Question

Calling Test1 in Excel gives 0 for any real A and B. Why does this occur?

Public Function Min(X As Double, y As Double, Optional y2 As Double, Optional y3 As Double) As Double
    Min = Application.WorksheetFunction.Min(X, y, y2, y3)
End Function


Function Test1(A As Double, B As Double)
    Test1 = Min(A, B)
End Function

In Excel: =Test1(5,2).

Answer 1

Optional parameters use their default value if not passed into the function.
For doubles the default value is 0. if you do not provide y2 or y3 essentially, you are testing Application.WorksheetFunction.Min(A,B,0,0).

If you want to change the default value, you can do something like this:

Public Function Min(X As Double, y As Double, Optional y2 As Double = 10000000, Optional y3 As Double = 10000000) As Double
    Min = Application.WorksheetFunction.Min(X, y, y2, y3)
End Function

which makes y2 and y3 default to 10000000 which would be your new built in artificial min.