FunctionContext.cs

class MyFunctionContext : IContext
{
    public MyFunctionContext()
    {
    }

    public double ResolveVariable(string name)
    {
        throw new InvalidDataException($"Unknown variable: '{name}'");
    }

    public double CallFunction(string name, double[] arguments)
    {
        if (name == "rectArea")
        {
            return arguments[0] * arguments[1];
        }

        if (name == "rectPerimeter")
        {
            return (arguments[0] + arguments[1]) * 2;
        }

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

[TestMethod]
public void Functions()
{
    var ctx = new MyFunctionContext();
    Assert.AreEqual(Parser.Parse("rectArea(10,20)").Eval(ctx), 200);
    Assert.AreEqual(Parser.Parse("rectPerimeter(10,20)").Eval(ctx), 60);
}