Keyboard Cursor Location on form in c# win. form

Note: code examples here have been tested, and verified, in Visual Studio 2013, compiled against .NET FrameWork 4.5, using the 'AnyCPU build option on a Win 8/64 machine.

After some research, I've found there is a known problem with using 'GetPositionFromCharIndex to determine the caret position (insertion cursor point) in a TextBox under certain circumstances. However, 'GetPositionFromCharIndex works fine with a RichTextBox.

With a TextBox:

To reliably get the caret point in a TextBox, it is necessary to use a WinAPI call:

[DllImport("user32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool GetCaretPos(out Point lpPoint);

Here's the original code I posted modifed to use the WinAPI 'GetCaretPos function:

private Point GetCursorTextBox(TextBox theTextBox)
{
    // see note below on this
    // Control currentActiveControl = ActiveControl;

    // see note below on why this check is here ...
    if (this.ActiveControl != theTextBox)
    {
        this.ActiveControl = theTextBox;
    }`
    
    // get the caret position which is the
    // row-column address of where in the
    // TextBox's Lines collection the insertion cursor is at
    Point chrTbLocation;

    GetCaretPos(out chrTbLocation);

    // index of the character insertion cursor is at
    int selStart = theTextBox.SelectionStart + theTextBox.SelectionLength;

    // line number the insertion cursor is at
    int line = theTextBox.GetLineFromCharIndex(selStart);

    // point in the Screen insertion cursor is at
    Point chrScrnLocation = theTextBox.PointToScreen(chrTbLocation);

    // point in the Form insertion cursor is at
    Point chrFrmLocation = this.PointToClient(chrScrnLocation);

    // for testing only ...

    // this should be identical to the value in 'selStart
    int charPosition = textBox1.GetCharIndexFromPosition(chrTbLocation);

    Console.WriteLine("Caret at: Line: {0} Column: {1} in TextBox: {2}", line, selStart, theTextBox.Name);
    Console.WriteLine("Screen Location: {0} Form Location: {1}", chrScrnLocation, chrFrmLocation);
    Console.WriteLine();

    // see note below on this
    // ActiveControl = currentActiveControl;

    // return the caret point in Form co-ordinates
    return chrFrmLocation;
}

Note that at the start of the method, a check is done to determine if the TextBox argument to the method is the current ActiveControl on the Form; if it is not, then the TextBox argument is made the ActiveControl. This is done because the call to 'GetCaretPos has no way of "knowing" which Control on the Form, if any, currently is the ActiveControl.

But, also, note that if you invoke the 'GetCursorTextBox method from an EventHandler of the TextBox passed in as an argument, such as in the TextChanged EventHandler, then that check is not really necessary because the TextChanged event is only going to be raised when the TextBox is the ActiveControl.

If having the ActiveControl changed by the 'GetCursorTextBox method presents a problem for you, then preserve the current ActiveControl at the start of the method, and set it back at the end of the method. Code to do this is shown commented out.

With a RichTextBox:

This code will work fine with a RichTextBox:

private Point GetCursorRichTextBox(RichTextBox theRichTextBox)
{
    // index of the character insertion cursor is at
    int selStart = theRichTextBox.SelectionStart + theRichTextBox.SelectionLength;

    // line number the insertion cursor is at
    int line = theRichTextBox.GetLineFromCharIndex(selStart);

    // point in the TextBox insertion cursor is at
    Point chrTbLocation = theRichTextBox.GetPositionFromCharIndex(selStart);

    // point in the Screen insertion cursor is at
    Point chrScrnLocation = theRichTextBox.PointToScreen(chrTbLocation);

    // point in the Form insertion cursor is at
    Point chrFrmLocation = this.PointToClient(chrScrnLocation);

    // for testing only ...

    // this should be identical to the value in 'selStart
    int charPosition = theRichTextBox.GetCharIndexFromPosition(chrTbLocation);

    Console.WriteLine("Caret at: Line: {0} Column: {1} in RichTextBox: {2}", line, selStart, theRichTextBox.Name);
    Console.WriteLine("Screen Location: {0} Form Location: {1}", chrScrnLocation, chrFrmLocation);
    Console.WriteLine();

    // return the caret point in Form co-ordinates
    return chrFrmLocation; 
}