Display scroll bar in textbox when contents are beyond the bounds C#

Consider using the RichTextBox -- it has that behavior built in.

Public Class TextBoxScrollbarPlugin
    Private WithEvents mTarget As TextBox

    ''' <summary>
    ''' After the Handle is created, mTarget.IsHandleCreated always returns
    ''' TRUE, even after HandleDestroyed is fired.
    ''' </summary>
    ''' <remarks></remarks>
    Private mIsHandleCreated As Boolean = False

    Public Sub New(item As TextBox)
        mTarget = item
        mIsHandleCreated = mTarget.IsHandleCreated
    End Sub

    Private Sub Update()
        If Not mTarget.IsHandleCreated Then
            Return
        ElseIf Not mIsHandleCreated Then
            Return
        End If
        Dim textBoxRect = TextRenderer.MeasureText(mTarget.Text,
                                                   mTarget.Font,
                                                   New Size(mTarget.Width, Integer.MaxValue),
                                                   TextFormatFlags.WordBreak + TextFormatFlags.TextBoxControl)

        Try
            If textBoxRect.Height > mTarget.Height Then
                mTarget.ScrollBars = ScrollBars.Vertical
            Else
                mTarget.ScrollBars = ScrollBars.None
            End If
        Catch ex As System.ComponentModel.Win32Exception
            'this sometimes throws a "failure to create window handle"
            'error.
            'This might happen if the TextBox is unvisible and/or
            'to small to display a toolbar.
            If mLog.IsWarnEnabled Then mLog.Warn("Update()", ex)
        End Try
    End Sub

    Private Sub mTarget_HandleCreated(sender As Object, e As System.EventArgs) Handles mTarget.HandleCreated
        mIsHandleCreated = True
    End Sub

    Private Sub mTarget_HandleDestroyed(sender As Object, e As System.EventArgs) Handles mTarget.HandleDestroyed
        mIsHandleCreated = False
    End Sub

    Private Sub mTarget_SizeChanged(sender As Object, e As System.EventArgs) Handles mTarget.SizeChanged
        Update()
    End Sub

    Private Sub mTarget_TextChanged(sender As Object, e As System.EventArgs) Handles mTarget.TextChanged
        Update()
    End Sub

End Class


Private mPlugins As New List(Of Object)
Private Sub Form_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    mPlugins.Add(New TextBoxScrollbarPlugin(txtBoxOne))
    mPlugins.Add(New TextBoxScrollbarPlugin(txtBoxTwo))
    mPlugins.Add(New TextBoxScrollbarPlugin(txtBoxThree))
End Sub  

Thanks dummy, it works! Here short version of dummy answer in c#

Call this code at the end of your SizeChanged and TextChanged handlers:

Size textBoxRect = TextRenderer.MeasureText(
    this.YourTextBox.Text, 
    this.YourTextBox.Font, 
    new Size(this.YourTextBox.Width, int.MaxValue),
    TextFormatFlags.WordBreak | TextFormatFlags.TextBoxControl);
try
{
    this.YourTextBox.ScrollBars = textBoxRect.Height > this.YourTextBox.Height ? 
        ScrollBars.Vertical : 
        ScrollBars.None;
} catch (System.ComponentModel.Win32Exception)
{
     // this sometimes throws a "failure to create window handle" error.
     // This might happen if the TextBox is unvisible and/or
     // too small to display a toolbar.
}