VariableTesting.cs

class MyContext : IContext
{
    public MyContext(double r)
    {
        _r = r;
    }

    double _r;

    public double ResolveVariable(string name)
    {
        switch (name)
        {
            case "pi": return Math.PI;
            case "r": return _r;
        }

        throw new InvalidDataException($"Unknown variable: '{name}'");
    }
}

[TestMethod]
public void Variables()
{
    var ctx = new MyContext(10);
    Assert.AreEqual(Parser.Parse("2 * pi * r").Eval(ctx), 2 * Math.PI * 10);
}