在 linq 查询 c# 中使用条件语句

Question

I have 2 queries. First gets the full name of a committee head if the committee has more that 1 member.

var result = await db.ExpertCommittees
    .Where(f => f.Id == committeeId)
    .Where(f => f.ExpertCommitteeMembers.Count > 1)
    .Select(f => f.ExpertCommitteeMembers
    .Where(m => m.IsCommitteeHead)
    .FirstOrDefault().Expert.FullName)
    .FirstOrDefaultAsync();

The second one gets the full name of the only committee member if the committee has only 1 member

var result2 = await db.ExpertCommittees
    .Where(f => f.Id == committeeId)
    .Where(f => f.ExpertCommitteeMembers.Count == 1)
    .Select(f => f.ExpertCommitteeMembers
    .FirstOrDefault().Expert.FullName)
    .FirstOrDefaultAsync();

Is it possible to check how many members does the committee have and then return the correct name all in the same query? Or do I first have to check how many members does the committee have and then run the appropriate query seperatly?

Answer 1

If I understand correctly you can try to let condition into inner linq where

var result = await db.ExpertCommittees
    .Where(f => f.Id == committeeId)
    .Select(f => f.ExpertCommitteeMembers.Where(m => 
                 (m.IsCommitteeHead && 
                 f.ExpertCommitteeMembers.Count > 1)||f.ExpertCommitteeMembers.Count == 1).FirstOrDefault().Expert.FullName)
    .FirstOrDefaultAsync();

Answer 2

you can Create a create anonymous object in linq query select. which will contain FullName and count.

var result = await db.ExpertCommittees
    .Where(f => f.Id == committeeId)
    .Where(f => f.ExpertCommitteeMembers.Count > 1)
    .Select(f => new
    {
        FullName = f.ExpertCommitteeMembers
    .Where(m => m.IsCommitteeHead)
    .FirstOrDefault().Expert.FullName,
        Count = f.ExpertCommitteeMembers.Count
    })
    .FirstOrDefaultAsync();

Answer 3

A generated Join, as it is generated from ExpertCommittees to ExpertCommitteeMembers, with a Navigation-Collection, will always do a Left Join , what you want is an Inner Join . It will give you only items with entities in both tables.

This will be something like

    db.ExpertCommittees.Join(db.ExpertCommittemember, x=>someid, y=>somid, 
          (comittee, member) => new { comittee, member});

But this will give you one line per member.... with the possibility to filter by "IsComitteeHead" or whatever.

You can append Grouping, or use a GroupJoin directly, to have a List of "Commitees", each with a List of it's members ...and only if there are members.

Without knowing the data structure, ID's, Foreign-Keys and table name, we cannot produce a valid query.