Console.Error Property

public:
 static property System::IO::TextWriter ^ Error { System::IO::TextWriter ^ get(); };

public static System.IO.TextWriter Error { get; }

static member Error : System.IO.TextWriter

Public Shared ReadOnly Property Error As TextWriter

Property Value

A TextWriter that represents the standard error output stream.

Examples

The following example is a command line utility named ExpandTabs that replaces tab characters in a text file with four spaces, the value defined by the tabSize variable. It redirects the standard input and output streams to files, but uses the Error property to write the standard error stream to the console. It can be launched from the command line by supplying the name of the file that contains tab characters and the name of the output file.

using System;
using System.IO;

public class ExpandTabs
{
 private const int tabSize = 4;
 private const string usageText = "Usage: EXPANDTABSEX inputfile.txt outputfile.txt";

 public static void Main(string[] args)
 {
 StreamWriter writer = null;

 if (args.Length < 2) {
 Console.WriteLine(usageText);
 return;
 }

 try {
 writer = new StreamWriter(args[1]);
 Console.SetOut(writer);
 Console.SetIn(new StreamReader(args[0]));
 }
 catch(IOException e) {
 TextWriter errorWriter = Console.Error;
 errorWriter.WriteLine(e.Message);
 errorWriter.WriteLine(usageText);
 return;
 }
 int i;
 while ((i = Console.Read()) != -1) {
 char c = (char)i;
 if (c == '\t')
 Console.Write(("").PadRight(tabSize, ' '));
 else
 Console.Write(c);
 }
 writer.Close();
 // Recover the standard output stream so that a
 // completion message can be displayed.
 StreamWriter standardOutput = new StreamWriter(Console.OpenStandardOutput());
 standardOutput.AutoFlush = true;
 Console.SetOut(standardOutput);
 Console.WriteLine("EXPANDTABSEX has completed the processing of {0}.", args[0]);
 return;
 }
}

open System
open System.IO

let tabSize = 4
let usageText = "Usage: EXPANDTABSEX inputfile.txt outputfile.txt"

[<EntryPoint>]
let main args =
 if args.Length < 2 then
 printfn $"{usageText}"
 else 
 try
 use writer = new StreamWriter(args[1])
 Console.SetOut writer
 Console.SetIn(new StreamReader(args[0]))
 let mutable i = Console.Read()
 while i <> -1 do
 let c = char i
 if c = '\t' then
 Console.WriteLine(("").PadRight(tabSize, ' '))
 else
 printf $"{c}"
 i <- Console.Read()
 // Recover the standard output stream so that a
 // completion message can be displayed.
 use standardOutput = new StreamWriter(Console.OpenStandardOutput())
 standardOutput.AutoFlush <- true
 Console.SetOut standardOutput
 printfn $"EXPANDTABSEX has completed the processing of {args[0]}."
 with :? IOException as e ->
 let errorWriter = Console.Error
 errorWriter.WriteLine e.Message
 errorWriter.WriteLine usageText
 0

Imports System.IO

Public Class ExpandTabs
 Private Const tabSize As Integer = 4
 Private Const usageText As String = "Usage: EXPANDTABSEX inputfile.txt outputfile.txt"
 
 Public Shared Sub Main(args() As String)
 Dim writer As StreamWriter = Nothing

 If args.Length < 2 Then
 Console.WriteLine(usageText)
 Exit Sub
 End If
 
 Try
 writer = New StreamWriter(args(1))
 Console.SetOut(writer)
 Console.SetIn(New StreamReader(args(0)))
 Catch e As IOException
 Console.Error.WriteLine(e.Message)
 Console.Error.WriteLine(usageText)
 Exit Sub
 End Try
 
 Dim i As Integer = Console.Read()
 While i <> -1 
 Dim c As Char = Convert.ToChar(i)
 If c = ControlChars.Tab Then
 Console.Write("".PadRight(tabSize, " "c))
 Else
 Console.Write(c)
 End If
 i = Console.Read()
 End While
 writer.Close()
 
 ' Reacquire the standard output stream so that a
 ' completion message can be displayed.
 Dim standardOutput As New StreamWriter(Console.OpenStandardOutput)
 standardOutput.AutoFlush = True
 Console.SetOut(standardOutput)
 Console.WriteLine("EXPANDTABSEX has completed the processing of {0}.", args(0))
 End Sub
End Class

The following example is a simple text file viewer that displays the contents of one or more text files to the console. If there are no command line arguments, or if any files passed as command line arguments do not exist, the example calls the SetError method to redirect error information to a file, calls the OpenStandardError method in the process of reacquiring the standard error stream, and indicates that error information was written to a file.

