使用LINQ比较2个对象列表以获取更新的属性

如何解决使用LINQ比较2个对象列表以获取更新的属性

我有2个相同类型的对象Student列表：

public class Student {
    public string StudentId { get; set; }
    public string Name { get; set; }
    public string Address { get; set; }
    public string PostCode { get; set; }
}

我有2个StudentData对象列表-去年列表和今年列表。我想查找学生姓名，地址或邮政编码已更改的数据。但是，我正在努力使用正确的LINQ查询。到目前为止，这是我尝试过的：

public DataChange<StudentData>[] Compare(StudentData[] previous,StudentData[] current) {
    Guard.AgainstNull(previous,nameof(previous));
    Guard.AgainstNull(current,nameof(current));

    var updatedName = previous
        .Where(d => current.Select(d2 => d2.StudentId )
        .Contains(d.StudentId )).Where(d => !current.Select(d2 => d2.Name)
        .Contains(d.Name)).Where(d => current.Select(d2 => d2.Address)
        .Contains(d.Address)).Select(DataChange<StudentData>.Updated).ToList();

    var updatedAddress = previous.Where(d => current
        .Select(d2 => d2.StudentId )
        .Contains(d.StudentId ))
        .Where(d => !current.Select(d2 => d2.Address)
        .Contains(d.Address))
        .Where(d => current.Select(d2 => d2.Name)
        .Contains(d.Name)).Select(DataChange<StudentData>.Updated).ToList();

    var updatedNameAndAddress = previous
        .Where(d => current.Select(d2 => d2.StudentId )
        .Contains(d.StudentId )).Where(d => !current.Select(d2 => d2.Address)
        .Contains(d.Address)).Where(d => !current.Select(d2 => d2.Name).Contains(d.Name))
        .Select(DataChange<StudentData>.Updated).ToList();
}

我这样做的原因是为了满足这些情况：

1. 名称已更新但地址未更新
2. 地址已更新但名称未更新

但是，这不能通过测试案例，即学生的姓名已从“ Fred Blogs”更新为“ Fred Blogger”，或将地址从“ 123 Address”更新为“ 123 Updated”。

有人可以指出正确的方向，以获取正确的LINQ查询/更流畅的方法吗？

解决方法

这样的事情怎么了？

public DataChange<StudentData>[] Compare(StudentData[] previous,StudentData[] current) {
    Guard.AgainstNull(previous,nameof(previous));
    Guard.AgainstNull(current,nameof(current));
    
    var updated = previous.Where(x => {
      var newOne = current.FirstOrDefault(n => n.StudentId == x.StudentId);
      if (newOne == null) {
        return true; // this is considered a change,not found in 'current',but practically it is a 'delete'
      }

     return x.Address != newOne.Address || x.Name != newOne.Name;
   }).Select(DataChange<StudentData>.Updated).ToList();
}

,

如果要查找姓名不同但地址不同的学生，则逻辑很简单：

1. 在StudentId上找到匹配项
2. 检查名称是否不同
3. 检查地址是否相同

可以重复此操作，以使地址不同但名称不相同，并针对不同的名称和不同的地址。在代码中，它将类似于：

List<StudentData> updatedNames = previous.Where(prev => current.Any(cur =>
    prev.StudentId == cur.StudentId &&     // Same student
    prev.Name != cur.Name &&               // Different name
    prev.Address == cur.Address))          // Same address
    .ToList();

List<StudentData> updatedaddresses = previous.Where(prev => current.Any(cur =>
    prev.StudentId == cur.StudentId &&     // Same student
    prev.Name == cur.Name &&               // Same name
    prev.Address != cur.Address))          // Different address
    .ToList();
        
List<StudentData> updatedNameAndAddresses = previous.Where(prev => current.Any(cur =>
    prev.StudentId == cur.StudentId &&     // Same student
    prev.Name != cur.Name &&               // Different name
    prev.Address != cur.Address))          // Different address
    .ToList();

,

使用GroupJoin：

return current.GroupJoin(
    previous,c => c.StudentId,p => p.StudentId,(c,ps) => {
        if (ps?.Any() != true)
            return DataChange<StudentData>.New();
        // otherwise,ps should contain single value
        // calculate the change in any appropriate way
        return DataChange<StudentData>.FromOldAndNew(ps.First(),c);
    }
)
// assuming null is returned for similar values
.Where(x => x is not null) // C# 9 feature,otherwise use !(x is null) or (x != null)
.ToArray();