StreamReader.DiscardBufferedData Method

Updated: December 2010

Clears the internal buffer.

Namespace:  System.IO
Assembly:  mscorlib (in mscorlib.dll)

public void DiscardBufferedData()

Use the DiscardBufferedData method to reset the internal buffer for the StreamReader object. You need to call this method only when the position of the internal buffer and the BaseStream do not match. These positions can become mismatched when you read data into the buffer and then seek a new position in the underlying stream. This method slows performance and should be used only when absolutely needed, such as when you want to read a portion of the contents of a StreamReader object more than once.

For a list of common I/O tasks, see Common I/O Tasks.

The following example shows a scenario where the DiscardBufferedData method must be called to synchronize the internal buffer and the underlying stream. The file in the example is used to illustrate position and consists of the text abcdefghijklmnopqrstuvwxyz. By calling DiscardBufferedData after the data is read, the example works as expected. After the first 15 characters are read, the position is reset to the offset value of 2 and all the remaining characters are read. If you remove the call to DiscardBufferedData, the example does not work as expected. The first 15 characters are read, but only the position of the underlying stream is reset. The internal buffer of the StreamReader object is still on the 16th character. Therefore, ReadToEnd returns all the characters in the buffer plus the characters in the underlying stream starting from the reset position.

using System;
using System.IO;

class Test
{
    public static void Main()
    {
        string path = @"c:\temp\alphabet.txt";

        using (StreamReader sr = new StreamReader(path))
        {
            char[] c = null;

            c = new char[15];
            sr.Read(c, 0, c.Length);
            Console.WriteLine("first 15 characters:");
            Console.WriteLine(c);
            // writes - "abcdefghijklmno"

            sr.DiscardBufferedData();
            sr.BaseStream.Seek(2, SeekOrigin.Begin);
            Console.WriteLine("\nBack to offset 2 and read to end: ");
            Console.WriteLine(sr.ReadToEnd());
            // writes - "cdefghijklmnopqrstuvwxyz" 
            // without DiscardBufferedData, writes - "pqrstuvwxyzcdefghijklmnopqrstuvwxyz"
        }
    }
}

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

Date

History

Reason

December 2010

Added example and clarified when to call DiscardBufferedData.

Customer feedback.

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