Subclasses can reimplement interfaces to effectively override non-virtual methods

ReimplementInterface.cs

void Main()
{
	BaseClass                 b = new BaseClass();
	DerivedClass              d = new DerivedClass();
	ReimplementedDerivedClass r = new ReimplementedDerivedClass();
	
	//
	
	b.GetX().Dump( b.GetType().Name ); // "1"
	
	d.GetX().Dump( d.GetType().Name ); // "2"
	
	r.GetX().Dump( r.GetType().Name ); // "2"
	
	//
	
	IFoo bFoo = b;
	bFoo.GetX().Dump( b.GetType().Name + " as IFoo" ); // "1"

	IFoo d1Foo = d;
	d1Foo.GetX().Dump( d.GetType().Name + " as IFoo"); // "1"

	IFoo d2Foo = r;
	d2Foo.GetX().Dump(r.GetType().Name + " as IFoo"); // "2" <-- okay, THIS output surprised me! 

	//
	
	BaseClass dAsBase = d; // I assumed that casting to a baseclass would "reset" the vtable pointer...
	BaseClass rAsBase = r;
	
	dAsBase.GetX().Dump( dAsBase.GetType().Name ); // "1" // non-virtual call
	
	rAsBase.GetX().Dump( rAsBase.GetType().Name ); // "1" // non-virtual call
	
	IFoo dAsBaseFoo = dAsBase;
	dAsBaseFoo.GetX().Dump( dAsBaseFoo.GetType().Name + " as IFoo" ); // "1" // DerivedClass's IFoo.GetX() is in BaseClass

	IFoo rAsBaseFoo = rAsBase;
	rAsBaseFoo.GetX().Dump(rAsBaseFoo.GetType().Name + " as IFoo"); // "2" // ReimplementedDerivedClass's IFoo.GetX() is in ReimplementedDerivedClass.
}

interface IFoo {
	
	Int32 GetX();
}

class BaseClass : IFoo {
	
	public Int32 GetX() => 1;
}

class DerivedClass : BaseClass
{
	public new Int32 GetX() => 2;
}

class ReimplementedDerivedClass : BaseClass, IFoo // <-- reimplements IFoo, even though `GetX` isn't virtual and is actually `new`.
{
	public new Int32 GetX() => 2;
}