C# Language Specification

Base class

When a partial class declaration includes a base class specification it must agree with all other parts that include a base class specification. If no part of a partial class includes a base class specification, the base class becomes System.Object (§10.1.4.1).

5. Base interfaces

The set of base interfaces for a type declared in multiple parts is the union of the base interfaces specified on each part. A particular base interface may only be named once on each part, but it is permitted for multiple parts to name the same base interface(s). There must only be one implementation of the members of any given base interface.

In the example

partial class C: IA, IB {...}

partial class C: IC {...}

partial class C: IA, IB {...}

the set of base interfaces for class C is IA, IB, and IC.

Typically, each part provides an implementation of the interface(s) declared on that part; however, this is not a requirement. A part may provide the implementation for an interface declared on a different part:

partial class X

{
int IComparable.CompareTo(object o) {...}
}

partial class X: IComparable

{
...
}

6.