using System;
using System.IO;

public class ViewTextFile
{
 public static void Main()
 {
 String[] args = Environment.GetCommandLineArgs();
 String errorOutput = "";
 // Make sure that there is at least one command line argument.
 if (args.Length <= 1)
 errorOutput += "You must include a filename on the command line.\n";

 for (int ctr = 1; ctr <= args.GetUpperBound(0); ctr++) {
 // Check whether the file exists.
 if (!File.Exists(args[ctr])) {
 errorOutput += String.Format("'{0}' does not exist.\n", args[ctr]);
 }
 else {
 // Display the contents of the file.
 StreamReader sr = new StreamReader(args[ctr]);
 String contents = sr.ReadToEnd();
 sr.Close();
 Console.WriteLine("*****Contents of file '{0}':\n\n",
 args[ctr]);
 Console.WriteLine(contents);
 Console.WriteLine("*****\n");
 }
 }

 // Check for error conditions.
 if (!String.IsNullOrEmpty(errorOutput)) {
 // Write error information to a file.
 Console.SetError(new StreamWriter(@".\ViewTextFile.Err.txt"));
 Console.Error.WriteLine(errorOutput);
 Console.Error.Close();
 // Reacquire the standard error stream.
 var standardError = new StreamWriter(Console.OpenStandardError());
 standardError.AutoFlush = true;
 Console.SetError(standardError);
 Console.Error.WriteLine("\nError information written to ViewTextFile.Err.txt");
 }
 }
}
// If the example is compiled and run with the following command line:
// ViewTextFile file1.txt file2.txt
// and neither file1.txt nor file2.txt exist, it displays the
// following output:
// Error information written to ViewTextFile.Err.txt
// and writes the following text to ViewTextFile.Err.txt:
// 'file1.txt' does not exist.
// 'file2.txt' does not exist.

open System
open System.IO

let args = Environment.GetCommandLineArgs()[1..]
let mutable errorOutput = ""
// Make sure that there is at least one command line argument.
if args.Length < 1 then
 errorOutput <- errorOutput + "You must include a filename on the command line.\n"

for file in args do
 // Check whether the file exists.
 if File.Exists file then
 errorOutput <- errorOutput + $"'{file}' does not exist.\n"
 else
 // Display the contents of the file.
 use sr = new StreamReader(file)
 let contents = sr.ReadToEnd()
 Console.WriteLine $"*****Contents of file '{file}':\n\n"
 Console.WriteLine contents
 Console.WriteLine "*****\n"

// Check for error conditions.
if not (String.IsNullOrEmpty errorOutput) then
 // Write error information to a file.
 Console.SetError(new StreamWriter(@".\ViewTextFile.Err.txt"))
 Console.Error.WriteLine errorOutput
 Console.Error.Close()
 // Reacquire the standard error stream.
 use standardError = new StreamWriter(Console.OpenStandardError())
 standardError.AutoFlush <- true
 Console.SetError standardError
 Console.Error.WriteLine "\nError information written to ViewTextFile.Err.txt"

// If the example is compiled and run with the following command line:
// ViewTextFile file1.txt file2.txt
// and neither file1.txt nor file2.txt exist, it displays the
// following output:
// Error information written to ViewTextFile.Err.txt
// and writes the following text to ViewTextFile.Err.txt:
// 'file1.txt' does not exist.
// 'file2.txt' does not exist.

Imports System.IO

Module ViewTextFile
 Public Sub Main()
 Dim args() As String = Environment.GetCommandLineArgs()
 Dim errorOutput As String = ""
 ' Make sure that there is at least one command line argument.
 If args.Length <= 1 Then
 errorOutput += "You must include a filename on the command line." +
 vbCrLf
 End If
 
 For ctr As Integer = 1 To args.GetUpperBound(0)
 ' Check whether the file exists.
 If Not File.Exists(args(ctr)) Then
 errorOutput += String.Format("'{0}' does not exist.{1}",
 args(ctr), vbCrLf)
 Else
 ' Display the contents of the file.
 Dim sr As New StreamReader(args(ctr))
 Dim contents As String = sr.ReadToEnd()
 sr.Close()
 Console.WriteLine("***** Contents of file '{0}':{1}{1}",
 args(ctr), vbCrLf)
 Console.WriteLine(contents)
 Console.WriteLine("*****{0}", vbCrLf)
 End If
 Next

 ' Check for error conditions.
 If Not String.IsNullOrEmpty(errorOutput) Then
 ' Write error information to a file.
 Console.SetError(New StreamWriter(".\ViewTextFile.Err.txt"))
 Console.Error.WriteLine(errorOutput)
 Console.Error.Close()
 ' Reacquire the standard error stream.
 Dim standardError As New StreamWriter(Console.OpenStandardError())
 standardError.AutoFlush = True
 Console.SetError(standardError)
 Console.Error.WriteLine("{0}Error information written to ViewTextFile.Err.txt",
 vbCrLf)
 End If
 End Sub
End Module
' If the example is compiled and run with the following command line:
' ViewTextFile file1.txt file2.txt
' and neither file1.txt nor file2.txt exist, it displays the
' following output:
' Error information written to ViewTextFile.Err.txt
' and writes the following text to ViewTextFile.Err.txt:
' 'file1.txt' does not exist.
' 'file2.txt' does not exist.

