找不到async修饰符所需的所有类型

Question

我得到以下两个编译器错误(Resharper 7.0.1报告没有错误):

未定义或导入预定义类型"System.Runtime.CompilerServices.IAsyncStateMachine"

找不到'async'修饰符所需的所有类型.您是否针对错误的框架版本,或缺少对程序集的引用？

但是这段代码不会编译:

public class Class1
{
    public Class1()
    {
        Um();
    }
    public async Task<DownloadStringCompletedEventArgs> Um()
    {
        var client = new WebClient();
        return await Observable.FromEvent<DownloadStringCompletedEventHandler, DownloadStringCompletedEventArgs>(x => client.DownloadStringCompleted += x,
                                    x => client.DownloadStringCompleted -= x);
    }
}

我已经看到MSBuild没有找到异步必需的引用,除了我有VS 2012 NOT VS 11 Beta - 虽然它已经安装.我也尝试了一个全新的程序集,没有指向应用程序的xaml命名空间.

Answer 1

您需要在Silverlight 5中支持Async Targeting Packasync.