使用LINQ的Select和Distinct方法[英] Using LINQ with Select and Distinct Methods

问题描述

If I use the following code in LINQPad to query my database I get the desired results:

LoadTables.Where(o=> o.Approver== "Name Name" ||o.Approver== "Name.Name").Select(o=>o.SubmittedBy).ToList().Distinct()

However, if I amend this and put it into my code, I get an error:

public IEnumerable<LoadTable> TableList;
TableList = _context.LoadTable.Where(o => o.Approver == GetADDetails.displayName || o.Approver == GetADDetails.userName).Select(o => o.SubmittedBy).ToList().Distinct();

The error returned is:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<string>' to 'System.Collections.Generic.IEnumerable<App.Models.LoadTable>' An explicit conversion exists.

Where am I going wrong?

For context, see previous quested here:

Using LINQ to loop through data and display in a table

Currently it's returing a new table for every match, I'm trying to get it to return a table per user.

推荐答案

Remove from the Linq expresion Select(o => o.SubmittedBy) and put Distinct()before of ToList():

public IEnumerable<LoadTable> TableList;
TableList = _context.LoadTable.Where(o => o.Approver == GetADDetails.displayName || 
  o.Approver == GetADDetails.userName).Distinct().ToList();

其他推荐答案

You are selecting a string property

**.Select(o => o.SubmittedBy)**

this return IEnumerable<string>

you need something like below

_context.LoadTable.Where(o => o.Approver == GetADDetails.displayName || o.Approver == GetADDetails.userName).Distinct().GroupBy(p => p.SubmittedBy).Select(grp => grp.FirstOrDefault());

其他推荐答案

Alas you forgot to tell us what kind of objects are in the sequence that is returned by _context.LoadTable. Looking at your code, it seems that it returns an enumerable sequence of objects, where every object has at least a property SubmittedBy.

Looking at your error, it seems that SubmittedBy is a string property.

If you had split your code into smaller pieces and used proper identifiers, you would soon have seen your problem.

Let's examine your code:

IEnumerable<LoadTable> TableList = _context.LoadTable
    .Where(o => o.Approver == GetADDetails.displayName || o.Approver == GetADDetails.userName)
    .Select(o => o.SubmittedBy)
    .ToList()
    .Distinct();

_context.LoadTable returns an IEnumerable sequence of items unknown by me, so let's assume it is a sequence of Notes:

IEnumerable<Note> notes = _context.LoadTable;

It could be that LoadTable returns an IEnumerable instead of IEnumerabl<Note>, in that case you should cast the loaded table.

The next statements:

IEnumerableM<Note> notesApprovedByUser = notes
    .Where(note => note.Approver == GetADDetails.displayName 
                || note.Approver == GetADDetails.userName);
IEnumerable<string> submitters = notesApprovedByUser
    .Select(note => note.SubmittedBy);
List<string> submitterList = submitters.ToList();
IEnumerable<string> distinctSubmitters = submitterList.Distinct();

It is easy to see that a sequence of strings can't easily be converted to a sequence of LoadTables.

The question is: do you want unique LoadTables, or do you want for every submitter all hist submitted notes? In that case you'll have to Groupby instead of Select:

.Where(note => ...)
.GroupBy(note => note.SubmittedBy,  // make groups of Notes submitted by the same submitter
   // parameter resultSelector: take every submitter and all notes that
   // were submitted by this submitter to make a new object
   (submittedBy, notesSubmittedByThisSubmitter) => new
   {
       // select the properties you plan to use
       Submitter = submittedBy
       LoadTables = notesSubmittedByThisSubmitter.Select(note => new LoadTable
       {
           ... again: select the properties you need
       })
       .ToList(),
   });

Remember: keep an IEnumerable<...> an IEnumerable<...> as long as possible. If not necessary, don't do aToList()before you return. If your query returns 1000 items, and your caller will only doFirstOrDefault, orTake(3).ToList()`, it would be a waste of processing power to create your complete list