对Delegate的误用及解决办法

      在开发Silverligh程式的时候，发现了一个问题。

      先看一段代码：目的很简单，就是在执行查询的时候进行异步处理。

public partial class test : UserControl
    {
        ISDApp01Xml.SDApp01XmlSoapClient client = new ISDApp01Xml.SDApp01XmlSoapClient();

        public test()
        {
            InitializeComponent();
        }

        private void btnQuery_Click(object sender, RoutedEventArgs e)
        {
            string sql = "select id,name,parentid,grade from pdm_basic_info";

            client.ExecuteQueryCompleted += client_ExecuteQueryCompleted;
            client.ExecuteQueryAsync(sql);

        }

        void client_ExecuteQueryCompleted(object sender, NewWordsDemo.ISDApp01Xml.ExecuteQueryCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                dgshow.ItemsSource = from item in e.Result.Descendants("row")
                                     select new BasicInfo
                                     {
                                         id = item.Attribute("ID").Value.ToString(),
                                         name = item.Attribute("NAME").Value.ToString(),
                                         parentid = item.Attribute("PARENTID").Value.ToString(),
                                         grade = item.Attribute("GRADE").Value.ToString(),
                                     };
                MessageBox.Show("Done");
            }
            else
            {
                MessageBox.Show("Error");
            }
        }
    }

    public class BasicInfo
    {
        public string id { set; get; }
        public string name { set; get; }
        public string parentid { set; get; }
        public string grade { set; get; }
    }

 

      结果发现：在执行第n次查询的时候 MessageBox.Show("Done...");会执行n次.

      原因：这里定义了一个公有的
      ISDApp01Xml.SDApp01XmlSoapClient client = new ISDApp01Xml.SDApp01XmlSoapClient();而后在查询动作时候，为ExecuteQueryCompleted事件添加执行方法，注意到这里用的是典型的多播委托的方式“+=”，也就是说，每执行一次查询，就为此事件添加一个同样的执行方法，从而造成在执行第n次查询的时候，实际上方法执行了n次。   
      解决：
      （1）把service进行私有实例化：在用到的时候再进行实例化，出了作用域，自动“释放”。
      （2）service公有，单是委托方法只能加载一次.
     1.

private void btnQuery_Click(object sender, RoutedEventArgs e)
        {
            string sql = "select id,name,parentid,grade from pdm_basic_info";

            ISDApp01Xml.SDApp01XmlSoapClient client = new ISDApp01Xml.SDApp01XmlSoapClient();
            client.ExecuteQueryCompleted += client_ExecuteQueryCompleted;
            client.ExecuteQueryAsync(sql);

        }

  2.

public partial class test : UserControl
    {
        ISDApp01Xml.SDApp01XmlSoapClient client = new ISDApp01Xml.SDApp01XmlSoapClient();

        public test()
        {
            InitializeComponent();
        }

        private void btnQuery_Click(object sender, RoutedEventArgs e)
        {
            string sql = "select id,name,parentid,grade from pdm_basic_info";

            client.ExecuteQueryAsync(sql);

        }

        void client_ExecuteQueryCompleted(object sender, NewWordsDemo.ISDApp01Xml.ExecuteQueryCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                dgshow.ItemsSource = from item in e.Result.Descendants("row")
                                     select new BasicInfo
                                     {
                                         id = item.Attribute("ID").Value.ToString(),
                                         name = item.Attribute("NAME").Value.ToString(),
                                         parentid = item.Attribute("PARENTID").Value.ToString(),
                                         grade = item.Attribute("GRADE").Value.ToString(),
                                     };
                MessageBox.Show("Done");
            }
            else
            {
                MessageBox.Show("Error");
            }
        }

        private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            client.ExecuteQueryCompleted += client_ExecuteQueryCompleted;
        }
    }