如何在C# CompareTo方法中妥善处理NULL属性？

我需要比较两个AdminEngineTableRowModel类的实例是否相等，编写的CompareTo方法存在缺陷：当NXSTDT等属性为NULL时，调用CompareTo会引发错误。我尝试过通过构造函数初始化所有属性，或在方法中逐个判断属性非空，但认为这类写法不够优雅，请问是否有更优的实现方式？

存在问题的代码：

public int CompareTo(AdminEngineTableRowModel other)
{
    int result;
    if (other != null)
    {
        result = NXSTDT.CompareTo(other.NXSTDT);
        if (result == 0)
            result = DALOC.CompareTo(other.DALOC);
        if (result == 0)
            result = DealerID.CompareTo(other.DealerID);
        if (result == 0)
            result = Status.CompareTo(other.Status);
        if (result == 0)
            result = OrderNumber.CompareTo(other.OrderNumber);
        if (result == 0)
            result = Dealership.CompareTo(other.Dealership);
        if (result == 0)
            result = Serial.CompareTo(other.Serial);
        if (result == 0)
            result = Model.CompareTo(other.Model);
        if (result == 0)
            result = OrderDate.CompareTo(other.OrderDate);
        if (result == 0)
            result = StartDate.CompareTo(other.StartDate);
        if (result == 0)
            result = Confirmed.CompareTo(other.Confirmed);
        if (result == 0)
            result = Deposit.CompareTo(other.Deposit);
        if (result == 0)
            result = Engine.CompareTo(other.Engine);
        if (result == 0)
            result = Color.CompareTo(other.Color);
        if (result == 0)
            result = Name.CompareTo(other.Name);
    }
    else
    {
        result = 1;
    }
    return result;
}

问题出在属性为NULL时，例如：

result = NULL.CompareTo(other.NXSTDT); // 这会引发错误

我曾尝试的改进写法：

public int CompareTo(AdminEngineTableRowModel other)
{
    int result;
    if (other != null)
    {
        if ((NXSTDT == null) && (other.NXSTDT != null))
            result = 1;
        if (result == 0)
            result = NXSTDT.CompareTo(other.NXSTDT);
        if ((DALOC == null) && (other.DALOC != null))
            result = 1;
        if (result == 0)
            result = DALOC.CompareTo(other.DALOC);
        // {snip}
    }
    else
    {
        result = 1;
    }
    return result;
}

但我认为这种写法不够规范，希望找到更好的解决办法。

方法1：使用Comparer<T>.Default处理null比较 #

.NET内置的Comparer<T>.Default已经原生处理了null的比较逻辑：null会被视为小于任何非null值，两个null相等。直接用它替代手动调用CompareTo，代码会简洁很多，且无需额外的null判断。

完整实现（替换[属性类型]为对应属性的实际类型，比如字符串用string、可空日期用DateTime?）：

public int CompareTo(AdminEngineTableRowModel other)
{
    if (other == null)
        return 1;

    int result = Comparer<string>.Default.Compare(NXSTDT, other.NXSTDT);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(DALOC, other.DALOC);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(DealerID, other.DealerID);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Status, other.Status);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(OrderNumber, other.OrderNumber);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Dealership, other.Dealership);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Serial, other.Serial);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Model, other.Model);
    if (result != 0) return result;

    result = Comparer<DateTime?>.Default.Compare(OrderDate, other.OrderDate);
    if (result != 0) return result;

    result = Comparer<DateTime?>.Default.Compare(StartDate, other.StartDate);
    if (result != 0) return result;

    result = Comparer<bool?>.Default.Compare(Confirmed, other.Confirmed);
    if (result != 0) return result;

    result = Comparer<decimal?>.Default.Compare(Deposit, other.Deposit);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Engine, other.Engine);
    if (result != 0) return result;

    result = Comparer<string>.Default.Compare(Color, other.Color);
    if (result != 0) return result;

    return Comparer<string>.Default.Compare(Name, other.Name);
}

方法2：封装通用null安全比较方法 #

如果需要自定义null的比较规则（比如把null视为大于非null值），可以封装一个通用方法复用，避免重复代码：

private int NullSafeCompare<T>(T a, T b) where T : IComparable<T>
{
    if (a == null && b == null)
        return 0;
    if (a == null)
        return 1; // 自定义规则：null大于非null，若要相反则返回-1
    return a.CompareTo(b);
}

然后在CompareTo中调用：

public int CompareTo(AdminEngineTableRowModel other)
{
    if (other == null)
        return 1;

    int result = NullSafeCompare(NXSTDT, other.NXSTDT);
    if (result != 0) return result;

    result = NullSafeCompare(DALOC, other.DALOC);
    if (result != 0) return result;

    // 后续属性依次类推...

    return NullSafeCompare(Name, other.Name);
}

方法3：LINQ链式比较（C# 6+） #

利用LINQ的FirstOrDefault方法，可以将所有属性的比较结果放入数组，直接取第一个非0的结果作为最终返回值，代码紧凑且逻辑清晰：

public int CompareTo(AdminEngineTableRowModel other)
{
    if (other == null)
        return 1;

    return new[]
    {
        Comparer<string>.Default.Compare(NXSTDT, other.NXSTDT),
        Comparer<string>.Default.Compare(DALOC, other.DALOC),
        Comparer<string>.Default.Compare(DealerID, other.DealerID),
        Comparer<string>.Default.Compare(Status, other.Status),
        Comparer<string>.Default.Compare(OrderNumber, other.OrderNumber),
        Comparer<string>.Default.Compare(Dealership, other.Dealership),
        Comparer<string>.Default.Compare(Serial, other.Serial),
        Comparer<string>.Default.Compare(Model, other.Model),
        Comparer<DateTime?>.Default.Compare(OrderDate, other.OrderDate),
        Comparer<DateTime?>.Default.Compare(StartDate, other.StartDate),
        Comparer<bool?>.Default.Compare(Confirmed, other.Confirmed),
        Comparer<decimal?>.Default.Compare(Deposit, other.Deposit),
        Comparer<string>.Default.Compare(Engine, other.Engine),
        Comparer<string>.Default.Compare(Color, other.Color),
        Comparer<string>.Default.Compare(Name, other.Name)
    }.FirstOrDefault(r => r != 0);
}

内容的提问来源于stack exchange，提问作者user153923