Why can I apply an indexer to an ICollection in VB.Net, but not in C#

Bizarrely enough, it looks like VB has special support for IEnumerable<T> and implicitly provides an indexer which actually calls Enumerable.ElementAtOrDefault. ICollection<T> extends IEnumerable<T>, so the same facility exists there. ICollection<T> doesn't provide a "real" indexer, hence the problem when you try using it from C#.

Sample program:

Option Strict On

Public Class Test
    Public Shared Sub Main(args As String())
      Dim x as System.Collections.Generic.ICollection(Of String) = args
      Console.WriteLine(x(0))
    End Sub
End Class

Generated IL for Main:

.method public static void  Main(string[] args) cil managed
{
  .entrypoint
  .custom instance void [mscorlib]System.STAThreadAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       15 (0xf)
  .maxstack  2
  .locals init 
      (class [mscorlib]System.Collections.Generic.IEnumerable`1<string> V_0)
  IL_0000:  ldarg.0
  IL_0001:  stloc.0
  IL_0002:  ldloc.0
  IL_0003:  ldc.i4.0
  IL_0004:  call       !!0
     [System.Core]System.Linq.Enumerable::ElementAtOrDefault<string>(
        class [mscorlib]System.Collections.Generic.IEnumerable`1<!!0>,
        int32)
  IL_0009:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_000e:  ret
} // end of method Test::Main

I find it very odd that VB provides this implicitly - it's really dangerous to make it look like it's fine to index into a collection which doesn't necessarily supply an efficient indexing operation.

Of course, you can call ElementAtOrDefault yourself, if you're happy with what that does.

Strictly viewed, ICollection<T> is an interface to an unordered collection of elements (more precisely, a collection whose elements cannot individually be accessed by their index). That is just by definition.

But you can still use LINQ's ElementAt(int index) extension method. That would just iterate through all elements index times every time you call it (so it is generally slower).

NOTE: ICollection<T> is not to be confused with Collection<T>. The latter implements IList<T> (among other things), which by definition does specify that each element can be accessed by its index.