I see language features like C#'s foreach loop (which requires the use of IEnumerable), and its using syntax (which uses IDisposable, knowing which method to call), and LINQ (which I assume uses a heap of .NET stuff too). All of this seems very tied-in to the .NET framework. So:
The best place to investigate this is the C# language specification. For example, in the case of enumerables, here are some snippets from the spec:
10.14.2 Enumerable interfaces
The enumerable interfaces are the non-generic interface System.Collections.IEnumerable and all instantiations of the generic interface System.Collections.Generic.IEnumerable. For the sake of brevity, in this chapter these interfaces are referenced as IEnumerable and IEnumerable, respectively.
...
10.14.5.1 The GetEnumerator method
An enumerable object provides an implementation of the GetEnumerator methods of the IEnumerable and IEnumerable interfaces. The two GetEnumerator methods share a common implementation that acquires and returns an available enumerator object. The enumerator object is initialized with the argument values and instance value saved when the enumerable object was initialized, but otherwise the enumerator object functions as described in §10.14.4.
This ties in nicely with how these interfaces are actually defined in the .NET BCL, although it would be completely possible for a different (i.e. non-.NET) framework to provide a similar, compatible implementation that a C# compiler could then target.
Portions of the spec that are hard to separate from the .NET framework are normally marked with:
Note: This section is applicable only to the Microsoft .NET implementation of C#.
On another note,foreachdoesn't actually requireIEnumerable; it can use a sort of 'duck-typing', which you can read about in section 8.8.4 of the spec.