Task.FromException Method

Definition

Namespace:

System.Threading.Tasks

Assemblies:

netstandard.dll, System.Runtime.dll

Overloads

FromException(Exception)	Creates a Task that has completed with a specified exception.
FromException<TResult>(Exception)	Creates a Task<TResult> that's completed with a specified exception.

FromException(Exception)

Source:

Task.cs

Creates a Task that has completed with a specified exception.

C#

public static System.Threading.Tasks.Task FromException(Exception exception);

Parameters

exception

Exception

The exception with which to complete the task.

Returns

Task

The faulted task.

Remarks

This method creates a Task object whose Status property is Faulted and whose Exception property contains exception. The method is commonly used when you immediately know that the work that a task performs will throw an exception before executing a longer code path. For an example, see the FromException<TResult>(Exception) overload.

FromException<TResult>(Exception)

Source:

Task.cs

Creates a Task<TResult> that's completed with a specified exception.

C#

public static System.Threading.Tasks.Task<TResult> FromException<TResult>(Exception exception);

Type Parameters

TResult

The type of the result returned by the task.

Parameters

exception

Exception

The exception with which to complete the task.

Returns

Task<TResult>

The faulted task.

Examples

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 FileInfo object and retrieves the value of its FileInfo.Length property for each file in the directory, the example simply calls the FromException<TResult>(Exception) method to create a faulted task if a particular subdirectory does not exist.

C#

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 (files.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;
                                                 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

Remarks

This method creates a Task<TResult> object whose Status property is Faulted and whose Exception property contains exception. The method is commonly used when you immediately know that the work that a task performs will throw an exception before executing a longer code path. The example provides an illustration.