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MemoryMappedFile.OpenExisting Method (String)

Opens an existing memory-mapped file that has the specified name in system memory.

Namespace:  System.IO.MemoryMappedFiles
Assembly:  System.Core (in System.Core.dll)

'Declaration
Public Shared Function OpenExisting ( _
	mapName As String _
) As MemoryMappedFile

Parameters

mapName
Type: System.String

The name of the memory-mapped file to open.

Return Value

Type: System.IO.MemoryMappedFiles.MemoryMappedFile
A memory-mapped file that has the specified name.

ExceptionCondition
ArgumentException

mapName is an empty string.

ArgumentNullException

mapName is Nothing.

FileNotFoundException

The file specified for mapName does not exist.

The memory-mapped file can be either a persisted memory-mapped file (associated with a file on disk) or non persisted.

Opening a Persisted Memory-Mapped File

The following example opens a memory-mapped file named ImgA that has already been created from a file on disk (as shown in the example for the CreateFromFile(String) method).

Imports System
Imports System.IO.MemoryMappedFiles
Imports System.Runtime.InteropServices

Class Program
    Public Shared Sub Main(ByVal args As String())
        ' Assumes another process has created the memory-mapped file. 
        Using mmf = MemoryMappedFile.OpenExisting("ImgA")
            Using accessor = mmf.CreateViewAccessor(4000000, 2000000)
                Dim colorSize As Integer = Marshal.SizeOf(GetType(MyColor))
                Dim color As MyColor

                ' Make changes to the view. 
                Dim i As Long = 0
                While i < 1500000
                    accessor.Read(i, color)
                    color.Brighten(30)
                    accessor.Write(i, color)
                    i += colorSize
                End While 
            End Using 
        End Using 
    End Sub 
End Class 

Public Structure MyColor
    Public Red As Short 
    Public Green As Short 
    Public Blue As Short 
    Public Alpha As Short 

    ' Make the view brigher. 
    Public Sub Brighten(ByVal value As Short)
        Red = CShort(Math.Min(Short.MaxValue, CInt(Red) + value))
        Green = CShort(Math.Min(Short.MaxValue, CInt(Green) + value))
        Blue = CShort(Math.Min(Short.MaxValue, CInt(Blue) + value))
        Alpha = CShort(Math.Min(Short.MaxValue, CInt(Alpha) + value))
    End Sub 
End Structure

Opening a Non-Persisted Memory-Mapped File

The following example opens a memory-mapped file used for inter-process communication. This code example is part of a larger example provided for the CreateNew(String, Int64) method.

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4

.NET Framework Client Profile

Supported in: 4

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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