Answer.

In Visual Basic, there are two ways to pass arguments to procedures: ByVal (by value, a copy) and ByRef (by reference, the actual object or variable). If the parameter is marked as ByRef, any changes made inside the procedure will reflect on the original variable outside of the procedure.

Using ByRef is particularly relevant when you need to return multiple results from a function or optimize the handling of large data structures (without copying them).

Example:

Sub Swap(ByRef a As Integer, ByRef b As Integer)
    Dim temp As Integer = a
    a = b
    b = temp
End Sub

Dim x As Integer = 10
Dim y As Integer = 20
Swap(x, y)
' x = 20, y = 10

Trick Question.

Question: "Is it safe to pass simple data types (Integer, String) by reference between different threads?"

Answer: No! When passing variables by reference (ByRef) between threads, data races can occur, as both procedures can change the variable at different times. This is unsafe and can lead to elusive bugs.

Example:

' In a multithreaded mode, it's possible that the values
' a and b will be incorrectly overwritten due to simultaneous access!

Examples of real errors due to ignorance of the nuances of the topic.

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Story:

In an old financial product, ByRef was used to pass counters between several modules. One of the developers accidentally passed a temporary expression by reference to a function instead of a variable. This caused the program to crash because a non-existent reference attempted to modify an uninitialized object.

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Story:

An engineering calculation system passed arrays by reference to save memory. However, someone modified the array inside a helper procedure. This led to elusive bugs – data was changed outside of the expected context.

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Story:

In a threaded data analyzer, there was an attempt to "optimize" the transmission of strings by reference for increased efficiency. Due to unsafe transmission of strings by reference simultaneously from multiple threads, unpredictable failures occurred and the synchronization mechanism broke down.