title: xUnit Testing says System.NullReferenceException
body: <p>Good day,</p>
  
  <p>I'm begginer in xUnit and I'm facing this error that says, "<code>System.NullReferenceException : Object reference not set to an instance of an object.</code>"</p>
  
  <p>Here's my code where it tests whether a rulename depending on the season.</p>
  
  <pre><code>public class UnitTest1
  {
      private CalculateDiscount calculateDiscount;
  
      [Fact]
      public void Setup()
      {
          calculateDiscount = new CalculateDiscount();
      }
  
      [Theory]
      [InlineData(10.00, "Summer", 100.00)]
      public void WhenRuleName_Is_Summer_Return_Valid(decimal amount, string ruleName, decimal expected)
      {
          var result = calculateDiscount.CalculateDiscountSalary(amount, ruleName);
  
          Assert.Equal(expected, result);
      }
  }
  </code></pre>
  
  <p>And here is my actual implementation</p>
  
  <pre><code>public class CalculateDiscount
  {
      public decimal CalculateDiscountSalary(decimal amount, string ruleName)
      {
          if (ruleName.Equals("Summer"))
          {
              return amount * 10.00M;
          }
          else if (ruleName.Equals("Winter"))
          {
              return amount * 15.00M;
          }
          else
          {
              return amount * 20.00M;
          }
      }
  }
  </code></pre>
  
  <p>Thank you in advance.</p>