xUnit测试系统。NullReferenceException

早上好,

我是xUnit的begginer,我面临的错误是,“ System.NullReferenceException : Object reference not set to an instance of an object. “”

这是我的代码,它测试规则名是否取决于季节。

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);
    }
}

这是我的实际实现

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;
        }
    }
}

提前谢谢你。