The overloading mechanism should be used in place of optional parameters for several reasons:

• Optional parameter values are baked into the method call site code, thus, if a default value has been changed, all referencing assemblies need to be rebuilt, otherwise the original values will be used.
• The Common Language Specification (CLS) allows compilers to ignore default parameter values, and thus require the caller to explicitly specify the values. For example, if you want to consume a method with default argument from another .NET compatible language (for instance C++/CLI), you will have to provide all arguments. When using method overloads, you could achieve similar behavior as default arguments.
• Optional parameters prevent muddying the definition of the function contract. Here is a simple example: if there are two optional parameters, when one is defined, is the second one still optional or mandatory?

Noncompliant Code Example

Sub Notify(ByVal Company As String, Optional ByVal Office As String = "QJZ") ' Noncompliant

End Sub

Compliant Solution

Sub Notify(ByVal Company As String)
  Notify(Company, "QJZ")
End Sub

Sub Notify(ByVal Company As String, ByVal Office As String)

End Sub

Exceptions

The rule ignores non externally visible methods.