mvc c＃结合两个以上的查询结果

Question

I have several generic queries that has the same structure of results and I'm trying to combine them in one variable so that I can pass them to the view.

var query1 = db1.table.Where(a => a.ID == filter1).Select(x=> new ViewModel { //columns}).LastOrDefault();
var query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel  { //columns}).LastOrDefault();
var query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel  { //columns}).LastOrDefault();

var combi = query1.Concat(query2);

I cannot combine the two query results, I tried using Concat or Union but it gives the error below.

'ViewModel' does not contain a definition for 'Concat' and no extension method 'Concat' accepting a first argument of type 'ViewModel' could be found (are you missing a using directive or an assembly reference?)

Thanks!

Answer 1

If you were to replace the var keyword with the actual types you'd see that you have this:

ViewModel query1 = db1.table.Where(a => a.ID == filter1).Select(x => new ViewModel { /* columns */ }).LastOrDefault();
ViewModel query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel { /* columns */ }).LastOrDefault();
ViewModel query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel { /* columns */ }).LastOrDefault();

In order to do a .Concat(...) it should be IQueryable<ViewModel> or IEnumerable<ViewModel> .

It's the .LastOrDefault() call that is causing you grief.

Try this:

IQueryable<ViewModel> query1 = db1.table.Where(a => a.ID == filter1).Select(x => new ViewModel { /* columns */ });
IQueryable<ViewModel> query2 = db1.table.Where(a => a.ID == filter2).Select(x=> new ViewModel { /* columns */ });
IQueryable<ViewModel> query3 = db1.table.Where(a => a.ID == filter3).Select(x=> new ViewModel { /* columns */ });

var combi = query1.Concat(query2).Concat(query3).LastOrDefault();