Question

有人可以给我看一小段代码，演示如何在c＃中异步调用方法吗？

Answer 1

如果你使用action.BeginInvoke（），你have to call EndInvoke某个地方 - 否则框架必须在堆上保存异步调用的结果，导致内存泄漏。

如果您不想使用async / await关键字跳转到C＃5，您可以使用.Net 4中的Task Parallels库。它比使用BeginInvoke / EndInvoke更好，并且提供了一种干净的方式忘记异步工作：

using System.Threading.Tasks;
...
void Foo(){}
...
new Task(Foo).Start();

如果您有调用参数的方法，则可以使用lambda来简化调用，而无需创建委托：

void Foo2(int x, string y)
{
    return;
}
...
new Task(() => { Foo2(42, "life, the universe, and everything");}).Start();

我很确定（但不可否认的是）C＃5 async / await语法只是Task库中的语法糖。

Answer 2

从.Net 4.5开始，您可以使用Task.Run简单地启动操作：

void Foo(string args){}
...
Task.Run(() => Foo("bar"));

http://blogs.msdn.com/b/pfxteam/archive/2011/10/24/10229468.aspx

Answer 3

这是一种方法：

// The method to call
void Foo()
{
}


Action action = Foo;
action.BeginInvoke(ar => action.EndInvoke(ar), null);

当然，如果方法具有不同的签名，则需要用其他类型的委托替换Action

Answer 4

如果您有能力使用新内容，请查看MSDN文章Asynchronous Programming with Async and Await。它被添加到.NET 4.5。

链接示例代码段（本身来自this MSDN sample code project）：

// Three things to note in the signature: 
//  - The method has an async modifier.  
//  - The return type is Task or Task<T>. (See "Return Types" section.)
//    Here, it is Task<int> because the return statement returns an integer. 
//  - The method name ends in "Async."
async Task<int> AccessTheWebAsync()
{ 
    // You need to add a reference to System.Net.Http to declare client.
    HttpClient client = new HttpClient();

    // GetStringAsync returns a Task<string>. That means that when you await the 
    // task you'll get a string (urlContents).
    Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");

    // You can do work here that doesn't rely on the string from GetStringAsync.
    DoIndependentWork();

    // The await operator suspends AccessTheWebAsync. 
    //  - AccessTheWebAsync can't continue until getStringTask is complete. 
    //  - Meanwhile, control returns to the caller of AccessTheWebAsync. 
    //  - Control resumes here when getStringTask is complete.  
    //  - The await operator then retrieves the string result from getStringTask. 
    string urlContents = await getStringTask;

    // The return statement specifies an integer result. 
    // Any methods that are awaiting AccessTheWebAsync retrieve the length value. 
    return urlContents.Length;
}

引用：

如果AccessTheWebAsync在调用GetStringAsync和等待其完成之间没有任何工作，则可以通过调用和等待以下单个语句来简化代码。

string urlContents = await client.GetStringAsync();

更多详情请见link。

Answer 5

public partial class MainForm : Form
{
    Image img;
    private void button1_Click(object sender, EventArgs e)
    {
        LoadImageAsynchronously("http://media1.santabanta.com/full5/Indian%20%20Celebrities(F)/Jacqueline%20Fernandez/jacqueline-fernandez-18a.jpg");
    }

    private void LoadImageAsynchronously(string url)
    {
        /*
        This is a classic example of how make a synchronous code snippet work asynchronously.
        A class implements a method synchronously like the WebClient's DownloadData(…) function for example
            (1) First wrap the method call in an Anonymous delegate.
            (2) Use BeginInvoke(…) and send the wrapped anonymous delegate object as the last parameter along with a callback function name as the first parameter.
            (3) In the callback method retrieve the ar's AsyncState as a Type (typecast) of the anonymous delegate. Along with this object comes EndInvoke(…) as free Gift
            (4) Use EndInvoke(…) to retrieve the synchronous call’s return value in our case it will be the WebClient's DownloadData(…)’s return value.
        */
        try
        {
            Func<Image> load_image_Async = delegate()
            {
                WebClient wc = new WebClient();
                Bitmap bmpLocal = new Bitmap(new MemoryStream(wc.DownloadData(url)));
                wc.Dispose();
                return bmpLocal;
            };

            Action<IAsyncResult> load_Image_call_back = delegate(IAsyncResult ar)
            {
                Func<Image> ss = (Func<Image>)ar.AsyncState;
                Bitmap myBmp = (Bitmap)ss.EndInvoke(ar);

                if (img != null) img.Dispose();
                if (myBmp != null)
                    img = myBmp;
                Invalidate();
                //timer.Enabled = true;
            };
            //load_image_Async.BeginInvoke(callback_load_Image, load_image_Async);             
            load_image_Async.BeginInvoke(new AsyncCallback(load_Image_call_back), load_image_Async);             
        }
        catch (Exception ex)
        {

        }
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        if (img != null)
        {
            Graphics grfx = e.Graphics;
            grfx.DrawImage(img,new Point(0,0));
        }
    }

如何在c＃中异步调用任何方法

5 个答案: