Capture selected text from active window

 sing System;
using System.Text;
using System.Runtime.InteropServices;

public class Example
{
[DllImport("user32.dll")]
static extern int GetFocus();

[DllImport("user32.dll")]
static extern bool AttachThreadInput(uint idAttach, uint idAttachTo, bool fAttach);

[DllImport("kernel32.dll")]
static extern uint GetCurrentThreadId();

[DllImport("user32.dll")]
static extern uint GetWindowThreadProcessId(int hWnd, int ProcessId);    

[DllImport("user32.dll") ]
static extern int GetForegroundWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern int SendMessage(int hWnd, int Msg, int wParam, StringBuilder lParam);     

const int WM_SETTEXT = 12;
const int WM_GETTEXT = 13;

public static void Main() 
{
        //Wait 5 seconds to give us a chance to give focus to some edit window,
        //notepad for example
        System.Threading.Thread.Sleep(5000);
        StringBuilder builder = new StringBuilder(500);

        int foregroundWindowHandle = GetForegroundWindow();
        uint remoteThreadId = GetWindowThreadProcessId(foregroundWindowHandle, 0);
    uint currentThreadId = GetCurrentThreadId();

    //AttachTrheadInput is needed so we can get the handle of a focused window in another app
    AttachThreadInput(remoteThreadId, currentThreadId, true);
        //Get the handle of a focused window
    int  focused = GetFocus();
        //Now detach since we got the focused handle
        AttachThreadInput(remoteThreadId, currentThreadId, false);

        //Get the text from the active window into the stringbuilder
        SendMessage(focused, WM_GETTEXT, builder.Capacity, builder);
        Console.WriteLine("Text in active window was " + builder);
        builder.Append(" Extra text");
        //Change the text in the active window
        SendMessage(focused, WM_SETTEXT, 0, builder);
        Console.ReadKey();
    }
}