How I get selected text in C#

There are two steps to do this:

1. Get the focused control. We can get the current activated window via GetForegroundWindow API, then call GetWindowThreadProcessId API to get the thread id of the current window. If it is not the current thread, call AttachThreadInput API to attach the thread to the current thread. With all these jobs done, we can call GetFocus API to get the focused control handle.

2. Get the text of the focused control. We can call SendMessageW API to send a WM_GETTEXT message to do this.

You can get detail from the code below:

private System.Windows.Forms.Timer _timer = new System.Windows.Forms.Timer();

private void Form2_Load(object sender, EventArgs e)
{
//Initialize a timer to update the control text.
_timer.Interval = 1000;
_timer.Tick += new EventHandler(_timer_Tick);
}
void _timer_Tick(object sender, EventArgs e)
{
try
{
textBox1.Text = GetTextFromFocusedControl();
}
catch (Exception exp)
{
textBox1.Text += exp.Message;
}
}

//Start to monitor and show the text of the related control.
private void button1_Click(object sender, EventArgs e)
{
_timer.Start();
}

//Get the text of the focused control
private string GetTextFromFocusedControl()
{
try
{
int activeWinPtr = GetForegroundWindow().ToInt32();
int activeThreadId = 0, processId;
activeThreadId = GetWindowThreadProcessId(activeWinPtr, out processId);
int currentThreadId = GetCurrentThreadId();
if (activeThreadId != currentThreadId)
AttachThreadInput(activeThreadId, currentThreadId, true);
IntPtr activeCtrlId = GetFocus();

return GetText(activeCtrlId);
}
catch (Exception exp)
{
return exp.Message;
}
}

//Get the text of the control at the mouse position
private string GetTextFromControlAtMousePosition()
{
try
{
Point p;
if (GetCursorPos(out p))
{
IntPtr ptr = WindowFromPoint(p);
if (ptr != IntPtr.Zero)
{
return GetText(ptr);
}
}
return "";
}
catch (Exception exp)
{
return exp.Message;
}
}

//Get the text of a control with its handle
private string GetText(IntPtr handle)
{
int maxLength = 100;
IntPtr buffer = Marshal.AllocHGlobal((maxLength + 1) * 2);
SendMessageW(handle, WM_GETTEXT, maxLength, buffer);
string w = Marshal.PtrToStringUni(buffer);
Marshal.FreeHGlobal(buffer);
return w;
}

[DllImport("user32.dll")]
public static extern bool GetCursorPos(out Point pt);

[DllImport("user32.dll", EntryPoint = "WindowFromPoint", CharSet = CharSet.Auto, ExactSpelling = true)]
public static extern IntPtr WindowFromPoint(Point pt);

[DllImport("user32.dll", EntryPoint = "SendMessageW")]
public static extern int SendMessageW([InAttribute] System.IntPtr hWnd, int Msg, int wParam, IntPtr lParam);
public const int WM_GETTEXT = 13;

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
internal static extern IntPtr GetForegroundWindow();

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true)]
internal static extern IntPtr GetFocus();

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern int GetWindowThreadProcessId(int handle, out int processId);

[DllImport("user32", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
internal static extern int AttachThreadInput(int idAttach, int idAttachTo, bool fAttach);
[DllImport("kernel32.dll")]
internal static extern int GetCurrentThreadId();

[DllImport("user32", CharSet = CharSet.Auto, SetLastError = true)]
internal static extern int GetWindowText(IntPtr hWnd, [Out, MarshalAs(UnmanagedType.LPTStr)] StringBuilder lpString, int nMaxCount);
Let me know if this does not help.
Aland Li[^]