VB Tutorial | Subs vs. Functions

Problem: Evil Type Coercion

VB 6.0 automatically converts between various types of data. This was done for performance and convenience
reasons. However, it can cause some unexpected errors that occur at runtime, rather than at compile time. In the
following code, VB type coercion is a convenient feature. Without it, you would have to explicitly convert the
text value to an integer.

'VB 6.0 Code
Dim iNum As Integer
iNum = txtAge.Text

This type conversion “feature” can hide real programming errors until runtime. Consider the following calls to a
method named EvilTest():

'More VB 6.0 Code
Sub SomeMethod()
Dim sString As String, iNumber As Integer
sString = "1"
iNumber = 2

' The sub call accidentally reverses the parameters.
' Ok this time...
EvilTest sString, iNumber

' But you will get a runtime error here!
' A compile error would be preferred.
sString = "Not OK this time!"
EvilTest sString, iNumber
End Sub

Sub EvilTest(ByVal iNum As Integer, ByVal sName As String)
  'Do something...
End Sub

VB supports the Option Strict statement to force explicit type conversion. Any attempt to assign variables that
are not the same type causes a compiler error. This is true, even if the values can be easily converted. Consider
the following:

'VB Code
Option Strict On

Public Class TestClass
Sub SomeMethod()
  Dim aString As String
  Dim aNumber As Integer
  aString = "1"
  aNumber = 2

' Both of these calls result in a compile error.
  EvilTest(aString, aNumber)
  aString = "Some string value"
  EvilTest(aString, aNumber)
End Sub

Sub EvilTest(num As Integer, name As String)
  'Do something
End Sub
End Class

You can (and should) enable Option Strict on a project-wide level. To do so, double-click on the My Project
icon within the Solution Explorer of Visual Studio. Once you have done so, click on the Compile tab and enable
Option Strict.

You also can (and probably should) set up Visual Studio to enable Option Strict for all new VB projects.
       •        To do so, in Visual Studio, click Tools | Options…
       •        Expand Projects and Solutions and click VB Defaults.
       •        Set Option Strict to On.

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