如何在 C# 中使用 compareTo 对空对象进行排序

Question

我有一个名为 UserView 的 class，我实现了他的 compareTo() 方法。

我有那个限制："If a UserView is compared with a null UserView, the null go before the other"

我有 compareTo():

 public int CompareTo(UserView other)
    {
        if (this == null && other != null) return -1;
        else if (this == null && other == null) return 0;
        else return Id.CompareTo(other.Id);
    }

而这个测试：

 [TestMethod]
    public void TestCompareWithNull()
    {
        UserView uv = new UserView(1, "pepe", "1234", "alumno", true);
        UserView uv2 = null;
        UserView uv3 = null;
        Assert.AreEqual(uv2.CompareTo(uv3), 0);
        Assert.AreEqual(uv2.CompareTo(uv), -1);
        Assert.AreEqual(uv.CompareTo(uv3), 1);
    }

当我从 uv2 调用 compareTo 时，它是空的，我有 NullReferenceException 所以...我怎样才能满足给定的限制？

Answer 1

如果你想比较 arbitraty 实例（包括两个 null 案例）你必须实现一个 static 方法，因为

  this == null

是任何成员函数中的不可能条件。因此，异常的直接原因是在 null 实例上调用成员函数：

UserView uv2 = null;
// Since uv2 is null, Null Exception will be thrown
uv2.CompareTo(uv3);

出路是static方法：

public int CompareTo(UserView other) 
{
    return UserView.Compare(this, other);   
}

// please, note "static" 
public static int Compare(UserView left, UserView right)
{
    if (left == null)
      if (right == null) 
        return 0;
      else 
        return -1;
    else if (right == null) 
      return 1; 
    else // Providing that Id can't be null (e.g. it's int)
      return left.Id.CompareTo(right.Id); 
}

....

UserView uv2 = null;
UserView uv3 = null;

// uv2.CompareTo(uv3) will throw Null Exception and
// ... static UserView.Compare will do
Assert.AreEqual(UserView.Compare(uv2, uv3), -1);

如何在 C# 中使用 compareTo 对空对象进行排序

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