Issue with ByVal and Arrays in Functions (VB.NET)-CodePudding

I've ran into an issue with my code for the last week or so, and its been killing me trying to figure out what's wrong with it. I've extracted and isolated the issue from my main project, but the issue still isn't apparent.

Essentially, I have a function that usually does a lot of stuff, but in this example just changes 1 element in an array called FalseTable. Now, I have set this variable to be ByVal, meaning the original variable (ie: TrueTable) shouldn't change, however, it does! Here is the full code:

Dim TrueTable(7) As Char
    Sub Main()
        Dim FalseTable(7) As Char
        For x = 0 To 7
            TrueTable(x) = "T"
        Next
        For x = 0 To 7
            FalseTable(x) = "F"
        Next

        Console.WriteLine("before")
        For x = 0 To 7
            Console.Write(TrueTable(x))
        Next
        Console.WriteLine()
        Test(TrueTable)
        Console.WriteLine("result")
        For x = 0 To 7
            Console.Write(TrueTable(x))
        Next
        Console.WriteLine()

        Console.ReadLine()
    End Sub

    Function Test(ByVal FalseTable() As Char) As Char()
        FalseTable(0) = "0"
        Return FalseTable
    End Function

Now, I used to think that it was the repetition of the name "FalseTable" in the function, however even if I change the function to:

Function Test(ByVal SomeTable() As Char) As Char()
        SomeTable(0) = "0"
        Return SomeTable
    End Function

And not modify the rest, the issue still persists - for some reason, TrueTable is being updated when it shouldn't due to the ByVal status.

Any help with this would be greatly appreciated; it's probably something stupid that I've overlooked, but it's pulling my hair out!!

Many thanks, Alfie :)

CodePudding user response：

To understand why that happens just imagine this scenario.

You have a regular TextBox1 (this will be your TrueTable), now you want to pass the object TextBox1 to a function, something like this:

Function Test(ByVal TextBoxAnything as TextBox) As String
    TextBoxAnything.Text = "0"
    Return ""
End Function

Do you understand that you're passing thru TextBox1 and once inside the function Test the object TextBoxAnything is just TextBox1, anything you do to TextBoxAnything you're just doing it to TextBox1. TextBoxAnything doesn't exist, it just points to Textbox1. That's why the value of your TrueTable is also changed.

CodePudding user response：

If you don't want to change the TrueTable, define another Array and copy TrueTable to it.

Here's the code you can refer to.

Dim TrueTable(7) As Char
Dim TrueTable2(7) As Char
Sub Main()
    For x = 0 To 7
        TrueTable(x) = "T"c
    Next

    Console.WriteLine("before")
    For x = 0 To 7
        Console.Write(TrueTable(x))
    Next
    Console.WriteLine()

    TrueTable.CopyTo(TrueTable2, 0)
    Test(TrueTable2)
    Console.WriteLine("result")
    For x = 0 To 7
        Console.Write(TrueTable(x))
    Next
    Console.WriteLine()

    Console.ReadLine()
End Sub

Result: