Console.ReadLine Method

public:
 static System::String ^ ReadLine();

[System.Runtime.Versioning.UnsupportedOSPlatform("android")]
[System.Runtime.Versioning.UnsupportedOSPlatform("browser")]
public static string? ReadLine();

[System.Runtime.Versioning.UnsupportedOSPlatform("browser")]
public static string? ReadLine();

public static string ReadLine();

[<System.Runtime.Versioning.UnsupportedOSPlatform("android")>]
[<System.Runtime.Versioning.UnsupportedOSPlatform("browser")>]
static member ReadLine : unit -> string

[<System.Runtime.Versioning.UnsupportedOSPlatform("browser")>]
static member ReadLine : unit -> string

static member ReadLine : unit -> string

Public Shared Function ReadLine () As String

Returns

The next line of characters from the input stream, or null if no more lines are available.

Attributes

UnsupportedOSPlatformAttribute

Exceptions

Examples

The following example requires two command line arguments: the name of an existing text file, and the name of a file to write the output to. It opens the existing text file and redirects the standard input from the keyboard to that file. It also redirects the standard output from the console to the output file. It then uses the Console.ReadLine method to read each line in the file, replaces every sequence of four spaces with a tab character, and uses the Console.WriteLine method to write the result to the output file.

using System;
using System.IO;

public class InsertTabs
{
 private const int tabSize = 4;
 private const string usageText = "Usage: INSERTTABS inputfile.txt outputfile.txt";
 public static int Main(string[] args)
 {
 if (args.Length < 2)
 {
 Console.WriteLine(usageText);
 return 1;
 }

 try
 {
 // Attempt to open output file.
 using (var writer = new StreamWriter(args[1]))
 {
 using (var reader = new StreamReader(args[0]))
 {
 // Redirect standard output from the console to the output file.
 Console.SetOut(writer);
 // Redirect standard input from the console to the input file.
 Console.SetIn(reader);
 string line;
 while ((line = Console.ReadLine()) != null)
 {
 string newLine = line.Replace(("").PadRight(tabSize, ' '), "\t");
 Console.WriteLine(newLine);
 }
 }
 }
 }
 catch(IOException e)
 {
 TextWriter errorWriter = Console.Error;
 errorWriter.WriteLine(e.Message);
 errorWriter.WriteLine(usageText);
 return 1;
 }

 // Recover the standard output stream so that a
 // completion message can be displayed.
 var standardOutput = new StreamWriter(Console.OpenStandardOutput());
 standardOutput.AutoFlush = true;
 Console.SetOut(standardOutput);
 Console.WriteLine($"INSERTTABS has completed the processing of {args[0]}.");
 return 0;
 }
}

open System
open System.IO

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

[<EntryPoint>]
let main args =
 if args.Length < 2 then
 Console.WriteLine usageText
 1
 else
 try
 // Attempt to open output file.
 use reader = new StreamReader(args[0])
 use writer = new StreamWriter(args[1])
 
 // Redirect standard output from the console to the output file.
 Console.SetOut writer
 
 // Redirect standard input from the console to the input file.
 Console.SetIn reader
 
 let mutable line = Console.ReadLine()
 while line <> null do
 let newLine = line.Replace(("").PadRight(tabSize, ' '), "\t")
 Console.WriteLine newLine
 line <- Console.ReadLine()

 // Recover the standard output stream so that a
 // completion message can be displayed.
 let standardOutput = new StreamWriter(Console.OpenStandardOutput())
 standardOutput.AutoFlush <- true
 Console.SetOut standardOutput
 Console.WriteLine $"INSERTTABS has completed the processing of {args[0]}."
 0

 with :? IOException as e -> 
 let errorWriter = Console.Error
 errorWriter.WriteLine e.Message
 errorWriter.WriteLine usageText
 1

Imports System.IO

Public Module InsertTabs
 Private Const tabSize As Integer = 4
 Private Const usageText As String = "Usage: INSERTTABS inputfile.txt outputfile.txt"
 
 Public Function Main(args As String()) As Integer
 If args.Length < 2 Then
 Console.WriteLine(usageText)
 Return 1
 End If
 
