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Console.KeyAvailable Property 
public static bool KeyAvailable { get; }

The property value is returned immediately; that is, the KeyAvailable property does not block input until a key press is available.

Use the KeyAvailable property in conjunction with only the ReadKey method, not the Read or ReadLine methods.


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// This example demonstrates the Console.KeyAvailable property. 
using System;
using System.Threading;

class Sample 
{
    public static void Main() 
    {
    ConsoleKeyInfo cki = new ConsoleKeyInfo();

    do {
        Console.WriteLine("\nPress a key to display; press the 'x' key to quit.");

// Your code could perform some useful task in the following loop. However,  
// for the sake of this example we'll merely pause for a quarter second. 

        while (Console.KeyAvailable == false)
            Thread.Sleep(250); // Loop until input is entered.
        cki = Console.ReadKey(true);
        Console.WriteLine("You pressed the '{0}' key.", cki.Key);
        } while(cki.Key != ConsoleKey.X);
    }
}
/*
This example produces results similar to the following text:

Press a key to display; press the 'x' key to quit.
You pressed the 'H' key.

Press a key to display; press the 'x' key to quit.
You pressed the 'E' key.

Press a key to display; press the 'x' key to quit.
You pressed the 'PageUp' key.

Press a key to display; press the 'x' key to quit.
You pressed the 'DownArrow' key.

Press a key to display; press the 'x' key to quit.
You pressed the 'X' key.
*/

----------
Pipes 


The document says clearly that InvalidOperationException is thrown when get_KeyAvailable is called when standard in is redirected.  How do I handle such situations?  Should I have a flag in my application that says whether input was redirected so I should avoid KeyAvailable?  And how would I even do such a thing?  I would hate it if someone decided to pipe to my application to automate some task and it crashes!


Preventing an InvalidOperationException



Console.KeyAvailable is used in conjunction with the Console.ReadKey method. In fact, both throw an InvalidOperationException if keyboard input is redirected from the console. 


It's difficult to say how you should handle a situation in which input is redirected without knowing how KeyAvailable and ReadKey are used. For example, if they are just used to ensure that a key is pressed after something is output to the console, then using a try/catch block and doing nothing to handle the exception is adequate. If they are central to a console application, then the best you can do is check for a redirected input stream before calling either KeyAvailable or ReadKey. In other cases, you may be able to change your program code to not use KeyAvailable or ReadKey, or you may only call them if standard input is not redirected.


Unfortunately, determining whether standard input has been redirected is not completely straightforward in the .NET Framework Version 3.5. The .NET Framework Version 4.5 includes a Console.IsInputRedirected property that indicates whether standard input has been redirected, but this is still beta software. In the .NET Framework 3.5, you have to use Windows API calls to determine whether standard input has been redirected. I've modified the example to show how this might be handled. Of course, in this case, since the entire point of the example is to illustrate Console.KeyAvailable and Console.ReadKey, the only practical way to respond to the fact that input is redirected is to display a message to the user and terminate the application. 


Here is the C# code:

using System;
using System.Runtime.InteropServices;
public class Example
{
   const int CONSOLE = 2;      // standard input or output stream
   const int STDIN = -10;
     
   [DllImport("kernel32.dll")] 
   private static extern int GetFileType(IntPtr h); 
   [DllImport("kernel32.dll")] 
   private static extern IntPtr GetStdHandle(int std); 

   public static void Main()
   {
      if (GetFileType(GetStdHandle(STDIN)) == CONSOLE) {
         ConsoleKeyInfo cki;
         Console.WriteLine("Press a key to display; press the 'x' key to quit."); 
         
         do {
            while (Console.KeyAvailable == false)
               System.Threading.Thread.Sleep(100);
               
            cki = Console.ReadKey(true);
            Console.Write(cki.KeyChar);      
         } while (cki.Key != ConsoleKey.X);
      }
      else {
         Console.WriteLine("Console input has been redirected.");  
      }   
   }
}


And here is the VB code:

Module Example
   Private Const CON As Integer = 2
   Private Const STDIN As Integer = -10
   
   Private Declare Function GetFileType Lib "kernel32" (h As IntPtr) As Integer
   Private Declare Function GetStdHandle Lib "kernel32" (std As Integer) As IntPtr
   
   Public Sub Main()
      If GetFileType(GetStdHandle(STDIN)) = CON Then
         Dim cki As ConsoleKeyInfo
         Console.WriteLine("Press a key to display; press the 'x' key to quit.") 
         
         Do 
            Do While Console.KeyAvailable = false
               System.Threading.Thread.Sleep(100)
            Loop
               
            cki = Console.ReadKey(true)
            Console.Write(cki.KeyChar)      
         Loop While cki.Key <> ConsoleKey.X
      Else 
         Console.WriteLine("Console input has been redirected.")  
      End If   
   End Sub
End Module