System.Linq中Union函数作用为将两个序列合并为一个不含重复数据序列，有两个重载函数，使用条件不相同

一、当IEnumerable<TSource>为简单类型（IEnumerable<string>、IEnumerable<int>等）

public static IEnumerable<TSource> Union<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second);

注意：参数first、second要处理成为不重复序列
示例：

IEnumerable<int> intList1= new List<int> { 1, 2, 3, 4, 5, 6 };
IEnumerable<int> intList2 = new List<int> { 5, 6, 7, 8, 9, 0 };
IEnumerable<int> resultList = intList1.Union(intList2).OrderBy(i => i);

输出结果：
0, 1, 2, 3, 4, 5, 6, 7, 8, 9

二、当IEnumerable<TSource>为复杂类型（IEnumerable<T>）

public static IEnumerable<TSource> Union<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer);

注意：参数first、second要处理成为不重复序列，参数comparer中IEqualityComparer为System.Collections.Generic中接口，接口是这样的

public interface IEqualityComparer<in T>
    {
        bool Equals(T x, T y);
        int GetHashCode(T obj);
    }

需要我们自己根据接口写实现类

 public class EntityComparer : IEqualityComparer<T>
    {
        public bool Equals(T a, T b)
        {
            if (Object.ReferenceEquals(a, b)) return true;
            if (Object.ReferenceEquals(a, null) || Object.ReferenceEquals(b, null))
                return false;

            return a.ID == b.ID && a.Name == b.Name && a.Grade == b.Grade;
        }

        public int GetHashCode(T a)
        {
            if (Object.ReferenceEquals(a, null)) return 0;
            int hashName = a.Name == null ? 0 : a.Name.GetHashCode();
            int hashCode = a.Grade.GetHashCode();

            return hashName ^ hashCode;
        }
    }

写的时候要将T 替换

示例：
定义一个User实体类

public class User
{
    public string ID { get; set; }
    public string Name { get; set; }
    public string Grade { get; set; }
}

定义一个接口实现类

public class EntityComparer : IEqualityComparer<User>
{
    public bool Equals(User a, User b)
    {
        if (Object.ReferenceEquals(a, b)) return true;
        if (Object.ReferenceEquals(a, null) || Object.ReferenceEquals(b, null))
            return false;

        return a.ID == b.ID && a.Name == b.Name && a.Grade == b.Grade;
    }

    public int GetHashCode(User a)
    {
        if (Object.ReferenceEquals(a, null)) return 0;
        int hashName = a.Name == null ? 0 : a.Name.GetHashCode();
        int hashCode = a.Grade.GetHashCode();

        return hashName ^ hashCode;
    }
}

IEnumerable<User> userList1= new List<User> { new User { ID = "0003", Name = "张三", Grade = "高三" }, new User { ID = "0002", Name = "王二", Grade = "高三" } };
IEnumerable<User> userList2= new List<User> { new User { ID = "0003", Name = "张三", Grade = "高三" }, new User { ID = "0004", Name = "李四", Grade = "高二" } };

IEqualityComparer<User> EC= new EntityComparer();

IEnumerable<User> resultList = userList1.Union(userList2, EC).OrderByDescending(u => u.ID);

输出结果：
ID = "0004", Name = "李四", Grade = "高二"
ID = "0003", Name = "张三", Grade = "高三"
ID = "0002", Name = "王二", Grade = "高三"

注意：
当需要将两个实体序列合并时，若实现第一个重载函数，得到序列会合并但会有重复值