.net c# ค่าว่างใน textbox คำนวนไม่ได้ช่วยหน่อยค่ะ winapp

             double num = 0;
             double.TryParse("ค่าที่จะเอามาแปลงเป็นตัวเลข", out num);
             // ถ้าแปลงได้ num จะเท่ากับเลขนั้น  ถ้าแปลงไม่ได้ num จะเท่ากับ 0

if (textBox24.Text == "0")
                {
                    textBox24.Text = "";
                    mum = 0;

                }
                else
                {
                    decimal txt24 = Decimal.Parse(textBox24.Text);
                    textBox24.Text = txt24.ToString();
                    mum = 1;
                }

Imports System.Drawing
Imports System.Windows.Forms
Imports System.Web.UI
'
<ToolboxBitmap(GetType(TextBox))> _
Public Class SexyTextBox : Inherits TextBox

    Public Enum TextBoxDisplayManager As Byte
        TextCharacter = 0
        TextNumber = 1
        TextInteger = 2
    End Enum

    Public Enum TextBoxDisplayPrecisionScale As Byte
        None = 0
        One = 1
        Two = 2
        Tree = 3
        Four = 4
    End Enum

    Public Enum TextBoxDisplayOnEnterMoveNext As Byte
        Yes
        No
    End Enum

    Public Enum TextBoxDisplayIsEnabledBinding As Byte
        Yes
        No
    End Enum

    Protected Overrides Sub OnValidating(ByVal e As System.ComponentModel.CancelEventArgs)
        If Me._DisplayManager = TextBoxDisplayManager.TextNumber Then
            Dim dblSubMybaseText As Double
            If String.IsNullOrEmpty(MyBase.Text) OrElse Not IsNumeric(MyBase.Text) Then
                dblSubMybaseText = 0
            Else
                dblSubMybaseText = CDbl(MyBase.Text)
            End If
            Select Case Me._DisplayPrecisionScale
                Case TextBoxDisplayPrecisionScale.One
                    MyBase.Text = String.Format("{0:N1}", dblSubMybaseText)
                Case TextBoxDisplayPrecisionScale.Two
                    MyBase.Text = String.Format("{0:N2}", dblSubMybaseText)
                Case TextBoxDisplayPrecisionScale.Tree
                    MyBase.Text = String.Format("{0:N3}", dblSubMybaseText)
                Case TextBoxDisplayPrecisionScale.Four
                    MyBase.Text = String.Format("{0:N4}", dblSubMybaseText)
                Case Else
                    MyBase.Text = String.Format("{0:N0}", dblSubMybaseText)
            End Select
        End If
    End Sub
End Class