What exactly is the difference between My.Computer

问题:

There is a lot of duplication of functions in the My.Computer.FileSystem and System.IO.File namespaces.

So what exactly is the difference between:

My.Computer.FileSystem.CopyFile(source, dest, True)

and:

System.IO.File.Copy(source, dest, True)

Is there a performance difference? What is everyone's opinion on which which has the edge on read-ability? I personally use the My.Computer Namespace but that is just habit now.

回答1:

My.* is simply a set of facade-pattern classes implemented for VB.NET that encompass common System.IO* (and other) operations. There is a very tiny performance hit since you're going through an extra layer of abstraction but you have to decide if it's worth optimizing for that. I would suggest using whichever way makes sense to you and others in your shop.

If you examine the code for My.Computer.FileSystem.CopyFile with .NET Reflector you will see that the method wraps many System.IO classes such as File and Directory and especially the File class' Copy, Move and Delete methods. Snippet:

'lots of other code snipped out for brevity and to show the use of System.IO classes...

Directory.CreateDirectory(FileSystem.GetParentPath(str))

   'snip

    If 
       ' snip
    Else
        File.Delete(str)
        File.Move(path, str)
    End If
Else
    File.Move(path, str)
End If
End Sub

回答2:

Virtually nothing.

My.Computer was added to VB as a more convenient and understandable abstraction layer to underlying functions. Some of it's methods add new functionality which in my opinion would be the only time you would use it over System.IO.File.

An example where My.Computer would add functionality over System.IO.File is the Network.DownloadFile method, wherein it has the ability to show a dialog to the user:

If showUI is set to True, a dialog box appears that shows the progress of the operation; the dialog box contains a Cancel button that can be used to cancel the operation. The dialog box is not modal, and therefore does not block user input to other windows in the program.

If you've already used System.IO.File in places I'd highly recommend against using My.Computer over it for consistency reasons. Namely, don't go around mixing calls to methods in My.Computer and System.IO.File, stick to one namespace!

回答3:

With regard to the

System.IO.Directory.Delete

and

FileSystem.DeleteDirectory 

method, there is quite an important difference.

Using

System.IO.Directory.Delete

a 'System.IO.IOException' will be thrown if the directory isn't empty. However, with

FileSystem.DeleteDirectory

the default action is to go ahead and delete the file unless you include an addidional parameter

This is from this page

Public Shared Sub DeleteDirectory ( directory As String, onDirectoryNotEmpty As DeleteDirectoryOption )

Parameters directory

Type: System.String Directory to be deleted. onDirectoryNotEmpty

Type: Microsoft.VisualBasic.FileIO.DeleteDirectoryOption Specifies what should be done when a directory that is to be deleted contains files or directories. Default is DeleteDirectoryOption.DeleteAllContents.

The other option is to specify DeleteDirectoryOption.ThrowIfDirectoryNonEmpty

There are also other differences, but this one, to me stands out a mile.

回答4:

The My namespace is a VB.Net construct which is intended, in part, to be a bridge between VB6 and .Net APIs. These methods will tend to have VB6 semantics + look and feel.

If you're a VB6 user transitioning to .Net I would use these methods as they will be closer to the behavior you are expecting. Otherwise I would stick with the standard .Net APIs of System.IO.File.Copy

EDIT

Several people have questioned if I'm mistaking the My namespace for the Microsoft.VisualBasic namespace. I'm not. The My namespace is a lot of things but one item it does is wrap certain calls into methods that forward into Microsoft.VisualBasic. For example if you type the following code into a VB.net project

My.Computer.FileSystem.CopyFile(source, dest)

It will result in the following set of events

• A call to MyProject.Computer.FileSystem.CopyFile will be embedded in the application
• The types MyProject and MyComputer will be generated into the assembly
• The MyComputer type simply derives from Microsoft.VisualBasic.Devices.Computer
• Hence the FileSystem.CopyFile method resolves down to FileSystemProxy.CopyFile which simply forwards to FileSystem.CopyFile

回答5:

Here's a difference that caused by app to malfunction:

My.Computer.FileSystem.WriteAllText inserts a 3 byte BOM (EF BB BF) at the beginning of the file, and system.io.file does not.

Thus, I replaced My.Computer.FileSystem.WriteAllText with system.io.file.WriteAllText and that fixed it.