= Interface =

An interface is a paradigm where an abstract class defines some set of abstract
functions that a class inheriting it can define. This is a core concept in C#.

In C# an interface can be declared with the `interface` keyword in its
declaration. However, there are several built in interfaces that are very
useful.

== Built in C# interfaces ==

=== System.IComparable ===

Something that wants to be IComparable must impliment
`public int CompareTo(object obj)`. This must return,
* Less than zero if the current instance preceds the object specified in sort
  order
* Zero if the current instance is in the same position in the sort order as the
  given object
* Greater than zero if the current instance follows the object given

All numeric types already have this, as do String Char and DateTime

=== System.IDisposable ===

Something that is IDisposable can be freed, it is very useful when working with
unmanaged resources, like database connections or filestreams. To impliment
this, a class must impliment `public void Dispose()`. This Dispose function
will be called if the object is used in a `using()` block statement for
example.

=== System.IEnumerable ===

Something that is IEnumerable can be used with an enumerator object. This
allows some object to work with LINQ queries. IEnumerable also requires
`IEnumerator<T>` to be defined. `IEnumerator<T>` also needs
[[#System.IDisposable]] to be implimented.

{{{
public class FooEnumerable : IEnumerable<string>
{
    private string _data;
    //must impliment
    public IEnumerator<string> GetEnumerator()
    {
        return new FooEnumerator(_data);
    }

    //must impliment
    private IEnumerator GetEnumerator1()
    {
        return this.GetEnumerator();
    }
    //now override the IEnumerator one
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator1();
    }
}

public class FooEnumerator : IEnumerator<string>
{
    //required
    public string Current
    {
        get
        {
            return "some val";
        }
    }
    
}
  }}}