Default Interface Implementation C# 8

defaultInterface.cs

interface ICalculator
{
    int Add(int a, int b) => a + b;
}

interface ISuperCalculator
{
    int Add(int a, int b) => a * 2 + b * 2;
}

public class Test : ICalculator, ISuperCalculator
{

}

var t = new Test();
if(t is ICalculator calc)
{
    var a = calc.Add(1, 2);
    Assert.Equal(3, a);
}
if (t is ISuperCalculator calc1)
{
    var a = calc1.Add(1, 2);
    Assert.Equal(6, a);
}