vb.netdirectcastctype

Directcast & Ctype differences with enums

 Public Enum Fruit
    Red_Apple = 1
    Oranges
    Ripe_Banana
End Enum
Private Sub InitCombosRegular()
    Dim d1 As New Dictionary(Of Int16, String)
    For Each e In [Enum].GetValues(GetType(Fruit))
        d1.Add(CShort(e), Replace(e.ToString, "_", " "))
    Next
    ComboBox1.DataSource = d1.ToList
    ComboBox1.DisplayMember = "Value"
    ComboBox1.ValueMember = "Key"
    ComboBox1.SelectedIndex = 0
End Sub

   'This fails
        Dim combo1 = DirectCast(ComboBox1.SelectedValue, Fruit) ' Fails
        'these both work
        Dim combo2 = DirectCast(CInt(ComboBox1.SelectedValue), Fruit) 'works
        Dim combo3 = CType(ComboBox1.SelectedValue, Fruit) 'works

Why does the CType work and the DirectCast does not with the same syntax? Yet if I cast the selectedValue to an int before I DirectCast, then it works

Regards

_Eric

Solution

  • The reason why is because CType and DirectCast are fundamentally different operations.

    DirectCast is a casting mechanism in VB.Net which allows for only CLR defined conversions. It is even more restrictive than the C# version of casting because it doesn't consider user defined conversions.

    CType is a lexical casting mechanism. It considers CLR rules, user defined conversions and VB.Net defined conversions. In short it will do anything and everything possible to create a valid conversion for an object to a specified type.

    In this particular case you are trying to convert a value to an Enum which does not have a CLR defined conversion and hence it's failing. The VB.Net runtime however was able to find a lexical conversion to satisfy the problem.

    A decent discussion on the differences exists here: