对 Delegate的误用及解决办法

最新推荐文章于 2022-04-24 15:06:08 发布

转载最新推荐文章于 2022-04-24 15:06:08 发布 · 738 阅读

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本文探讨了在Silverlight开发中使用多播委托时出现的一个常见问题：执行查询时，由于不当的事件处理方法附加导致的重复执行现象。文中通过具体代码示例展示了问题的原因，并提供了两种解决方案。

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);

        }

Code
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;
        }
    }