Question

I’m trying to create a generic IsEmpty extension but am having problems specifically with dictionaries:

The functions I have created:

<Extension()> Function IsEmpty(null As DBNull) As Boolean
  Return True
End Function

<Extension()> Function IsEmpty(d As Date) As Boolean
  Return d = New Date
End Function

<Extension()> Function IsEmpty(s As String) As Boolean
  Return s Is Nothing OrElse s.Length = 0
End Function

<Extension()> Function IsEmpty(a As Array) As Boolean
  Return a Is Nothing OrElse a.Length = 0
End Function

<Extension()> Function IsEmpty(collection As ICollection) As Boolean
  Return collection Is Nothing OrElse collection.Count = 0
End Function

<Extension()> Function IsEmpty(Of T)(val As T) As Boolean
  Return False
End Function

For my purposes, booleans and numerics are never “empty”.

The problem is that dictionaries are using the generic (Of T)(val As T) function instead of the (collection As ICollection) function.

i.e.

Dim dict As New Dictionary(Of String,String)
Response.Write(dict.IsEmpty)

output is “False”

I tried using (Of K,V)(collection As ICollection(Of KeyValuePair(Of K,V))) to no avail.

What am I doing wrong?

Answer 1

The problem is that your overload:

Function IsEmpty(Of T)(val As T) As Boolean

becomes more specific than:

Function IsEmpty(collection As ICollection) As Boolean

The compiler will choose the generic overload first, because it can specify T to be a Dictionary(of String, String) which is more specific than an ICollection. If you change the generic method to simply take an object, the ICollection overload will be more specific, and be selected by the compiler:

Function IsEmpty(val As Object) As Boolean