Task.FromResult<TResult> Method

.NET Framework 4.6 and 4.5

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

Namespace:  System.Threading.Tasks
Assemblies:   System.Threading.Tasks (in System.Threading.Tasks.dll)
  mscorlib (in mscorlib.dll)

public static Task<TResult> FromResult<TResult>(
	TResult result
)

Type Parameters

TResult

The type of the result returned by the task.

Parameters

result
Type: TResult

The result to store into the completed task.

Return Value

Type: System.Threading.Tasks.Task<TResult>
The successfully completed task.

This method creates a Task<TResult> object whose Task<TResult>.Result property is result and whose Status property is TaskStatus.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.

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<TResult> 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."));
      }
      else {
         string[] files = Directory.GetFiles(filePath);
         if (filePath.Length == 0)
            return Task.FromResult(0L);
         else 
            return Task.Run( () => { long total = 0;
                                     Parallel.ForEach(files, (fileName) => {
                                                 var fs = new FileStream(fileName, FileMode.Open,
                                                                         FileAccess.Read, FileShare.ReadWrite,
                                                                         256, true);
                                                 long length = fs.Length;
                                                 total = Interlocked.Add(ref total, length);
                                                 fs.Close(); } );
                                     return total;
                                   } );
      }
   }
}
// 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

.NET Framework

Supported in: 4.6, 4.5

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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