Question

I am updating a legacy application, and it was reading a dll from another project for a Dictionary(of Guid, String) of items and using them.

The requirements have changed, and the method that was returning the Dictionary is now returning an IList.

This is the odd behavior of this; the intellisense is not throwing a cast error, nor is the compiler. It does not throw an error until runtime when it tries to set the Dictionary to the IList.

Example:

        Dim someDictionary As Dictionary(Of Integer, String) = New Dictionary(Of Integer, String)
        Dim someList As IList(Of Integer)
        someDictionary = someList

Any idea as to why the compiler is not catching this?

Answer 1

It gives an error when "Option Strict" is On:

Option Strict On

Imports System.Collections.Generic

Public Class Test
    Public Shared Sub Main()
        Dim someDictionary As Dictionary(Of Integer, String) = _
               New Dictionary(Of Integer, String)
        Dim someList As IList(Of Integer) = Nothing
        someDictionary = someList
    End Sub
End Class

Error:

error BC30512: Option Strict On disallows implicit conversions from ‘System.Collections.Generic.IList(Of Integer)’ to ‘System.Collections.Generic.Dictionary(Of Integer, String)’.

   someDictionary = someList

I suggest you change your project to enable Option Strict, to help catch this sort of thing 🙂