 Try
 ' Attempt to open output file.
 Using writer As New StreamWriter(args(1))
 Using reader As New StreamReader(args(0))
 ' Redirect standard output from the console to the output file.
 Console.SetOut(writer)
 ' Redirect standard input from the console to the input file.
 Console.SetIn(reader)
 Dim line As String = Console.ReadLine()
 While line IsNot Nothing
 Dim newLine As String = line.Replace("".PadRight(tabSize, " "c), ControlChars.Tab)
 Console.WriteLine(newLine)
 line = Console.ReadLine()
 End While
 End Using
 End Using
 Catch e As IOException
 Dim errorWriter As TextWriter = Console.Error
 errorWriter.WriteLine(e.Message)
 errorWriter.WriteLine(usageText)
 Return 1
 End Try

 ' Recover 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($"INSERTTABS has completed the processing of {args(0)}.")
 Return 0
 End Function 
End Module

Remarks

The ReadLine method reads a line from the standard input stream. (For the definition of a line, see the paragraph after the following list.) This means that:

• If the standard input device is the keyboard, the ReadLine method blocks until the user presses the Enter key.

  One of the most common uses of the ReadLine method is to pause program execution before clearing the console and displaying new information to it, or to prompt the user to press the Enter key before terminating the application. The following example illustrates this.

  using System;
  
  public class Example
  {
   public static void Main()
   {
   Console.Clear();
  
   DateTime dat = DateTime.Now;
  
   Console.WriteLine("\nToday is {0:d} at {0:T}.", dat);
   Console.Write("\nPress any key to continue... ");
   Console.ReadLine();
   }
  }
  // The example displays output like the following:
  // Today is 10/26/2015 at 12:22:22 PM.
  //
  // Press any key to continue...
  

  open System
  
  Console.Clear()
  
  let dat = DateTime.Now
  
  printfn $"\nToday is {dat:d} at {dat:T}." 
  printf "\nPress any key to continue... "
  Console.ReadLine() |> ignore
  
  // The example displays output like the following:
  // Today is 12/28/2021 at 8:23:50 PM.
  //
  // Press any key to continue...
  

  Module Example
   Public Sub Main()
   Console.Clear()
  
   Dim dat As Date = Date.Now
  
   Console.WriteLine()
   Console.WriteLine("Today is {0:d} at {0:T}.", dat)
   Console.WriteLine()
   Console.Write("Press any key to continue... ")
   Console.ReadLine()
   End Sub
  End Module
  ' The example displays output like the following:
  ' Today is 10/26/2015 at 12:22:22 PM.
  ' 
  ' Press any key to continue...
  

• If standard input is redirected to a file, the ReadLine method reads a line of text from a file. For example, the following is a text file named ReadLine1.txt:

  
  This is the first line.
  This is the second line.
  This is the third line.
  This is the fourth line.
  
  

  The following example uses the ReadLine method to read input that is redirected from a file. The read operation terminates when the method returns null, which indicates that no lines remain to be read.

  using System;
  
  public class Example
  {
   public static void Main()
   {
   if (!Console.IsInputRedirected) {
   Console.WriteLine("This example requires that input be redirected from a file.");
   return;
   }
  
   Console.WriteLine("About to call Console.ReadLine in a loop.");
   Console.WriteLine("----");
   String s;
   int ctr = 0;
   do {
   ctr++;
   s = Console.ReadLine();
   Console.WriteLine("Line {0}: {1}", ctr, s);
   } while (s != null);
   Console.WriteLine("---");
   }
  }
  // The example displays the following output:
  // About to call Console.ReadLine in a loop.
  // ----
  // Line 1: This is the first line.
  // Line 2: This is the second line.
  // Line 3: This is the third line.
  // Line 4: This is the fourth line.
  // Line 5:
  // ---
  

  open System
  
  if not Console.IsInputRedirected then
   printfn "This example requires that input be redirected from a file."
  
  printfn "About to call Console.ReadLine in a loop."
  printfn "----"
  
  let mutable s = ""
  let mutable i = 0
  
  while s <> null do
   i <- i + 1
   s <- Console.ReadLine()
   printfn $"Line {i}: {s}"
  printfn "---"
  
  
  // The example displays the following output:
  // About to call Console.ReadLine in a loop.
  // ----
  // Line 1: This is the first line.
  // Line 2: This is the second line.
  // Line 3: This is the third line.
  // Line 4: This is the fourth line.
  // Line 5:
  // ---
  

  Module Example
   Public Sub Main()
   If Not Console.IsInputRedirected Then
   Console.WriteLine("This example requires that input be redirected from a file.")
   Exit Sub 
   End If
  
   Console.WriteLine("About to call Console.ReadLine in a loop.")
   Console.WriteLine("----")
   Dim s As String
   Dim ctr As Integer
   Do
   ctr += 1
   s = Console.ReadLine()
   Console.WriteLine("Line {0}: {1}", ctr, s)
   Loop While s IsNot Nothing
   Console.WriteLine("---")
   End Sub
  End Module
  ' The example displays the following output:
  ' About to call Console.ReadLine in a loop.
  ' ----
  ' Line 1: This is the first line.
  ' Line 2: This is the second line.
  ' Line 3: This is the third line.
  ' Line 4: This is the fourth line.
  ' Line 5:
  ' ---
  

  After compiling the example to an executable named ReadLine1.exe, you can run it from the command line to read the contents of the file and display them to the console. The syntax is:

  ReadLine1 < ReadLine1.txt
  

A line is defined as a sequence of characters followed by a carriage return (hexadecimal 0x000d), a line feed (hexadecimal 0x000a), or the value of the Environment.NewLine property. The returned string does not contain the terminating character(s). By default, the method reads input from a 256-character input buffer. Because this includes the Environment.NewLine character(s), the method can read lines that contain up to 254 characters. To read longer lines, call the OpenStandardInput(Int32) method.

The ReadLine method executes synchronously. That is, it blocks until a line is read or the keyboard combination (followed by on Windows), is pressed. The In property returns a TextReader object that represents the standard input stream and that has both a synchronous TextReader.ReadLine method and an asynchronous TextReader.ReadLineAsync method. However, when used as the console's standard input stream, the TextReader.ReadLineAsync executes synchronously rather than asynchronously and returns a Task<String> only after the read operation has completed.

If this method throws an OutOfMemoryException exception, the reader's position in the underlying Stream object is advanced by the number of characters the method was able to read, but the characters already read into the internal ReadLine buffer are discarded. Since the position of the reader in the stream cannot be changed, the characters already read are unrecoverable, and can be accessed only by reinitializing the TextReader. If the initial position within the stream is unknown or the stream does not support seeking, the underlying Stream also needs to be reinitialized. To avoid such a situation and to produce robust code, you should use the KeyAvailable property and ReadKey method and store the read characters in a pre-allocated buffer.

If the key combination (followed by on Windows) is pressed when the method is reading input from the console, the method returns null. This enables the user to prevent further keyboard input when the ReadLine method is called in a loop. The following example illustrates this scenario.

using System;

public class Example
{
 public static void Main()
 {
 string line;
 Console.WriteLine("Enter one or more lines of text (press CTRL+Z to exit):");
 Console.WriteLine();
 do {
 Console.Write(" ");
 line = Console.ReadLine();
 if (line != null)
 Console.WriteLine(" " + line);
 } while (line != null);
 }
}
// The following displays possible output from this example:
// Enter one or more lines of text (press CTRL+Z to exit):
//
// This is line #1.
// This is line #1.
// This is line #2
// This is line #2
// ^Z
//
// >

open System

printfn "Enter one or more lines of text (press CTRL+Z to exit):\n"

let mutable line = ""

while line <> null do
 printf " "
 line <- Console.ReadLine()
 if line <> null then
 printfn $" {line}"


// The following displays possible output from this example:
// Enter one or more lines of text (press CTRL+Z to exit):
//
// This is line #1.
// This is line #1.
// This is line #2
// This is line #2
// ^Z
//
// >

Module Example
 Public Sub Main()
 Dim line As String
 Console.WriteLine("Enter one or more lines of text (press CTRL+Z to exit):")
 Console.WriteLine()
 Do 
 Console.Write(" ")
 line = Console.ReadLine()
 If line IsNot Nothing Then Console.WriteLine(" " + line)
 Loop While line IsNot Nothing 
 End Sub
End Module
' The following displays possible output from this example:
' Enter one or more lines of text (press CTRL+Z to exit):
' 
' This is line #1.
' This is line #1.
' This is line #2
' This is line #2
' ^Z
' 
' >

See also

• Read()
• Write(String, Object)
• WriteLine()