实体框架 - 按降序排序，然后对该结果执行 Distinct 返回错误的结果

Question

I currently have a class that looks like this

class Student
{
    [Key]
    public int StudentLogId { get; set; }
    public string Guid { get; set; }
    public string info { get; set; }
}

Now what I would like to do is to sort the result by descending based on the Id (last ones on the top) and then from those results get the distinct values (in short the sort order is important). I noticed that if I do this

//Order the students in descending order based on their id

List<string> lst = StudentLogHistoryModel.OrderByDescending(s => s.StudentLogId).Select(u => u.Guid).ToList<string>();

The above returns the correct result in the right order now I would like to only get the unique values so I tried adding to the above query and came up with this

List<string> lst = StudentLogHistoryModel.OrderByDescending(s => s.StudentLogId).Select(s => s.Guid).Distinct().ToList<string>();

However I noticed that in the second one the order got messed up. Any suggestions on why the order does not get preserved? How can I fix this?

What I mean by order getting messed up was suppose the first result i got this [A,A,B,B,D] I was expecting to get [A,B,D] after the second query. However I get something like [D,A,B]

Answer 1

As per the docs :

The query behavior that occurs as a result of executing an expression tree that represents calling Distinct(IQueryable) depends on the implementation of the type of the source parameter. The expected behavior is that it returns an unordered sequence of the unique items in source.

The key bit there is 'unordered'. IQueryable's Distinct does not guarantee that order is maintained. As such, you need to OrderBy after the Distinct .