a kind of async programming in c#, need to reference definition

void Main()
{
Run d=new Run(RunHandler);
IAsyncResult result= d.BeginInvoke(new AsyncCallback(CallBack),new string[]{"sdf","sdffd"});
IAsyncResult res=d.BeginInvoke(r=>{},"");
//i.e. asyncresult is a wrapperclass that wraps the state
d.EndInvoke(d.BeginInvoke(re=>{Console.WriteLine (re.AsyncState);},"async state"));
d.EndInvoke(result);
Console.WriteLine (3);
}
void RunHandler(){
Console.WriteLine (1);
}
void CallBack(IAsyncResult result){
Console.WriteLine (result.AsyncState);
Console.WriteLine (2);}

// Define other methods and classes here
public delegate void Run();

1
1

5String[] (2 items)4
sdf
sdffd

2
1
3

void Main()
{
Console.WriteLine (Geta().Result);
}

// Define other methods and classes here
async Task<String> Get(){
return "aa";
}
async Task<string> Geta(){
var a=await Get();
return "bb"+a;
}