WebForms gridview 上的异步行为

Question

在将遗留应用程序移植到 async\await 模式时，我们遇到了一些我们无法跟踪的奇怪行为。我们将页面异步指令设置为 true，并将事件处理程序添加到 RowDataBound。如果 EventHandler 中的代码包含等待调用，则在继续处理下一行之前不会等待。

为了便于理解，这里有一个复述：

public partial class WorkingCorrectlyWebForm: System.Web.UI.Page
{
    private int _index;
    protected void Page_Load(object sender, EventArgs e)
    {
        var grid = new GridView();
        grid.RowDataBound += GridOnRowDataBound;
        grid.DataSource = new[]
        {
            new {Name = "Person1", Age = 23},
            new {Name = "Person2", Age = 33},
            new {Name = "Person3", Age = 15}
        };

        grid.DataBind();
    }

    private async void GridOnRowDataBound(object sender, GridViewRowEventArgs gridViewRowEventArgs)
    {
        if (gridViewRowEventArgs.Row.RowType != DataControlRowType.DataRow)
            return;

        var localIndex = ++_index;

        HttpContext.Current.Response.Write($"starting #{localIndex} <br />");

        await Task.Delay(1000); //HERE IS THE PROBLEMATIC LINE

        //removing the remark from the following line will make code behave "synchronously" as expected.
        // Task.Delay(1000).Wait(); 

        HttpContext.Current.Response.Write($"exiting #{localIndex} <br />");
    }
}

此代码将生成与此类似的输出（而不是按顺序“开始\退出”每个项目）：

从 #1 开始

从 #2 开始

从 #3 开始

退出#3

退出#1

退出#2

为什么会这样？为什么我没有看到有序的“开始”和“退出”消息。

Answer 1

如果您想要串行行为，这应该可以：

    private void GridOnRowDataBound(object sender, GridViewRowEventArgs gridViewRowEventArgs)
    {
        if (gridViewRowEventArgs.Row.RowType != DataControlRowType.DataRow)
            return;

        var localIndex = ++_index;

        HttpContext.Current.Response.Write($"starting #{localIndex} <br />");

        Thread.Sleep(1000);
        // or             Task.Delay(1000).Wait();


        HttpContext.Current.Response.Write($"exiting #{localIndex} <br />");
    }

或尝试：

    private void GridOnRowDataBound(object sender, GridViewRowEventArgs gridViewRowEventArgs)
    {
        RegisterAsyncTask(new PageAsyncTask(() => Bob(gridViewRowEventArgs)));
        HttpContext.Current.Response.Write($"b");

    }

    private async Task Bob(GridViewRowEventArgs gridViewRowEventArgs)
    {
        if (gridViewRowEventArgs.Row.RowType != DataControlRowType.DataRow)
            return;

        var localIndex = ++_index;

        HttpContext.Current.Response.Write($"starting #{localIndex} <br />");

        await Task.Delay(1000);

        HttpContext.Current.Response.Write($"exiting #{localIndex} <br />");
    }