从调用方法获得退货 [英] Get a RETURN from a Invoke method

问题描述

我正在尝试从另一个线程上的列表框项目中读取值.

I am trying to read value from a listbox item that is on another thread.

我试图制作一个新方法来运行invoke命令，我可以设法通过调用方法像添加一样将命令发送到列表框，但是我似乎无法获得响应，我似乎无法获得项目，我尝试了几种方法，一旦将其从空变为字符串，事情就开始变得毛茸茸了...

I tried to make a new method to run the invoke command, I can manage to send a command to the listbox like add via the invoke method but i cant seem to get a response, i cant seem to get the value of the item, i have tried a few ways, once i change it from a void to a string things start to get hairy...

  thread t1 = new thread(thethread)
    t1.start()

    public void thethread()
    {
    string text = readListBoxSelected(listBox1) + " lala" ;
    }



    public static string readListBoxSelected(ListBox listbox)
    {
        if (listbox.InvokeRequired)
        {
            return (string)listbox.Invoke(
              new Func<String>(() => readListBoxSelected(listbox))
            );
        }
        else
        {

            string varText = listbox.SelectedValue.ToString();
            return varText;
        }
        }

上面是我正在尝试做的一个例子.

Above is a example of what i am trying to do.

这是错误:

System.NullReferenceException原为 用户代码未处理
Message =对象引用未设置为 对象的实例.
来源= * * StackTrace: 在 * * .在e:\ documents中的Form1.readListBoxSelected(ListBox listbox)和 设置\斯科特\我的文档\视觉 工作室 2010 \ Projects ** * * * \ Form1.cs:line 133 在 *** .Form1.<> c_ DisplayClass5.b _3() 在e:\ documents and settings \ scott \ my 文件\视觉工作室 2010 \ Projects ** * * ** \ Form1.cs:line 127 InnerException:

System.NullReferenceException was unhandled by user code
Message=Object reference not set to an instance of an object.
Source=** StackTrace: at **.Form1.readListBoxSelected(ListBox listbox) in e:\documents and settings\scott\my documents\visual studio 2010\Projects*****\Form1.cs:line 133 at ***.Form1.<>c_DisplayClass5.b_3() in e:\documents and settings\scott\my documents\visual studio 2010\Projects******\Form1.cs:line 127 InnerException:

我想我的错恰恰是它所说的对象引用未设置为对象实例" .......据我所知，我所有的变量似乎都被声明为公平，我该如何纠正这个吗?

I imagine what is wrong is exactly what it says "Object reference not set to an instance of an object"....... All my variables seem to be declared as fair as i am aware, how can i correct this??

我感觉到我正在错误地处理整个事情.... 0_o 提前致谢， 斯科特

I get the feeling i am going about the entire thing wrongly.... 0_o Thanks in Advance, Scott

推荐答案

尝试一下

public static string readListBoxSelected(ListBox listbox)
    {
        if (listbox.InvokeRequired)
        {
            return (string)listbox.Invoke(
              new Func<String>(() => readListBoxSelected(listbox))
            );
        }
        else
        {
if(istbox.SelectedValue != null)

            return  listbox.SelectedValue.ToString();
else
return String.Empty
        }
        }

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