我正在学习如何在C#中进行异步编程的教程，但遇到了一个错误，我不确定该如何解决。以下是链接：http://msdn.microsoft.com/en-us/library/hh191443.aspx，错误信息如下：
Cannot find all types required by the 'async' modifier.
Are you targeting the wrong framework version, or missing a reference to an assembly?
我正在针对.NET 4.0框架进行开发，但不确定是否需要其他程序集。
这是代码：
public async Task<string> AccessTheWebAsync(Class1 class1, Class2 class2)
{
// GetStringAsync returns a Task<string>. That means that when you await the
// task you'll get a List<string> (urlContents).
Task<string[]> listTask = GetList(class1);
// send message task
// You can do work here that doesn't rely on the string from GetStringAsync.
//CompareService();
// 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[] listContents = await listTask;
// The return statement specifies an integer result.
// Any methods that are awaiting AccessTheWebAsync retrieve the length value.
return listContents;
}
public Task<string[]> GetList(Class1 class1)
{
var taskArray = Task<string[]>.Factory.StartNew(() => GenerateResults(class1));
return taskArray;
}
public string[] GenerateResults(Class1 class1)
{
string[] results = new string[2];
results[1] = "";
results[2] = "";
return results;
}
