Can I override a property in c#? How?

You need to use virtual keyword

abstract class Base
{
  // use virtual keyword
  public virtual int x
  {
    get { throw new NotImplementedException(); }
  }
}

or define an abstract property:

abstract class Base
{
  // use abstract keyword
  public abstract int x { get; }
}

and use override keyword when in the child:

abstract class Derived : Base
{
  // use override keyword
  public override int x { get { ... } }
}

If you're NOT going to override, you can use new keyword on the method to hide the parent's definition.

abstract class Derived : Base
{
  // use new keyword
  public new int x { get { ... } }
}

Change property signature as shown below:

Base class

public virtual int x 
{ get { /* throw here*/ } }

Derived class

public override int x 
{ get { /*overriden logic*/ } }

If you do not need any implementation in Base class just use abstract property.

Base:

public abstract int x { get; }

Derived:

public override int x { ... }

I would suggest you using abstract property rather than trhowing NotImplemented exception in getter, abstact modifier will force all derived classes to implement this property so you'll end up with compile-time safe solution.

Make the base property abstract and override or use the new keyword in the derived class.

abstract class Base
{
  public abstract int x { get; }
}

class Derived : Base
{
  public override int x
  {
    get { //Actual Implementaion }
  }
}

Or

abstract class Base
{
  public int x { get; }
}

class Derived : Base
{
  public new int x
  {
    get { //Actual Implementaion }
  }
}