Export (0) Print
Expand All

FileStream.Lock Method

Prevents other processes from changing the FileStream while permitting read access.

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

'Declaration
Public Overridable Sub Lock ( _
	position As Long, _
	length As Long _
)
'Usage
Dim instance As FileStream
Dim position As Long
Dim length As Long

instance.Lock(position, length)
public void Lock (
	long position, 
	long length
)
public function Lock (
	position : long, 
	length : long
)

Parameters

position

The beginning of the range to lock. The value of this parameter must be equal to or greater than zero (0).

length

The range to be locked.

Exception typeCondition

ArgumentOutOfRangeException

position or length is negative.

ObjectDisposedException

The file is closed.

IOException

The process cannot access the file because another process has locked a portion of the file.

The following table lists examples of other typical or related I/O tasks.

To do this...

See the example in this topic...

Create a text file.

How to: Write Text to a File

Write to a text file.

How to: Write Text to a File

Read from a text file.

How to: Read Text from a File

Append text to a file.

How to: Open and Append to a Log File

File.AppendText

FileInfo.AppendText

Rename or move a file.

File.Move

FileInfo.MoveTo

Copy a file.

File.Copy

FileInfo.CopyTo

Get the size of a directory.

FileInfo.Length

Get the attributes of a file.

File.GetAttributes

Set the attributes of a file.

File.SetAttributes

Create a subdirectory.

CreateSubdirectory

Read from a binary file.

How to: Read and Write to a Newly Created Data File

Write to a binary file.

How to: Read and Write to a Newly Created Data File

See the files in a directory.

Name

Sort files in a directory by size.

GetFileSystemInfos

The following code example demonstrates how to lock part of a file so another process cannot access that part of the file even though it has read/write access to the file. Run the program simultaneously in different command windows and investigate using the different console input options.

Imports Microsoft.VisualBasic
Imports System
Imports System.IO
Imports System.Text

Public Class FStreamLock

    Shared Sub Main()
    
        Dim uniEncoding As New UnicodeEncoding()
        Dim lastRecordText As String = _
            "The last processed record number was: "
        Dim textLength As Integer = _
            uniEncoding.GetByteCount(lastRecordText)
        Dim recordNumber As Integer = 13
        Dim byteCount As Integer = _
            uniEncoding.GetByteCount(recordNumber.ToString())
        Dim tempString As String 

        Dim aFileStream As New FileStream( _
            "Test#@@#.dat", FileMode.OpenOrCreate, _
            FileAccess.ReadWrite, FileShare.ReadWrite)
        
        Try
            ' Write the original file data.
            If aFileStream.Length = 0 Then
                tempString = _
                    lastRecordText + recordNumber.ToString()
                aFileStream.Write(uniEncoding.GetBytes(tempString), _
                    0, uniEncoding.GetByteCount(tempString))
            End If

            ' Allow the user to choose the operation.
            Dim consoleInput As Char = "R"C
            Dim readText(CInt(aFileStream.Length)) As Byte
            While consoleInput <> "X"C

                Console.Write(vbcrLf & _
                    "Enter 'R' to read, 'W' to write, 'L' to " & _ 
                    "lock, 'U' to unlock, anything else to exit: ")

                tempString = Console.ReadLine()
                If tempString.Length = 0 Then
                    Exit While
                End If
                consoleInput = Char.ToUpper(tempString.Chars(0))
                Select consoleInput
                
                    ' Read data from the file and 
                    ' write it to the console.
                    Case "R"C
                        Try
                            aFileStream.Seek(0, SeekOrigin.Begin)
                            aFileStream.Read( _
                                readText, 0, CInt(aFileStream.Length))
                            tempString = New String( _
                                uniEncoding.GetChars( _
                                readText, 0, readText.Length))
                            Console.WriteLine(tempString)
                            recordNumber = Integer.Parse( _
                                tempString.Substring( _
                                tempString.IndexOf(":"C) + 2))

                        ' Catch the IOException generated if the 
                        ' specified part of the file is locked.
                        Catch ex As IOException
                            Console.WriteLine("{0}: The read " & _
                                "operation could not be performed " & _
                                "because the specified part of the" & _
                                " file is locked.", _
                                ex.GetType().Name)
                        End Try
                        Exit Select

                    ' Update the file.
                    Case "W"C
                        Try
                            aFileStream.Seek(textLength, _
                                SeekOrigin.Begin)
                            aFileStream.Read( _
                                readText, textLength - 1, byteCount)
                            tempString = New String( _
                                uniEncoding.GetChars( _
                                readText, textLength - 1, byteCount))
                            recordNumber = _
                                Integer.Parse(tempString) + 1
                            aFileStream.Seek( _
                                textLength, SeekOrigin.Begin)
                            aFileStream.Write(uniEncoding.GetBytes( _
                                recordNumber.ToString()), 0, byteCount)
                            aFileStream.Flush()
                            Console.WriteLine( _
                                "Record has been updated.")

