Vb.Net Textbox Auto Add S.No. With Full Coding

Source Code

  Form Load Coding 
   TextBox4.Text = "S.No. "
        TextBox4.SelectionStart = TextBox4.Text.Length
        End Form load 
Private prefix As String = "S.No."
    Private Sub TextBox4_TextChanged(sender As Object, e As EventArgs) Handles TextBox4.TextChanged
        RemoveHandler TextBox4.TextChanged, AddressOf TextBox4_TextChanged

        ' Check karein ke spaces hata kar text prefix se start ho raha hai ya nahi
        If Not TextBox4.Text.TrimStart().StartsWith(prefix) Then
            ' Agar prefix delete ho jaye to purana text ya sirf prefix wapas le aayein
            TextBox4.Text = prefix
            TextBox4.SelectionStart = TextBox4.Text.Length
        End If

        AddHandler TextBox4.TextChanged, AddressOf TextBox4_TextChanged

    End Sub

    Private Sub TextBox4_KeyDown(sender As Object, e As KeyEventArgs) Handles TextBox4.KeyDown
        Dim cursorPosition As Integer = TextBox4.SelectionStart

        ' Yeh maloom karne ke liye ke prefix kahan se shuru ho raha hai (spaces ke baad)
        Dim prefixStartIndex As Integer = TextBox4.Text.IndexOf(prefix)

        ' 1. Backspace Protection: Agar cursor prefix ke start par ya andar ho
        If e.KeyCode = Keys.Back Then
            If cursorPosition <= prefixStartIndex + prefix.Length AndAlso cursorPosition > prefixStartIndex Then
                ' Prefix ke andar delete karne se rokein
                e.SuppressKeyPress = True
            End If
        End If

        ' 2. Delete Key Protection
        If e.KeyCode = Keys.Delete Then
            If cursorPosition >= prefixStartIndex AndAlso cursorPosition < prefixStartIndex + prefix.Length Then
                e.SuppressKeyPress = True
            End If
        End If
    End Sub