Integrating Windows API into C# code

[StanislavPetrov]

I have Windows API code which is written in C++.Sorry,if i'm wrong.How to integrate it into C# code.I mean how to make it work?This code must get the word under the mouse cursor in RichTextBox control.

[DllImport("user32")] public static extern int GetCursorPos(ref POINTAPI lpPoint)
[DllImport("user32")] public static extern int SendMessage(ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As Integer)
[DllImport("user32")] public static extern int WindowFromPoint(int xPoint, int yPoint)

const object WM_GETTEXT = 13;
const object WM_GETTEXTLENGTH = 14;

POINTAPI P;
long lRet;
long hHandle;
string aText;
long lTextlen;
string aText;
lRet = GetCursorPos(P);
hHandle = WindowFromPoint(P.x, P.y);
lTextlen = SendMessage(hHandle, WM_GETTEXTLENGTH, 0, 0);
if (lTextlen)
{
if (lTextlen > 1024) lTextlen = 1024;
lTextlen += 1
aText = Space(lTextlen);
lRet = SendMessage(hHandle, WM_GETTEXT, lTextlen, aText);
aText = aText.Substring(lRet);
}