using System;
using System.Collections.Generic;
using System.Text;

namespace FizzBuzz
{
    public class FizzBuzzLogic
    {
        public  string IsDevBy(int liczba)
        {

            if (liczba <= 0)
            {
                throw new ArgumentException("Liczba mniejsza od 0");
            }

            string result = "";
            if (liczba % 3 == 0)
            {
                result += "Fizz";
            }
            if (liczba % 5 == 0)
            {
                result += "Buzz";
            }
            else if (result == "" && liczba >= 0)
            {
                return liczba.ToString();
            }

            return result;
        }

    }
}


----------


using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;

namespace FizzBuzzTest
{
    [TestClass]
    public class UnitTest1
    {

        [TestMethod]
        public void IsDevBy3_ReturnTrue() {
            var devided = new FizzBuzz.FizzBuzzLogic();
            var result = devided.IsDevBy(6);
            Assert.IsTrue(result=="Fizz");
        }

        [TestMethod]
        public void IsDevBy5_ReturnTrue()
        {
            var devided = new FizzBuzz.FizzBuzzLogic();
            var result = devided.IsDevBy(10);
            Assert.IsTrue(result == "Buzz");
        }

        [TestMethod]
        public void IsDevBy5And3_ReturnTrue()
        {
            var devided = new FizzBuzz.FizzBuzzLogic();
            var result = devided.IsDevBy(15);
            Assert.AreEqual("FizzBuzz", result);
        }

        [TestMethod]
        public void IsNotDevBy3And5_ReturnTrue()
        {
            var devided = new FizzBuzz.FizzBuzzLogic();
            var result = devided.IsDevBy(11);
            Assert.AreEqual("11", result);
        }

        [TestMethod]
        [ExpectedException(typeof(ArgumentException))]
        public void IsLessThanZero_ReturnTrue()
        {
            var devided = new FizzBuzz.FizzBuzzLogic();
            var result = devided.IsDevBy(-10);
        }



    }
}