(pt);
  return ab;
}

public void GetValue()  
{

   SendMessageW(utilityHandle1, TVM_GETITEMHEIGHT, handle,Testing());

}

--------------------- C++ code --------------

CString *st = (CString*)lParam;

MessageBox(NULL,*st,L"stringvalue",NULL);

Here *st value is empty.

David Heffernan · Accepted Answer · 2015-10-21 06:34:27Z

3

You seem to be abusing TVM_GETITEMHEIGHT. Why not use a custom message.

CString is a C++ class. It is not binary compatible with a BSTR.

Personally I would use Marshal.StringToCoTaskMemUni in the C# and cast to wchar_t* in the C++. Remember to destroy the unmanaged memory after you've used it when SendMessageW returns, by calling Marshal.FreeCoTaskMem.

edited Oct 21, 2015 at 6:34

answered Oct 21, 2015 at 6:25

David Heffernan

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6 Comments

Rajendar Manda Over a year ago

I am sending custom message in place of TVM_GETITEMHEIGHT. I am new to c++ I have used Marshal.StringToUni in c# but I do not know how to convert 'LPARAM' to 'wchar_t*' and to 'CString' Can you please tell c++ code to covert 'LPARAM' to 'CString'

David Heffernan Over a year ago

You do know how to do this. You have the code in your answer. Cast the lParam to wchar_t*. With (wchar_t*)lParam. Or reinterpret_cast<wchar_t*>)(lParam). You don't need a CString to call MessageBoxW.

Barmak Shemirani Over a year ago

He should be able to use CString s = (const wchar_t*)lParam;? though it is not necessary unless you need to do some string formatting with CString

Rajendar Manda Over a year ago

I have tried those three but message in MessageBox is coming as empty.c# code is 'IntPtr bn = Marshal.StringToCoTaskMemUni("passingstring");' --- 'SendMessageW(utilityHandle, Win32Utilities.RegisterWindowMessage("requiredcontrol"), handle,bn )' ---Marshal.FreeCoTaskMem(bn);-----In C++: wchar_t* cvb = reinterpret_cast<wchar_t*>(lParam); ----- MessageBox(NULL,cvb,L"stringvalue",NULL); ------ here cvb value is empty. Am I missing anything here ?

David Heffernan Over a year ago

And you changed the C# code to call StringToCoTaskMemUni?

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