在确定值之前使用C＃变量(C# variable is used before it's value is determined)

在确定值之前使用C＃变量(C# variable is used before it's value is determined)

此代码应从网页获取一条信息。 我的问题是它没有显示出来，并且不知道为什么。

我想以某种方式让它等待文件完成而不在那个之外创建一个函数。

我想从更大的文件中修复的实际代码：

public static string GetNews() { WebBrowser page = new WebBrowser(); string data = null; page.Navigate(launcherScriptAddress); page.DocumentCompleted += delegate { data = page.Document.GetElementById("news").InnerText; // can't return `data` from here }; return data; // returns null because it doesn't wait for document to be completed }

This code should take a piece of information from a webpage. My problem is that it doesn't show correnctly and don't know why.

I want to make it somehow to wait for document completion without creating a function outside that one.

The actual code that I want to fix from a larger file:

public static string GetNews() { WebBrowser page = new WebBrowser(); string data = null; page.Navigate(launcherScriptAddress); page.DocumentCompleted += delegate { data = page.Document.GetElementById("news").InnerText; // can't return `data` from here }; return data; // returns null because it doesn't wait for document to be completed }

最满意答案

这不会像你尝试的那样工作。 您的函数在执行page.DocumentCompleted-delegate之前返回。

因此，您唯一能做的就是将回调传递给在您的委托中执行的GetNews函数。

同步< - >异步的旧问题。

此外，您应该交换语句.Navigate和.DocumentCompleted + =以确保“page”在开始加载任何内容之前设置了回调。

[编辑]为此，您需要创建一个委托并更改您的功能：

public delegate void NewsCallback( string dataReceived );

public static void GetNews( NewsCallback callback )
{
    WebBrowser page = new WebBrowser();
    string data = null;
    page.Navigate(launcherScriptAddress);
    page.DocumentCompleted += delegate {
       data = page.Document.GetElementById("news").InnerText;
       callback( data );
    };
}
 
 之后，您可能想以这种方式调用它： 
 
void CallMyNews(){
    GetNews( (dataReceived) => {
      DoSomeStuffWith(dataReceived);
    } );
}
This won't work the way you try to do it. Your function returns way before the page.DocumentCompleted-delegate is being executed.
 
So the only thing you can do is pass a callback to your GetNews-function that gets executed within your delegate.
 
The old problem with sync <-> async.
 
Besides, you should swap the statements .Navigate and .DocumentCompleted += to make sure "page" has the callback set before it even starts to load anything.
 
[edit] To do that, you need to create a delegate and change your function:
 
public delegate void NewsCallback( string dataReceived );

public static void GetNews( NewsCallback callback )
{
    WebBrowser page = new WebBrowser();
    string data = null;
    page.Navigate(launcherScriptAddress);
    page.DocumentCompleted += delegate {
       data = page.Document.GetElementById("news").InnerText;
       callback( data );
    };
}
 
After that, you may want to call it this way:
 
void CallMyNews(){
    GetNews( (dataReceived) => {
      DoSomeStuffWith(dataReceived);
    } );
}