StreamReader.DiscardBufferedData Method


Clears the internal buffer.

 void DiscardBufferedData();
public void DiscardBufferedData ();
member this.DiscardBufferedData : unit -> unit
Public Sub DiscardBufferedData ()


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:");
            // writes - "abcdefghijklmno"

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

Module Module1

    Sub Main()
        Dim path As String = "c:\temp\alphabet.txt"

        Dim sr As StreamReader = New StreamReader(path)
        Dim c(14) As Char

        sr.Read(c, 0, c.Length)
        Console.WriteLine("first 15 characters:")
        ' writes - "abcdefghijklmno"

        sr.BaseStream.Seek(2, SeekOrigin.Begin)
        Console.WriteLine(Environment.NewLine & "Back to offset 2 and read to end: ")
        ' writes - "cdefghijklmnopqrstuvwxyz"
        ' without DiscardBufferedData, writes - "pqrstuvwxyzcdefghijklmnopqrstuvwxyz"
    End Sub

End Module


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 necessary, 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.

Applies to

See also