File.Open Method (String, FileMode, FileAccess, FileShare)


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Opens a FileStream on the specified path, having the specified mode with read, write, or read/write access and the specified sharing option.

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

Public Shared Function Open (
	path As String,
	mode As FileMode,
	access As FileAccess,
	share As FileShare
) As FileStream


Type: System.String

The file to open.

Type: System.IO.FileMode

A FileMode value that specifies whether a file is created if one does not exist, and determines whether the contents of existing files are retained or overwritten.

Type: System.IO.FileAccess

A FileAccess value that specifies the operations that can be performed on the file.

Type: System.IO.FileShare

A FileShare value specifying the type of access other threads have to the file.

Return Value

Type: System.IO.FileStream

A FileStream on the specified path, having the specified mode with read, write, or read/write access and the specified sharing option.

Exception Condition

path is a zero-length string, contains only white space, or contains one or more invalid characters as defined by InvalidPathChars.


access specified Read and mode specified Create, CreateNew, Truncate, or Append.


path is null.


The specified path, file name, or both exceed the system-defined maximum length. For example, on Windows-based platforms, paths must be less than 248 characters, and file names must be less than 260 characters.


The specified path is invalid, (for example, it is on an unmapped drive).


An I/O error occurred while opening the file.


path specified a file that is read-only and access is not Read.


path specified a directory.


The caller does not have the required permission.


mode is Create and the specified file is a hidden file.


mode, access, or share specified an invalid value.


The file specified in path was not found.


path is in an invalid format.

The path parameter is permitted to specify relative or absolute path information. Relative path information is interpreted as relative to the current working directory. To obtain the current working directory, see GetCurrentDirectory.

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

The following example opens a file with read-only access and with file sharing disallowed.

Imports System
Imports System.IO
Imports System.Text

Public Class Test
  Public Shared Sub Main()
    Dim path As String = "c:\temp\MyTest.txt"

    ' Create the file if it does not exist. 
    If Not File.Exists(path) Then
      ' Create the file.
      Using fs As FileStream = File.Create(path)
        Dim info As Byte() = New UTF8Encoding(True).GetBytes("This is some text in the file.")

        ' Add some information to the file.
        fs.Write(info, 0, info.Length)
      End Using
    End If

    ' Open the stream and read it back.
    Using fs As FileStream = File.Open(path, FileMode.Open, FileAccess.Read, FileShare.None)
      Dim b(1024) As Byte
      Dim temp As UTF8Encoding = New UTF8Encoding(True)

      Do While fs.Read(b, 0, b.Length) > 0

        ' Try to get another handle to the same file. 
        Using fs2 As FileStream = File.Open(path, FileMode.Open)
          ' Do some task here.
        End Using
      Catch e As Exception
        Console.Write("Opening the file twice is disallowed.")
        Console.WriteLine(", as expected: {0}", e.ToString())
      End Try

    End Using

  End Sub
End Class


for reading from and writing to the specified file. Associated enumerations: FileIOPermissionAccess.Read, FileIOPermissionAccess.Write

Universal Windows Platform
Available since 10
.NET Framework
Available since 1.1
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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