Using DllImport to get string from unmanaged c++ dll

I need to call a unmanaged c++ function which returns a string from c#.

The code shown below (TestApp) works fine for TestDll1 where the GetStringFromDll() returns a char*.

However what should I do if the unmanaged c++ function returns CString/LPTSTR instead of char* (see TestDll2.cpp and TestDll3.cpp).

I tried using TestApp shown below for TestDll2.cpp and the ReceivedStr only contains the first character 'T' instead of the "Testing". Can anybody help?

// Sample program in c# to call unmanaged code
using System;
using System.Runtime.InteropServices;

class TestApp
{
[DllImport("TestDll1.dll")]
static extern void GetStringFromDll();

public static void Main()
{
string ReceivedStr = GetStringFromDll();
}
}

// TestDll1.cpp
// Written in unmanaged c++

__declspec(dllexport) char* GetStringFromDll()
{
static char* pszReturn;
pszReturn = "Testing";

return pszReturn;
}

// TestDll2.cpp
// Written in unmanaged c++

__declspec(dllexport) LPTSTR GetStringFromDll()
{
LPTSTR pszReturn;
pszReturn = _T("Testing");

return pszReturn;
}

// TestDll3.cpp
// Written in unmanaged c++

__declspec(dllexport) CString GetStringFromDll()
{
CString pszReturn;
pszReturn = "Testing";

return pszReturn;
}