Linked Textboxes - any action in one is automatically shared with the other

The code links two textboxes, but it can be easily tweaked to handle more.

If you highlight a selection of text in one TextBox, the other will do the same in sync. You would have to change HideSelection to False for both TextBoxes in their properties to see that.

You can backspace the whole selection in both at the same time, or single characters by either the mouse or keyboard.

Also pressing the Return key in one will create new lines in the other.

Pasting is not working in this version, but I'll get it together tomorrow.

The reason for linking the textboxes this way instead of simply TextBox1.text = Textbox2.text and visa-versa is because that simple approach will slow down or flicker the copying textbox if the length of the other textbox is too long. My code doesn't copy an entire textbox this way, but only where changes are made using SelectionStart and SelectionLength.

Code:

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Public Class Form1


    Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        ' Detects backspace key pressed within TextBox1
        If (e.KeyChar) = ControlChars.Back Then
            ' Checks TextBox1 selection length to see if more than one character was selected
            If TextBox1.SelectionLength > 1 Then
                Try
                    ' TextBox2 backspaces the same number of text selected and backspaced in TextBox1
                    TextBox2.Focus()
                    SendKeys.Send("{DELETE}")
                Catch ex As Exception
                End Try
            Else
                Try
                    ' TextBox2 backspaces single characters at the same position of the character backspaced in TextBox1
                    TextBox2.Select(TextBox1.SelectionStart - 1, TextBox1.SelectionLength + 1)
                    TextBox2.SelectedText = ""
                Catch ex As Exception
                End Try
            End If
        Else
            ' Detects if ESC key was pressed
            If (e.KeyChar) = Chr(27) Then
                ' Nothing here to prevent ESC key character from
                ' being entered into TextBoxes
            Else
                ' Detects if Return key was pressed
                If e.KeyChar = Chr(Keys.Return) Then
                    ' Creates new line in TextBox1 since
                    ' it must send focus to TextBox2 in order to
                    ' create a new line there by sending ENTER and
                    ' to set TextBox2's caret position

                    TextBox2.AppendText(Environment.NewLine + "")
                    TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)

                Else
                    ' TextBox2 writes the same Text entered into TextBox1
                    Try
                        TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)
                        TextBox2.SelectedText = e.KeyChar
                    Catch ex As Exception
                        TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)
                        TextBox2.SelectedText = e.KeyChar
                    End Try
                End If
            End If
        End If
    End Sub


    Private Sub TextBox1_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyUp
        TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)
    End Sub


    Private Sub TextBox1_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDown
        TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)
    End Sub


    Private Sub TextBox1_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseUp
        TextBox2.Select(TextBox1.SelectionStart, TextBox1.SelectionLength)
    End Sub


    Private Sub TextBox2_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox2.KeyPress
        ' Detects backspace key pressed within TextBox2
        If (e.KeyChar) = ControlChars.Back Then
            ' Checks TextBox2 selection length to see if more than one character was selected
            If TextBox2.SelectionLength > 1 Then
                Try
                    ' TextBox1 backspaces the same number of text selected and backspaced in TextBox2
                    TextBox1.Focus()
                    SendKeys.Send("{DELETE}")
                Catch ex As Exception
                End Try
            Else
                Try
                    ' TextBox1 backspaces single characters at the same position of the character backspaced in TextBox2
                    TextBox1.Select(TextBox2.SelectionStart - 1, TextBox2.SelectionLength + 1)
                    TextBox1.SelectedText = ""
                Catch ex As Exception
                End Try
            End If
        Else
            ' Detects if ESC key was pressed
            If (e.KeyChar) = Chr(27) Then
                ' Nothing here to prevent ESC key character from
                ' being entered into TextBoxes
            Else
                ' Detects if Return key was pressed
                If e.KeyChar = Chr(Keys.Return) Then
                    ' Creates new line in TextBox2 since
                    ' it must send focus to TextBox1 in order to
                    ' create a new line there by sending ENTER and
                    ' to set TextBox1's caret position

                    TextBox1.AppendText(Environment.NewLine + "")
                    TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)

                Else
                    ' TextBox2 writes the same Text entered into TextBox1
                    Try
                        TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)
                        TextBox1.SelectedText = e.KeyChar
                    Catch ex As Exception
                        TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)
                        TextBox1.SelectedText = e.KeyChar
                    End Try
                End If
            End If
        End If

    End Sub


    Private Sub TextBox2_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox2.KeyUp
        TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)
    End Sub


    Private Sub TextBox2_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox2.MouseDown
        TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)
    End Sub


    Private Sub TextBox2_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles TextBox2.MouseUp
        TextBox1.Select(TextBox2.SelectionStart, TextBox2.SelectionLength)
    End Sub


End Class

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Your code ignores the fact that selection can be made using the keyboard as well as the mouse.

Oops, forgot about that!

I've updated the code to include the keyboard for selecting text. :)