                        ' Catch the IOException generated if the 
                        ' specified part of the file is locked.
                        Catch ex As IOException
                            Console.WriteLine( _
                                "{0}: The write operation could " & _
                                "not be performed because the " & _
                                "specified part of the file is " & _
                                "locked.", ex.GetType().Name)
                        End Try
                        Exit Select

                    ' Lock the specified part of the file.
                    Case "L"C
                        Try
                            aFileStream.Lock(textLength - 1, byteCount)
                            Console.WriteLine("The specified part " & _
                                "of file has been locked.")
                        Catch ex As IOException
                            Console.WriteLine( _
                                "{0}: The specified part of file " & _
                                "is already locked.", _
                                ex.GetType().Name)
                        End Try
                        Exit Select

                    ' Unlock the specified part of the file.
                    Case "U"C
                        Try
                            aFileStream.Unlock( _
                                textLength - 1, byteCount)
                            Console.WriteLine("The specified part " & _
                                "of file has been unlocked.")
                        Catch ex As IOException
                            Console.WriteLine( _
                                "{0}: The specified part of file " & _
                                "is not locked by the current " & _
                                "process.", ex.GetType().Name)
                        End Try
                        Exit Select

                    ' Exit the program.
                    Case Else
                        consoleInput = "X"C
                        Exit While
                End Select
            End While

        Finally
            aFileStream.Close()    
        End Try

    End Sub
End Class

import System.*;
import System.IO.*;
import System.Text.*;

class FStreamLock
{
    public static void main(String[] args)
    {
        UnicodeEncoding uniEncoding =  new UnicodeEncoding();
        String lastRecordText = "The last processed record number was: ";
        int textLength = uniEncoding.GetByteCount(lastRecordText);
        int recordNumber = 13;
        int byteCount = uniEncoding.GetByteCount(
            (new Integer(recordNumber)).ToString());
        String tempString;
                    
        FileStream fileStream =  new FileStream
            ("Test#@@#.dat", FileMode.OpenOrCreate, FileAccess.ReadWrite,
            FileShare.ReadWrite);
        try {
            // Write the original file data.
            if ( fileStream.get_Length() == 0  ) {
                tempString = lastRecordText +(
                    new Integer( recordNumber)).ToString();
                fileStream.Write(uniEncoding.GetBytes(tempString),
                    0, uniEncoding.GetByteCount(tempString));
            }

            // Allow the user to choose the operation.
            char consoleInput = 'R';
            ubyte readText[] = new ubyte[(int) fileStream.get_Length() ];
            while((consoleInput != 'X')) {
                Console.Write(("\nEnter 'R' to read,'W' to write, 'L' to "
                    + "lock, 'U' to unlock, anything else to exit: "));
                if (((String)(tempString =
                        Console.ReadLine())).get_Length() == 0) {
                    break ;
                }
                consoleInput = Char.ToUpper( tempString.get_Chars(0));
                switch(consoleInput) {
                    // Read data from the file and 
                    // write it to the console.
                    case 'R' : 
                        try {
                            fileStream.Seek(0, SeekOrigin.Begin);
                            fileStream.Read(readText, 0, 
                                (int)(fileStream.get_Length()));
                            tempString = new String(
                                uniEncoding.GetChars(readText,
                                0, readText.length ));
                            Console.WriteLine(tempString);
                            recordNumber = Int32.Parse(
                                tempString.Substring(
                                tempString.IndexOf(':') + 2));
                        }                            
                        // Catch the IOException generated if the 
                        // specified part of the file is locked.
                        catch(IOException e) {                            
                            Console.WriteLine("{0}: The read "
                                + "operation could not be performed "
                                + "because the specified part of the "
                                + "file is locked.", e.GetType().get_Name());
                        }
                        break;
                    // Update the file.
                    case 'W' : 
                        try {
                            fileStream.Seek(textLength, SeekOrigin.Begin);
                            fileStream.Read(readText, 
                                textLength - 1, byteCount);
                            tempString = new String(uniEncoding.GetChars(
                                readText, textLength - 1, byteCount));
                            recordNumber = Int32.Parse(tempString)+1;
                            fileStream.Seek(textLength, SeekOrigin.Begin);
                            fileStream.Write(uniEncoding.GetBytes(
                                (new Integer( recordNumber)).ToString()),
                                0, byteCount);
                            fileStream.Flush();
                            Console.WriteLine("Record has been updated.");
                        }

                        // Catch the IOException generated if the 
                        // specified part of the file is locked.
                        catch(IOException e) {                        
                            Console.WriteLine(
                                "{0}: The write operation could not "
                                + "be performed because the specified "
                                + "part of the file is locked.", 
                                e.GetType().get_Name());
                        }
                        break;

                    // Lock the specified part of the file.
                    case 'L' : 
                        try {
                            fileStream.Lock(textLength - 1, byteCount);
                            Console.WriteLine(("The specified part "
                                + "of file has been locked."));
                        }
                        catch(IOException e) {                            
                            Console.WriteLine
                                ("{0}: The specified part of file is"
                                + " already locked.", e.GetType().get_Name());
                        }
                        break;

                    // Unlock the specified part of the file.
                    case 'U' : 
                        try {
                            fileStream.Unlock(textLength - 1, byteCount);
                            Console.WriteLine(("The specified part "
                                + "of file has been unlocked."));
                        }
                        catch(IOException e) {                            
                            Console.WriteLine(
                                "{0}: The specified part of file is "
                                + "not locked by the current process.",
                                e.GetType().get_Name());
                        }
                        break;

                    // Exit the program.
                    default :                        
                        // Exit the program.
                        consoleInput = 'X';
                        break;
                }                    
            }
        }
        finally {
            fileStream.Dispose();
        }
    } //main
} //FStreamLock

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

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

.NET Framework

Supported in: 2.0, 1.1, 1.0

Community Additions

ADD
Show:
© 2014 Microsoft