如何計算C＃中更新的列表框中的條目？

Question

我有一個Windows窗體，其中包含一個列表框（Listbox1），一個標籤（label1）和一個按鈕（button1）。我已經指派一個click事件button1，代碼如下：

public void button1_Click(object sender, EventArgs e) 
{     
    label1.Text = "Parsing entries && initializing comms ..."; 

    apples = new Task(Apple); 
    apples.Start(); 
    Task.WaitAll(apples); 

    label1.Text = "No. of items: " + Listbox1.Items.Count.ToString(); 

    if (Listbox1.Items.Count >= 2) 
    { 
     Listbox1.SetSelected(1, true); 
    } 
} 

public void Apple() {  
//Send 1st command - 90000 
command = "90000"; 
CommPort com = CommPort.Instance; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 

//Send 2nd command - 90001 
command = "90001"; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 

//Send 3rd command - 90002 
command = "90002"; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 

//Send 4th command - 90003 
command = "90003"; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 

//Send 5th command - 90004 
command = "90004"; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 

//Send 6th command - 90005 
command = "90005"; 

if (command.Length > 0) 
{ 
command = ConvertEscapeSequences(command); 
com.Send(command); 
} 
Thread.Sleep(100); //allow 100ms delay for receiving response from serial port 
//Listbox1 eventually contains some (~6) entries 
} 

然而，當我點擊button1，label1顯示文本No. of items: 0，雖然Listbox1實際上包含6個項目。爲什麼代碼在Listbox1中有6個項目時返回0？

Answer 1

您不應該在UI線程上使用阻塞調用，例如WaitAll，因爲這會導致死鎖，更好的選擇是使用async-await。

關於異步等待here的最佳實踐有一篇很好的文章。

您只能使用await一個async mehtod裏面，所以你將需要button1_Clickasync然後await調用Apple。

，你await需要返回的方法Task或Task<T>所以Apple的返回值將需要改變其中的一個。您可以使用Task.Run向線程池發送任何同步阻塞調用，但必須在線程池上運行的Task.Run委託外部的主線程上訪問任何GUI元素。

public async void button1_Click(object sender, EventArgs e) 
{     
    label1.Text = "Parsing entries && initializing comms ..."; 
    await Apple(); 

    label1.Text = "No. of items: " + ((Listbox1.Items.Count).ToString()); 

    if (Listbox1.Items.Count >= 2) 
    { 
     Listbox1.SetSelected(1, true); 
    } 
} 

public async Task Apple() 
{ 
    await Task.Run(()=> 
     { 
      // serial comms work ...    
     } 

    // add items to list here ... 
} 

如果你的串行通信的API supprts async，即SendAsync，你可以await的SendAsync調用和使用await Task.Delay(100)做異步睡覺，你不需要Task.Run。