使用LINQ与Select和Distinct方法

Question

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.

Answer 1

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();

Answer 2

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());

Answer 3

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