Update for 22-02-22 15:30

tech/C-sharp.wiki

@@ -104,7 +104,7 @@ declare a class as `abstract`.
 
 To override a class add the modifier `override`.
 
-== Interfaces ==
+== [[interface]] ==
 
 An interface is a fully abstract class that can be inherited by other classes.
 It provides some set functions to be implimented by the child class.
@@ -127,6 +127,8 @@ It provides some set functions to be implimented by the child class.
     }
   }}}
 
+See [[interface]]
+
 == Examples ==
 
 === Hello World ===

tech/interface.wiki

@@ -0,0 +1,37 @@
+= 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.