How does VB.NET compiler choose which extension overload to run?

Question

Got an interesting oddity - thought someone might be able to help.

This came out of some fun with nullable types from this question:

How to check if an obje

Answer 1

Overload 2 will ONLY work as an extension on explicitly defined Nullable(of T)'s. For example:

    Dim y As New Nullable(Of Integer)
    y.IsNullable()

This is because extension methods extend the type (or a base type), which in this case is Nullable(of T). Calling a.IsNullable() will never call overload 2. That's the easy part to figure out. This means the real question is why would overload 2 be called instead of overload 1 as a standard overloaded method call.

The CLR will determine which Overload to use by performing a "Better Conversion" check, where it implicitly converts the value(s) passed in to the type of the parameter(s) defined in the overloaded methods and then go down a checklist of rules to determine the best method to use.

From the MSDN Better Conversion Article:

If S is T1, C1 is the better conversion.

If S is T2, C2 is the better conversion.

Puting this code into Visual Studio will shows you that Overload 2 is the better conversion because the integer a (S) is the implicitly converted Nullable(of Integer) version of a (T2).

    ' a is an integer! 
    Dim a As Integer = 123

    Dim objValueType As ValueType = 123 'Or CType(a, ValueType)
    Dim objNullable As Nullable(Of Integer) = 123 'Or CType(a, Nullable(Of Integer))

    'Oh No, a compiler error for implicit conversion done for overload 1!
    Dim bolValueTypeConversionIsBetter As Boolean = (objValueType = a)

    'No error as long as Option Strict is off and it will equal True.
    Dim bolNullableConversionIsBetter As Boolean = (objNullable = a)