Task.Result.txt
20210327

Task.FromResult<TResult>(TResult) Method
public static System.Threading.Tasks.Task<TResult> FromResult<TResult> (TResult result);

Creates a Task<TResult> that's completed successfully with the specified result.

The following example is a command-line utility that calculates the number of bytes in the files in each directory whose name is passed as a command-line argument. 
Rather than executing a longer code path that instantiates a FileStream object and retrieves the value of its FileStream.Length property for each file in the directory, 
the example simply calls the FromResult method to create a task whose Task<TResult>.Result property is zero (0) if a directory has no files.

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
   public static void Main()
   {
      string[] args = Environment.GetCommandLineArgs();
      if (args.Length > 1) {
         List<Task<long>> tasks = new List<Task<long>>();
         for (int ctr = 1; ctr < args.Length; ctr++)
            tasks.Add(GetFileLengthsAsync(args[ctr]));

         try {
            Task.WaitAll(tasks.ToArray());
         }
         // Ignore exceptions here.
         catch (AggregateException) {}

         for (int ctr = 0 ; ctr < tasks.Count; ctr++) {
            if (tasks[ctr].Status == TaskStatus.Faulted)
               Console.WriteLine("{0} does not exist", args[ctr + 1]);
            else
               Console.WriteLine("{0:N0} bytes in files in '{1}'",
                                 tasks[ctr].Result, args[ctr + 1]);
         }
      }
      else {
         Console.WriteLine("Syntax error: Include one or more file paths.");
      }
   }

   private static Task<long> GetFileLengthsAsync(string filePath)
   {
      if (! Directory.Exists(filePath)) {
         return Task.FromException<long>(new DirectoryNotFoundException("Invalid directory name.")); // CodeHelpe 若有錯誤, 則可建立 Task.FromException<T> 回傳
      }
      else {
         string[] files = Directory.GetFiles(filePath);
         if (files.Length == 0)
            return Task.FromResult(0L); // CodeHelper Task.FromResult<T> 建立(已完成的 Task.FromResult<T>)回傳.
         else
            return Task.Run( () => { long total = 0;
                                     Parallel.ForEach(files, (fileName) => {  // CodeHelper Parallel.ForEach()
                                                 var fs = new FileStream(fileName, FileMode.Open,
                                                                         FileAccess.Read, FileShare.ReadWrite,
                                                                         256, true);
                                                 long length = fs.Length;
                                                 Interlocked.Add(ref total, length);
                                                 fs.Close(); } );
                                     return total; // CodeHelper return T for Task<T>. 正常情況下, 只要回傳 T
                                   } );
      }
   }
}
// When launched with the following command line arguments:
//      subdir . newsubdir
// the example displays output like the following:
//       0 bytes in files in 'subdir'
//       2,059 bytes in files in '.'
//       newsubdir does not exist

Remarks
This method creates a Task<TResult> object whose Task<TResult>.Result property is result and whose Status property is RanToCompletion. The method is commonly used when the return value of a task is immediately known without executing a longer code path. The example provides an illustration.

To create a Task object that does not return a value, retrieve the Task object from the CompletedTask property.