reverting from derivative to base class

I've initialized an object '_currDie' in the Die class, ran an if statement to change the object to a derivative class object, then run a method.

The problem is that when it leaves the if statement, it seems to be reverting back to the base class.

namespace CharacterSheet
{
public partial class DiceRoller : Form
{        
     Die _currDie;
    public DiceRoller()
    {
        InitializeComponent();
    }

    private void chooseSides_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (chooseSides.SelectedItem.ToString() == "D4")
        {
            D4 _currDie = new D4();
        }

        if (chooseSides.SelectedItem.ToString() == "D6")
        {
            D6 _currDie = new D6();
        }

        if (chooseSides.SelectedItem.ToString() == "D8")
        {
            D8 _currDie = new D8();
        }

        if (chooseSides.SelectedItem.ToString() == "D10")
        {
            D10 _currDie = new D10();
        }

        if (chooseSides.SelectedItem.ToString() == "D20")
        {
            D20 _currDie = new D20();
        }

        if (chooseSides.SelectedItem.ToString() == "Percentile")
        {
            Percentile _currDie = new Percentile();
        }



    }


    private void Roll_Click(object sender, EventArgs e)
    {
        _currDie.Roll();
        string currResult = Convert.ToString(_currDie.RollResult);

        MessageBox.Show(currResult);

    }



}
}

Here is the base class

namespace CharacterSheet
{
public class Die
{

    public int Sides { get; set; }

    private Random rng = new Random();

    public int rollResult;

    public int RollResult
    {
        get
        {
            return rollResult;
        }
    }

    public virtual void Roll()
    {
        rollResult = rng.Next(Sides) + 1;

    }


}
}

And the derivative class I've been testing with

namespace CharacterSheet
{
public class D4:Die
{
  public D4 ()
    {
        Sides = 4;
    }

}
}

I set break points at the first if statement and as I step through I can clearly see that _currDie changes from a D4 object to a Die object on the _currDie.Roll(); At which point I get a System.NullReferenceException

I've tried instantiating _currDie without defining the class, but then the method gets errors because there is no Roll method for the object.