CODEWITHSUNDEEP
.netvb.netreferencecopy
Maxime
Maxime

Reputation: 15

Copy a reference VB.NET

I want to keep my reference to a variable and I don't know how in .NET

First, I receive a reference to an Object like this

Public Function Subscribe(ByRef var As Object) As Integer  

    MyGlobalVar = var   

End Function

I want to keep this reference in a global variable so I can access it later and change the original object.

But every time I do that .NET create a copy, so it does not work.

The only solution I found is to pass an array of Object but this is ugly and i'm searching for other solutions.

Thank you

Upvotes: 1

Views: 2018

Answers (2)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

There are no reference variables in .NET, only ByRef parameters. You can pass a reference to a variable into a method, but once the method ends, that reference is gone as well.

You are correct that using a single-item array for its mutability is a hack. You can build your own "mutable reference" class instead, with get and set operations changing the object inside your reference class. You can also make that class generic on the type of the object that it holds.

However, good chances are that the need to use a global variable is a consequence of some poor design choice that you made earlier. It is hard to tell for sure without seeing the rest of your design, but if you could think of a way to eliminate that global, the problem of needing to store a reference will be solved as well. For example, knowing that

I'm in a DLL and want the user to pass me the reference of his variable of any type so I can update it each second

you should change the API that you present to the user from this

Public Function Subscribe(ByRef var As Object) As Integer

to this:

Public Function Subscribe(callback As Action(Of Object) ) As Integer

The users of your code will pass a delegate for you to call when you want to update the value of interest. Then it would be up to them to assign the value that you pass to them to whatever variable they may choose.

You can store the Action(Of Object) delegate, and call it each time that you want to update the value. It will be up to you to update it each second, or skip updates whenever the value does not need to change. Overall, this would achieve a better separation between your code and the code of your callers.

Upvotes: 0

user2471238
user2471238

Reputation:

If you mean “actually change what the original variable points to”, no, that’s just not possible. A cleaner way might be to put it in a class, but it really depends on your situation.

Since you look like you need events, though, use ’em!

Public Event SomethingHappened As EventHandler(Of Object) ' Or whatever type

And if you’d really like:

Public Sub Subscribe(cb As EventHandler(Of Object))
    AddHandler SomethingHappened, cb
End Sub

Once a second:

RaiseEvent SomethingHappened(42)

Upvotes: 1

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