Question

I am trying to do something like this in C#. I found out how to call Win32 methods from C# using P/Invoke from this link. However I met some difficulties in implementing P/Invoke.

For example, one of the methods that I would like to access is PdhOpenQuery,
signature:

PDH_STATUS PdhOpenQuery(
  __in   LPCTSTR szDataSource,
  __in   DWORD_PTR dwUserData,
  __out  PDH_HQUERY *phQuery
);

I figure the corresponding C# declaration should be something like this

    [DllImport("Pdh.dll")]
    static extern PDH_STATUS PdhOpenQuery(LPCTSTR szDataSource, 
        DWORD_PTR dwUserData, out PDH_HQUERY *phQuery);

My questions:

What is LPCTSTR, and to what data type does it map in C#?
How to map a pointer type DWORD_PTR? The pinvoke article says DWORD maps to UInt32, but how about pointers?
I think PDH_STATUS and PDH_HQUERY are specific struct to the library (I’m not sure yet). how do I map these?

What is the correct method declaration, and how do you call it correctly?

Answer 1

What is LPCTSTR, and to what data type
does it map in C#?

LPCTSTR is a typedef for const TCHAR*.

TCHAR is an attempt to abstract away the fact that the Windows API exists in both “ANSI” (char strings in a locale-specific encoding) and “Unicode” (UTF-16) versions. There is no actual PdhOpenQuery function; there is a PdhOpenQueryA function that takes an ANSI string and a PdhOpenQueryW function that takes a UTF-16 string.

C# uses UTF-16 strings, so you’ll want to prefer the “W” version of these functions. Use PdhOpenQueryW. Then the first parameter has C++ type const wchar_t*. The C# type is [MarshalAs(UnmanagedType.LPWStr)] string.

How to map a pointer type DWORD_PTR?
The pinvoke article says DWORD maps to
UInt32, but how about pointers?

DWORD_PTR isn’t a pointer. It’s an unsigned integer big enough to hold a pointer. The equivalent C# type is System.UIntPtr.

I think PDH_STATUS and PDH_HQUERY are
specific struct to the library (I’m
not sure yet). how do I map these?

PDH_STATUS appears to be just an int.

PDH_HQUERY is a pointer to a handle (another pointer), but you can just pretend it’s an integer and use IntPtr.

Putting it all together, your declaration should be:

[DllImport("Pdh.dll")]
static extern int PdhOpenQueryW(
    [MarshalAs(UnmanagedType.LPWStr)] string szDataSource, 
    UIntPtr dwUserData,
    out IntPtr phQuery);