Argument passed ByVal to VB.NET Function and manipulated there

Question

In this Microsoft sample an array Of Doubles is passed to the functions MultiplyMatricesSequential(...) and MultiplyMatricesParallel(...) as argument r

Answer 1

When you pass an object ByVal to a function, you put a pointer to it on the stack. Then function can then modify the inner parts of the object, but not replace it with a new object.

When you pass an object ByRef, you instead put a pointer to the objects pointer on the stack. The function can now replace the entire object with a new one.

If you send an intrinsic value, like an Int32, to a function ByVal the value is put on the stack and can't be edited at all by the function.

Answer 2

The distinction is between "value types" and "reference types". Value types are defined as "Structure" (VB.NET) or "Struct" (C#) whereas reference types are defined as "Class". Primitive types such as integer, double and boolean are value types. Arrays are reference types. As @Mattias Åslund pointed out, whether passed ByVal or ByRef, with reference types you are always passing a pointer.

If you did want to manipulate the passed array but return the original array to the calling procedure, you would need to make a copy into a new locally declared array. Beware of the overhead, however, if passing very large arrays.

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim myOriginalArray As String() = New String() {"Hello", "to", "you", "Michael"}
    ManipulateArray(myOriginalArray)
    MessageBox.Show("myOriginalArray = " & myOriginalArray(0) & " " & myOriginalArray(1) & " " & myOriginalArray(2) & " " & myOriginalArray(3))
End Sub

Private Sub ManipulateArray(ByVal myOriginalArray As String())
    Dim myCopyArray(myOriginalArray.GetUpperBound(0)) As String
    myOriginalArray.CopyTo(myCopyArray, 0)
    myCopyArray(3) = "Sarah"
    MessageBox.Show("myCopyArray = " & myCopyArray(0) & " " & myCopyArray(1) & " " & myCopyArray(2) & " " & myCopyArray(3))
End Sub