AddHandler命令WillStart/RemveHandler命令WillStart不工作

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using CadApp = Autodesk.AutoCAD.ApplicationServices.Application;
[assembly: CommandClass(typeof(CommandWillStartHandler.Commands))]
[assembly: ExtensionApplication(typeof(CommandWillStartHandler.Commands))]
namespace CommandWillStartHandler
{
    public class Commands : IExtensionApplication
    {
        private static string _savedLayer = null;
        private const string LINE_LAYER = "MyLine";
        #region IExtensionApplication Interface
        public void Initialize()
        {
            Document dwg = CadApp.DocumentManager.MdiActiveDocument;
            Editor ed = dwg.Editor;
            try
            {
                ed.WriteMessage("\nInitialising custom add-in...");
                AddCommandHandler();
                ed.WriteMessage("\nInitializing custom add-in completed.");
            }
            catch
            {
                ed.WriteMessage("\nInitializing custom add-in failed");
            }
            finally
            {
                Autodesk.AutoCAD.Internal.Utils.PostCommandPrompt();
            }
        }
        public void Terminate()
        {
        }
        private void AddCommandHandler()
        {
            CadApp.DocumentManager.DocumentCreated += DocumentManager_DocumentCreated;
            foreach (Document d in CadApp.DocumentManager)
            {
                d.CommandWillStart += Document_CommandWillStart;
                d.CommandEnded += Document_CommandEnded;
                d.CommandCancelled += Document_CommandCancelled;
            }
        }
        private void DocumentManager_DocumentCreated(object sender, DocumentCollectionEventArgs e)
        {
            e.Document.CommandWillStart += Document_CommandWillStart;
            e.Document.CommandEnded += Document_CommandEnded;
            e.Document.CommandCancelled += Document_CommandCancelled;
        }
        private void Document_CommandEnded(object sender, CommandEventArgs e)
        {
            Document doc = CadApp.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;
            ed.WriteMessage("\nCommand {0} ended.", e.GlobalCommandName);
            if (!string.IsNullOrEmpty(_savedLayer))
            {
                CadApp.SetSystemVariable("CLAYER", _savedLayer);
                _savedLayer = null;
            }
        }
        private void Document_CommandCancelled(object sender, CommandEventArgs e)
        {
            Document doc = CadApp.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;
            ed.WriteMessage("\nCommand {0} cancelled.", e.GlobalCommandName);
            if (!string.IsNullOrEmpty(_savedLayer))
            {
                CadApp.SetSystemVariable("CLAYER", _savedLayer);
                _savedLayer = null;
            }
        }
        private void Document_CommandWillStart(object sender, CommandEventArgs e)
        {
            Document doc = CadApp.DocumentManager.MdiActiveDocument;
            Editor ed = doc.Editor;
            ed.WriteMessage("\nCommand {0} will start.", e.GlobalCommandName);
            //Whenever "LINE" command starts, the active layer is set to
            //LINE_LAYER so that the line is drawn on that layer
            if (e.GlobalCommandName.ToUpper() == "LINE")
            {
                _savedLayer = CadApp.GetSystemVariable("CLAYER").ToString();
                CadApp.SetSystemVariable("CLAYER", LINE_LAYER);
            }
        }
        #endregion
    }
}

Imports Autodesk.AutoCAD.ApplicationServices
Imports Autodesk.AutoCAD.ApplicationServices.Core.Application
Imports Autodesk.AutoCAD.DatabaseServices
Imports Autodesk.AutoCAD.Geometry
Imports Autodesk.AutoCAD.EditorInput
Imports Autodesk.AutoCAD.ApplicationServices.Application
Imports System.Windows.Forms
Imports Autodesk.AutoCAD.Colors
Imports System.IO
Imports Autodesk.AutoCAD.Runtime
Public Class Form1
    Implements IExtensionApplication
    Private CurrentLayerName As String
    Private _SelectedLayer As LayerData
    Private Shared acDoc As Document
    Public Sub Initialize() Implements IExtensionApplication.Initialize
            Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog("Initialize")
        acDoc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument
    End Sub
        
    Public Sub Terminate() Implements IExtensionApplication.Terminate
    End Sub
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        _SelectedLayer = New LayerData
      End Sub
    Private Sub myOnButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles myOnButton.Click
         AddHandler acDoc.CommandWillStart, AddressOf SwapTheLayer
    End Sub
        
    Private Sub OFF_Button_Click(ByVal sender As Object, ByVal e As EventArgs) Handles OFF_Button.Click
        RemoveHandler acDoc.CommandWillStart, AddressOf SwapTheLayer
    End Sub
    Private Sub SwapTheLayer(ByVal senderObj As Object, ByVal e As CommandEventArgs)
        CurrentLayerName = Autodesk.AutoCAD.ApplicationServices.Application.GetSystemVariable("CLAYER")
        Dim importedLayer As Array = _SelectedLayer.ToArray
        Select Case e.GlobalCommandName
            Case "MTEXT"
                If checkIfLayerExistInDrawing(MText_TextBox.Text) = True Then
                    Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog("layer exist")
                    Autodesk.AutoCAD.ApplicationServices.Application.SetSystemVariable("CLAYER", MText_TextBox.Text.ToString)
                Else
                    Dim myCreateLay As New CreateLayer(importedLayer(0).MyLayerName,
                                                       importedLayer(0).MyLayerDescription,
                                                       importedLayer(0).MyLayerColor,
                                                       importedLayer(0).MyLayerPlot,
                                                       importedLayer(0).MyLayerLinetypeName,
                                                       importedLayer(0).MyLayerLineweight)
                    Autodesk.AutoCAD.ApplicationServices.Core.Application.SetSystemVariable("CLAYER", MText_TextBox.Text)
                    Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog("layer do not exist in drawing")
                End If
                Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog("AddHandler CommandEnded")
                AddHandler acDoc.CommandEnded, AddressOf BackToOriginalLayer
                AddHandler acDoc.CommandCancelled, AddressOf BackToOriginalLayer
                AddHandler acDoc.CommandFailed, AddressOf BackToOriginalLayer
            End Select
    End Sub
    Private Sub BackToOriginalLayer(ByVal senderObj As Object, ByVal e As CommandEventArgs)
        Select Case e.GlobalCommandName
            Case Is = "MTEXT"
                Autodesk.AutoCAD.ApplicationServices.Application.SetSystemVariable("CLAYER", CurrentLayerName)
                Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog("RemoveHandler CommandEnded")
                RemoveHandler acDoc.CommandEnded, AddressOf BackToOriginalLayer
                RemoveHandler acDoc.CommandCancelled, AddressOf BackToOriginalLayer
                RemoveHandler acDoc.CommandFailed, AddressOf BackToOriginalLayer
        End Select
    End Sub
    Private Function checkIfLayerExistInDrawing(ByVal myLayerName As String)
        Dim LayerExist As Boolean
        '' Get the current document and database
        Dim acCurDb As Database = acDoc.Database
        '' Start a transaction
        Using acTrans As Transaction = acCurDb.TransactionManager.StartTransaction()
            '' Open the Layer table for read
            Dim acLyrTbl As LayerTable
            acLyrTbl = acTrans.GetObject(acCurDb.LayerTableId, OpenMode.ForRead)
            'layer exist in drawing...
            If acLyrTbl.Has(myLayerName) = False Then
                LayerExist = False
            Else
                LayerExist = True
            End If
        End Using
        Return LayerExist
    End Function
End Class