ערוך

שתף באמצעות

Using Statement (Visual Basic)

  • מאמר

Declares the beginning of a Using block and optionally acquires the system resources that the block controls.

Syntax

Using { resourcelist | resourceexpression }
    [ statements ]
End Using

Parts

Term Definition
resourcelist Required if you do not supply resourceexpression. List of one or more system resources that this Using block controls, separated by commas.
resourceexpression Required if you do not supply resourcelist. Reference variable or expression referring to a system resource to be controlled by this Using block.
statements Optional. Block of statements that the Using block runs.
End Using Required. Terminates the definition of the Using block and disposes of all the resources that it controls.

Each resource in the resourcelist part has the following syntax and parts:

resourcename As New resourcetype [ ( [ arglist ] ) ]

-or-

resourcename As resourcetype = resourceexpression

resourcelist Parts

Term Definition
resourcename Required. Reference variable that refers to a system resource that the Using block controls.
New Required if the Using statement acquires the resource. If you have already acquired the resource, use the second syntax alternative.
resourcetype Required. The class of the resource. The class must implement the IDisposable interface.
arglist Optional. List of arguments you are passing to the constructor to create an instance of resourcetype. See Parameter List.
resourceexpression Required. Variable or expression referring to a system resource satisfying the requirements of resourcetype. If you use the second syntax alternative, you must acquire the resource before passing control to the Using statement.

Remarks

Sometimes your code requires an unmanaged resource, such as a file handle, a COM wrapper, or a SQL connection. A Using block guarantees the disposal of one or more such resources when your code is finished with them. This makes them available for other code to use.

Managed resources are disposed of by the .NET Framework garbage collector (GC) without any extra coding on your part. You do not need a Using block for managed resources. However, you can still use a Using block to force the disposal of a managed resource instead of waiting for the garbage collector.

A Using block has three parts: acquisition, usage, and disposal.

  • Acquisition means creating a variable and initializing it to refer to the system resource. The Using statement can acquire one or more resources, or you can acquire exactly one resource before entering the block and supply it to the Using statement. If you supply resourceexpression, you must acquire the resource before passing control to the Using statement.

  • Usage means accessing the resources and performing actions with them. The statements between Using and End Using represent the usage of the resources.

  • Disposal means calling the Dispose method on the object in resourcename. This allows the object to cleanly terminate its resources. The End Using statement disposes of the resources under the Using block's control.

Behavior

A Using block behaves like a Try...Finally construction in which the Try block uses the resources and the Finally block disposes of them. Because of this, the Using block guarantees disposal of the resources, no matter how you exit the block. This is true even in the case of an unhandled exception, except for a StackOverflowException.

The scope of every resource variable acquired by the Using statement is limited to the Using block.

If you specify more than one system resource in the Using statement, the effect is the same as if you nested Using blocks one within another.

If resourcename is Nothing, no call to Dispose is made, and no exception is thrown.

Structured Exception Handling Within a Using Block

If you need to handle an exception that might occur within the Using block, you can add a complete Try...Finally construction to it. If you need to handle the case where the Using statement is not successful in acquiring a resource, you can test to see if resourcename is Nothing.

Structured Exception Handling Instead of a Using Block

If you need finer control over the acquisition of the resources, or you need additional code in the Finally block, you can rewrite the Using block as a Try...Finally construction. The following example shows skeleton Try and Using constructions that are equivalent in the acquisition and disposal of resource.

Using resource As New resourceType
    ' Insert code to work with resource.
End Using

' For the acquisition and disposal of resource, the following  
' Try construction is equivalent to the Using block.
Dim resource As New resourceType
Try
    ' Insert code to work with resource.
Finally
    If resource IsNot Nothing Then
        resource.Dispose()
    End If
End Try

Note

The code inside the Using block should not assign the object in resourcename to another variable. When you exit the Using block, the resource is disposed, and the other variable cannot access the resource to which it points.

Example

The following example creates a file that is named log.txt and writes two lines of text to the file. The example also reads that same file and displays the lines of text:

Because the TextWriter and TextReader classes implement the IDisposable interface, the code can use Using statements to ensure that the file is correctly closed after the write and read operations.

Private Sub WriteFile()
    Using writer As System.IO.TextWriter = System.IO.File.CreateText("log.txt")
        writer.WriteLine("This is line one.")
        writer.WriteLine("This is line two.")
    End Using
End Sub

Private Sub ReadFile()
    Using reader As System.IO.TextReader = System.IO.File.OpenText("log.txt")
        Dim line As String

        line = reader.ReadLine()
        Do Until line Is Nothing
            Console.WriteLine(line)
            line = reader.ReadLine()
        Loop
    End Using
End Sub

See also