我如何添加按钮/插件如果且仅当它'还没有

我改变代码使用堆栈面板，并得到这个，从重复点击

我如何实现它，以便它只添加按钮/插件，如果它是空白的，而不是添加它们，如果堆栈面板已经填充??

下面是后面的代码:

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
using PluginContracts;
using System;
using System.IO;
using Microsoft.Win32;
namespace SimplePlugin
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Dictionary<string, IPlugin> _Plugins;

        public MainWindow()
        {
            InitializeComponent();

        }
       private void AssembleComponents(object sender)
       {
             _Plugins = new Dictionary<string, IPlugin>();
            //ICollection<IPlugin> plugins = PluginLoader.LoadPlugins("Plugins");
            ICollection<IPlugin> plugins = GenericPluginLoader<IPlugin>.LoadPlugins("Plugins");
            foreach(var item in plugins)
            {
                Button b = new Button();
                //if exists an object, 
                if (b!= null)
                {

                    //delete that object, instantiate new object
                _Plugins.Add(item.Name, item);
                b.Content = item.Name;
                b.Click += b_Click;
                stackPanel.Children.Add(b);
                b = null;
                //PluginGrid.Children.Add(b);    
                }


            }

       }

        private void b_Click(object sender, RoutedEventArgs e)
        {

            Button b = sender as Button;
            if(b != null)
            {
                string key = b.Content.ToString();
                if(_Plugins.ContainsKey(key))
                {
                    IPlugin plugin = _Plugins[key];
                    plugin.Do();

                }
            }

        }
        private void addPlugin_Click(object sender, RoutedEventArgs e)
        {
            var fileDialog = new OpenFileDialog();
            fileDialog.Multiselect = true;
            fileDialog.Filter = "All files (*.*)|*.*|DLL files (*.dll)|*.dll|CS Files (*.cs)|*.cs"; 

            if (fileDialog.ShowDialog() == true)
            {
                string filename = fileDialog.FileName;
                var ext = System.IO.Path.GetExtension(filename);
                 // ListBox lbFiles = new ListBox();
                //this.Controls.Add(lbFiles);
                //lbFiles.Size = new System.Drawing.Size(200, 100);
                //lbFiles.Location = new System.Drawing.Point(10, 10);
                lbFiles.Items.Add(System.IO.Path.GetFileName(filename));
              //  
                CopyToDir(filename);
            }
        }
        //private void lbFiles_SelectionChanged(object sender, SelectionChangedEventArgs e)
        //{
        //    AssembleComponents();
        //}
        private void CopyToDir(string filename)
        {
            //  txtBox.Text = "Hello World";  
            string path = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "Plugins");
            Console.WriteLine(path);
            //Check the directory exists
            if (!Directory.Exists(path))
            {
                Directory.CreateDirectory(path);
            }
            try 
            {  
            FileInfo fi = new FileInfo(filename);
            if (!File.Exists(System.IO.Path.Combine(path, fi.Name)))
            {
                File.Copy(fi.FullName, System.IO.Path.Combine(path, fi.Name));
            }
            }
            catch (Exception ex)
            {
                throw ex; 
            }
        }

        //Approach 1
        //if exists an object, delete that object, instantiate new object
        private void lbFiles_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
           AssembleComponents(sender);

        }  
        //Approach 2 
        private void ClearBtn_Click(object sender, RoutedEventArgs e)
        { 
            lbFiles.Items.Clear();

        }
    }
}

if (stackpanel.Children.Count <= 0)
{
    _Plugins.Add(item.Name, item);
    stackPanel.Children.Add(b);
}