Is there a VB.NET equivalent of C# out parameters?

Question

Does VB.NET have a direct equivalent to C# out function parameters, where the variable passed into a function does not need to be initialised?

Resolve

Answer 1

No, there is no equivalent of the out keyword in VB.

However, VB does automatically initialise all local variables in a method, so you can use ByRef without needing to explicitly initialise the variable first.

Example:

Sub Main()
  Dim y As Integer
  Test(y)
End Sub

Sub Test(ByRef x As Integer)
  x = 42
End Sub

(If you examine code in the framework (for example Double.TryParse), you may see the <OutAttribute> added to parameters, but that only makes a difference when the call is marshalled for COM interop or platform invoke.)

Dare

Is there any way to avoid the compilation warnings, without disabling the warnings. – Resolve

Yes I've read about the <OutAttribute>, fortunately I'm migrating a classic ASP app to VB.NET, it's not COM interop or platform invoke. I have hundreds of compiler warnings to deal with. – Resolve

@Spolto: I tested this in both VS 2008 and VS 2010, with explicit and strict mode on, and I don't get any warnings. I added the example above. – Dare

@Spolto: If you are translating VBScript to VB, you should make sure to set Explicit and Strict mode on. It will get you more error messages, but most will point to the source of problems (e.g. variable declared without type) rather than secondary problems (e.g. variable declared without type becomes Object, so it can't be used for a ByRef x As Integer parameter). – Dare

@Guffa: I don't know if it's a version thing, but I too get compiler warnings when passing uninitialized reference type variables as ByRef parameters. (It doesn't happen with value type parameters.) – Vicenary

I'll post a representative code example tomorrow, I don't have access to Visual Studio right now. – Resolve

@Dan Tao, Spolto: That seems to be the difference, I get a warning with reference types also. The inability to specify out parameters is a limitation in the language, and you just have to initialise the variables to get rid of the warnings. Even assigning Nothing to them will get rid of the warning eventhough it doesn't change the result. – Dare

@Guffa: Yes, I've been assigning Nothing to them so far. It's just time consuming as I'm having to do it hundreds of times in a large legacy website. Thank you for investigating. – Resolve

The downvote may have been because the <Out()> attribute is equivalent to the C# version, as my answer points out. – Larghetto

@MarkHurd: Then the downvote is unjust, because I have already covered that the Out attribute is not equivalent to the C# out keyword. – Dare

@Guffa: In that case please add an answer to my question correcting my answer, showing the differences between a C# out parameter and a VB.NET <Out()> ByRef parameter. – Larghetto

Answer 2

No, there is no equivalent construct that allows a non-initialised variable to be passed to a method without a warning, but, as mentioned in my question and answer specifying an <Out()> attribute on a ByRef parameter definition, although VB ignores it, is treated by C# as an out parameter.

So, I would pre-initialise reference variables to Nothing and specify <Out()> ByRef to signify the intention (that will work if C# users ever access your methods).

If you feel you know when you intend to access the default Nothing in otherwise unassigned reference variables you can set the "Warning configuration" "Use of variable prior to assignment" to "None" at the Project level (Project Properties > Compile, and you probably want to set Configuration to "All Configurations" before changing this setting), or, in VS2015 (VB.NET 14), you can use #Disable Warning BC42030.

Larghetto

This is significant. I had a VB subclass of MembershipProvider and then a C# subclass of the VB subclass. The C# code was not recognizing the fact that the abstract methods in the MembershipProvider had already been implemented until I applied the attribute in the VB class for parameters that were specified as out in the MembershipProvider base class. – Ldopa

@RichardCollette That's probably worth being an answer to my linked question! – Larghetto

Answer 3

C# version

  void TestFunc(int x, ref int y, out int z) {
  x++;  
  y++;
  z = 5;
}

Vb.net version

    Sub TestFunc(ByVal x As Integer, ByRef y As Integer, ByRef z As Integer)
  x += 1
  y += 1 
  z = 5 
End Sub

Found the answer here

Update

As stated in the comment do not forget to initialze your parameter that will be used in the out slot

Drakensberg

In general, I agree that ByRef is the closest thing to the out. However, ByRef will still throw a warning if you pass a variable uninitialized, as the question asks. – Hanging

My downvote was from quite a while ago: The web site you've linked to is very general; it does not list specific issues, differences and technicalities. Also your answer still does not answer the question. – Larghetto

Answer 4

I had the problem in VB.NET that I called a function "by ref" that passed an array back.

Even though the compiler flagged it as a warning it was fine. The fix is super simple and probably good programming practice.

I changed

Dim m_arr_values() as Integer

fnRetArray(m_arr_values)

to

' Even though 'Nothing' is the default value, setting it
' stops the compiler complaining.
Dim m_arr_values() as Integer = Nothing

fnRetArray(m_arr_values)

It also helps when coding if variable names are descriptive...

Sub fnCreatePalette(ByRef arr_in_pal() As color, ByRef arr_out_pal() as uinteger)
    ...
End Sub

Canada

Hungarian notation is against .NET framework design guideline. – Glidebomb

Answer 5

VB has the attribute which should be the same as C# out but today you still get a warning even if you use it. There are details about fixing it in vblang area of github. https://github.com/dotnet/vblang/issues/67.

Changsha

Answer 6

You can use the pass by reference method in VB.NET.

You need the Out parameter mechanism in C#, because it doesn't let you use any variable without initializing it.

VB.NET doesn't need a special keyword as it automatically does it by itself.

Just use ByRef.

Gavan

This does not answer the question, and it's wrong with respect to C#. – Resolve

The byref allows you to not initialize, and allows you to change the params value. But, as opposed to the C# out param, it DOES ALLOW you to initialize the parameter with a value and use it in the function, whereas in C# the out keyword forces you to use this ONLY as an out-parameter and NOT as input to the function. Also, if you do not change or set a value to this param within you function, the compiler will not catch that as an error, as opposed to C# where a compilation error will be issued. – Caryl

Answer 7

Use keyword ByRef before variable.

Chor

ByRef is equivalent to ref parameters in C#, which need to be initialised before being passing into a function. – Resolve