How value from different class/form can be changed without ByRef - vb.net

Question

Let's assume following example below:
When I call Form2 from Form1 and pass _name of Form1 value. When I show Form1's _name = Alex? I didn't change pname in Form2 and constructor doesn't contains ByRef.
Example code:
Public Form1
Public _name as String
Sub New
_name = "John"
Dim bla as New Form2(_name)
'now _name=Alex !!
End Sub
End Class
Public Form2
Property _name2 as String
Sub New(pname as String) 'no ByVal !!
_name2 = pname 'even if would be ByVal no pname changed !
_name2 = "Alex"
End Sub
End Class
Why is that happening?

Answer 1

Use Shared in variable _name:
Class Form1
Public Shared _name As String
Public Sub New()
_name = "John"
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
' Message "John"
MessageBox.Show(_name)
Dim bla As New Form2()
bla.Show()
' Message "Alex"
MessageBox.Show(_name)
End Sub
End Class
Public Class Form2
Property _name2 As String
Sub New()
_name2 = "Alex"
Form1._name = _name2
End Sub
Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
' message Alex
MessageBox.Show(_name2)
' message Alex
MessageBox.Show(Form1._name)
End Sub
End Class
Or
When the modifier is not entered in the signature of a constructor (method), the default is byVal. If you want to change the value of the variable in form2 you must inform byref in the form2 constructor signature:
Sub New (byref _name as String)

Answer 2

To change the underlying value of an argument you must use the ByRef modifer:
Specifies that an argument is passed in such a way that the called procedure can change the value of a variable underlying the argument in the calling code.
This differs slightly from ByVal:
Specifies that an argument is passed in such a way that the called procedure or property cannot change the value of a variable underlying the argument in the calling code.
By not specifying a modifier in VB.NET the compiler by default will use ByVal.
It would be good to note here that although VB.NET uses ByVal by default if not specified, VBA does not and instead by default uses ByRef. Beware of this should you ever port code from one to the other.
Furthermore you are changing name2 when instead you should be changing pname if you want to change the underlying value.
Have a look at the following code based on your example:
Public Class Form1
Private _name As String
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
_name = "John"
Dim bla As New Form2(_name)
bla.Show()
Debug.WriteLine(_name)
End Sub
End Class
Public Class Form2
Private Property _name2 As String
Public Sub New(ByRef pname As String)
InitializeComponent()
_name2 = pname
pname = "Alex"
End Sub
End Class
Before passing the value to Form2 the name is "John":
At this point I am changing the value of pname to "Alex":
Note that although I have changed the value of pname To "Alex", _name2 is still set to "John".
Notice how the value of _name changes to "Alex" because of the change made to pname:
I'm not sure what it is you're trying to achieve here but hopefully this example gives you a better understanding. The alternative way would be to use a shared variable as explained in the other answer.