CSharp - Implementing Interface Members Virtually

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Introduction

An implicitly implemented interface member is sealed by default.

It must be marked virtual or abstract in the base class to be overridden.

For example: 

interface Printable { void Print(); }

class PDFDocument : Printable
{
       public virtual void Print() => Console.WriteLine ("PDFDocument.Print");
}

class RichPDFDocument : PDFDocument
{
       public override void Print() => Console.WriteLine ("RichPDFDocument.Print");
}

Calling the interface member through either the base class or the interface calls the subclass's implementation: 

RichPDFDocument r = new RichPDFDocument();
r.Print();                          // RichPDFDocument.Print
((Printable)r).Print();             // RichPDFDocument.Print
((PDFDocument)r).Print();               // RichPDFDocument.Print

An explicitly implemented interface member cannot be marked virtual.

And it cannot be overridden. But it can be reimplemented.