The dot operator is used for member access. For example, consider the following class:
class Simple { public int a; public void b() {...} } Simple s = new Simple();
The variable s has two members, a and b; to access them, use the dot operator:
s.a = 6; // set member variable a; s.b(); // invoke member function b;
The dot is also used to form qualified names, names that specify the namespace or interface (for example) to which they belong.
System.Console.WriteLine("hello"); // class Console in namespace System
The using directive makes some name qualification optional:
using System; ... System.Console.WriteLine("hello"); Console.WriteLine("hello"); // same thing
But when an identifier is ambiguous, it must be qualified:
using System; using OtherSystem; // a namespace containing another Console class ... System.Console.WriteLine( "hello" ); // must qualify Console
C# Operators | CLR 7.5.4 Member access