Information about Interfaces in C# - what they can have and what not

This is taken directly from C# reference document from Microsoft (I recommend reading it)

• An interface declaration may declare zero or more members. 
• The members of an interface must be methods, properties, events, or indexers. 
• An interface cannot contain constants, fields, operators, instance constructors, destructors, or types, nor can an interface contain static members of any kind.

All interface members implicitly have public access. It is a compile-time error for interface member declarations to include any modifiers. In particular, interfaces members cannot be declared with the modifiers abstract, public, protected, internal, private, virtual, override, or static.