Note that the StreamWriter.AutoFlush property is set to true before reacquiring the error stream. This ensures that output will be sent to the console immediately rather than buffered.

Remarks

This standard error stream is set to the console by default. It can be set to another stream with the SetError method. After the standard error stream is redirected, it can be reacquired by calling the OpenStandardError method.

In console applications whose informational output is often redirected to a file, the standard error stream available through the Error property can be used to display information to the console even if output is redirected. The following example displays product tables for 10 numbers at a time starting with 1. After every set of 10 numbers, the Error property is used to ask the user whether to display the next set. If the standard output is redirected to a file, the user is still asked whether the routine should generate the next set of products.

using System;

public class Example
{
 public static void Main()
 {
 int increment = 0;
 bool exitFlag = false;

 while (!exitFlag) {
 if (Console.IsOutputRedirected)
 Console.Error.WriteLine("Generating multiples of numbers from {0} to {1}",
 increment + 1, increment + 10);

 Console.WriteLine("Generating multiples of numbers from {0} to {1}",
 increment + 1, increment + 10);
 for (int ctr = increment + 1; ctr <= increment + 10; ctr++) {
 Console.Write("Multiples of {0}: ", ctr);
 for (int ctr2 = 1; ctr2 <= 10; ctr2++)
 Console.Write("{0}{1}", ctr * ctr2, ctr2 == 10 ? "" : ", ");

 Console.WriteLine();
 }
 Console.WriteLine();

 increment += 10;
 Console.Error.Write("Display multiples of {0} through {1} (y/n)? ",
 increment + 1, increment + 10);
 Char response = Console.ReadKey(true).KeyChar;
 Console.Error.WriteLine(response);
 if (!Console.IsOutputRedirected)
 Console.CursorTop--;

 if (Char.ToUpperInvariant(response) == 'N')
 exitFlag = true;
 }
 }
}

open System

let mutable increment = 0
let mutable exitFlag = false

while not exitFlag do
 if Console.IsOutputRedirected then
 Console.Error.WriteLine $"Generating multiples of numbers from {increment + 1} to {increment + 10}"

 Console.WriteLine $"Generating multiples of numbers from {increment + 1} to {increment + 10}"

 for i = increment + 1 to increment + 10 do
 Console.Write $"Multiples of {i}: "
 for j = 1 to 10 do
 Console.Write $"""{i * j}{if j = 10 then "" else ", "}"""

 Console.WriteLine()
 Console.WriteLine()

 increment <- increment + 10
 Console.Error.Write $"Display multiples of {increment + 1} through {increment + 10} (y/n)? "
 let response = Console.ReadKey(true).KeyChar
 Console.Error.WriteLine response
 if not Console.IsOutputRedirected then
 Console.CursorTop <- Console.CursorTop - 1 

 if Char.ToUpperInvariant response = 'N' then
 exitFlag <- true

Module Example
 Public Sub Main()
 Dim increment As Integer = 0
 Dim exitFlag As Boolean = False
 
 Do While Not exitFlag
 If Console.IsOutputRedirected Then
 Console.Error.WriteLine("Generating multiples of numbers from {0} to {1}",
 increment + 1, increment + 10)
 End If
 Console.WriteLine("Generating multiples of numbers from {0} to {1}",
 increment + 1, increment + 10)
 For ctr As Integer = increment + 1 To increment + 10
 Console.Write("Multiples of {0}: ", ctr)
 For ctr2 As Integer = 1 To 10
 Console.Write("{0}{1}", ctr * ctr2, If(ctr2 = 10, "", ", "))
 Next
 Console.WriteLine()
 Next
 Console.WriteLine()
 
 increment += 10
 Console.Error.Write("Display multiples of {0} through {1} (y/n)? ",
 increment + 1, increment + 10)
 Dim response As Char = Console.ReadKey(True).KeyChar
 Console.Error.WriteLine(response)
 If Not Console.IsOutputRedirected Then
 Console.CursorTop = Console.CursorTop - 1
 End If
 If Char.ToUpperInvariant(response) = "N" Then exitFlag = True
 Loop
 End Sub
End Module

See also

• In
• Out
• IsErrorRedirected
• SetError(TextWriter)