差集運算 -

差集運算指的是兩份清單中，不一樣的部分；依照A差集B，或是B差集A，會有不同的結果

忘了在哪邊看到的，大概就是【我有的你沒有】這樣的概念，朗朗上口挺好記

這邊可以看一下WIKI上面的圖可能會比較好理解一點

  	var list1 = new[] { 1, 2, 3, 4, 5 };
	var list2 = new[] { 4, 5, 6, 7, 8 };

	var array = list1.Except(list2).ToArray();
	var result = string.Join(",", array); // 1,2,3

但是如果今天要比較計算的是物件，就不能直接用Except來做了，需要告訴程式怎麼樣去比較物件，透過繼承IEqualityComparer來實作比較的方法，下面是LINQPAD的範例

void Main()
{
    var list1 = new List<User>
    {
        new User {Name = "AAA", Age = 20},
        new User {Name = "BBB", Age = 20},
        new User {Name = "CCC", Age = 20},
        new User {Name = "DDD", Age = 20},
        new User {Name = "EEE", Age = 20},
    };
    var list2 = new List<User>
    {
        new User {Name = "DDD", Age = 20},
        new User {Name = "EEE", Age = 20},
        new User {Name = "FFF", Age = 20},
    };

    var result = list1.Except(list2, new UserEqualityComparer()).ToList();
	result.Dump();
}

// Define other methods and classes here
class UserEqualityComparer : IEqualityComparer<User>
{
    public bool Equals(User x, User y)
    {
        return x.Name == y.Name && x.Age == y.Age;
    }

    public int GetHashCode(User obj)
    {
        var name = obj.Name == null ? 0 : obj.Name.GetHashCode();
        var age = obj.Age.GetHashCode();
        return name ^ age;
    }
}
class User
{
    public string Name { get; set; }
    public int Age { get; set; }
}

linq