ByRef Vs ByVal Clarification

Answer :

I think you're confusing the concept of references vs. value types and ByVal vs. ByRef. Even though their names are a bit misleading, they are orthogonal issues.

ByVal in VB.NET means that a copy of the provided value will be sent to the function. For value types (Integer, Single, etc.) this will provide a shallow copy of the value. With larger types this can be inefficient. For reference types though (String, class instances) a copy of the reference is passed. Because a copy is passed in mutations to the parameter via = it won't be visible to the calling function.

ByRef in VB.NET means that a reference to the original value will be sent to the function (1). It's almost like the original value is being directly used within the function. Operations like = will affect the original value and be immediately visible in the calling function.

Socket is a reference type (read class) and hence passing it with ByVal is cheap. Even though it does perform a copy it's a copy of the reference, not a copy of the instance.

(1) This is not 100% true though because VB.NET actually supports several kinds of ByRef at the callsite. For more details, see the blog entry The many cases of ByRef

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Remember that ByVal still passes references. The difference is that you get a copy of the reference.

So, on my overloaded constructor, I am accepting a reference to an instance of a System.Net.Sockets.Socket, yes?

Yes, but the same would be true if you asked for it ByVal instead. The difference is that with ByVal you get a copy of the reference — you have new variable. With ByRef, it's the same variable.

It is my understanding that the instance in memory is copied

Nope. Only the reference is copied. Therefore, you're still working with the same instance.

Here's a code example that explains it more clearly:

Public Class Foo    Public Property Bar As String    Public Sub New(ByVal Bar As String)        Me.Bar = Bar    End Sub End Class  Public Sub RefTest(ByRef Baz As Foo)      Baz.Bar = "Foo"      Baz = new Foo("replaced") End Sub  Public Sub ValTest(ByVal Baz As Foo)     Baz.Bar = "Foo"     Baz = new Foo("replaced") End Sub  Dim MyFoo As New Foo("-") RefTest(MyFoo) Console.WriteLine(MyFoo.Bar) ''# outputs replaced  ValTest(MyFoo) Console.WriteLine(MyFoo.Bar) ''# outputs Foo 

My understanding has always been that the ByVal/ByRef decision really matters most for value types (on the stack). ByVal/ByRef makes very little difference at all for reference types (on the heap) UNLESS that reference type is immutable like System.String. For mutable objects, it doesn't matter if you pass an object ByRef or ByVal, if you modify it in the method the calling function will see the modifications.

Socket is mutable, so you can pass any which way you want, but if you don't want to keep modifications to the object you need to make a deep copy yourself.

Module Module1      Sub Main()         Dim i As Integer = 10         Console.WriteLine("initial value of int {0}:", i)         ByValInt(i)         Console.WriteLine("after byval value of int {0}:", i)         ByRefInt(i)         Console.WriteLine("after byref value of int {0}:", i)          Dim s As String = "hello"         Console.WriteLine("initial value of str {0}:", s)         ByValString(s)         Console.WriteLine("after byval value of str {0}:", s)         ByRefString(s)         Console.WriteLine("after byref value of str {0}:", s)          Dim sb As New System.Text.StringBuilder("hi")         Console.WriteLine("initial value of string builder {0}:", sb)         ByValStringBuilder(sb)         Console.WriteLine("after byval value of string builder {0}:", sb)         ByRefStringBuilder(sb)         Console.WriteLine("after byref value of string builder {0}:", sb)          Console.WriteLine("Done...")         Console.ReadKey(True)     End Sub      Sub ByValInt(ByVal value As Integer)         value += 1     End Sub      Sub ByRefInt(ByRef value As Integer)         value += 1     End Sub      Sub ByValString(ByVal value As String)         value += " world!"     End Sub      Sub ByRefString(ByRef value As String)         value += " world!"     End Sub      Sub ByValStringBuilder(ByVal value As System.Text.StringBuilder)         value.Append(" world!")     End Sub      Sub ByRefStringBuilder(ByRef value As System.Text.StringBuilder)         value.Append(" world!")     End Sub  End Module