gistfile1.txt

 void Main()
   {
       var result = MakeCalls("ABCDEFGHIJKLMNOP", c => CallWebService(c), 2).Result;
	   result.Dump();
   }

   public async System.Threading.Tasks.Task<string> CallWebService(char c)
   {
       await System.Threading.Tasks.Task.Delay(2000);
       return c.ToString() + DateTime.Now.Second.ToString();
   }
   

   public async System.Threading.Tasks.Task<IEnumerable<TReturn>> MakeCalls<T, TReturn>(IEnumerable<T> items, Func<T, System.Threading.Tasks.Task<TReturn>> func, int maxDegreeeOfParallelism)
   {

       var allTasks = new List<System.Threading.Tasks.Task<TReturn>>();
       var throttler = new SemaphoreSlim(maxDegreeeOfParallelism);
       foreach (var item in items)
       {
           var localItem = item;
           // do an async wait until we can schedule again
           await throttler.WaitAsync();

           allTasks.Add(System.Threading.Tasks.Task.Run(async () =>
           {
               try
               {
                   return await func(localItem);
               }
               finally
               {
                   throttler.Release();
               }
           }));
       }

       // won't get here until all urls have been put into tasks
       await System.Threading.Tasks.Task.WhenAll(allTasks);

       // won't get here until all tasks have completed in some way
       // (either success or exception)
       return allTasks.Select(t => t.Result);
}