How to convert a query to lambda based

[Mou_kol]

var usersWithRoles = (from user in context.Users
select new
{
UserId = user.Id,
Username = user.UserName,
Email = user.Email,
RoleNames = (from userRole in user.Roles
join role in context.Roles on userRole.RoleId
equals role.Id
select role.Name).ToList()
}).ToList().Select(p => new Users_in_Role_ViewModel()

                              {  
                                  UserId = p.UserId,  
                                  Username = p.Username,  
                                  Email = p.Email,  
                                  Role = string.Join(",", p.RoleNames)  
                              });

guide me how could i compose the above query using lambda if possible. thanks

[OriginalGriff]

You already have a Lambda in there:

Select(p => new Users_in_Role_ViewModel()

So what help do you need? And why are you converting things to List<T> so often? All that does is "collapse" the iteration, which wastes time and memory space...

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[Richard Deeming]

Something like this should work:

var usersWithRoles = context.Users
.Select(user => new
{
UserId = user.Id,
Username = user.UserName,
Email = user.Email,
RoleNames = user.Roles.Join(context.Roles, userRole => userRole.RoleId, role => role.Id, (userRole, role) => role.Name)
})
.AsEnumerable()
.Select(p => new Users_in_Role_ViewModel
{
UserId = p.UserId,
Username = p.Username,
Email = p.Email,
Role = string.Join(",", p.RoleNames)
});

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[Mou_kol]

thank you sir :)

[Mou_kol]

you said :- why are you converting things to List so often? All that does is "collapse" the iteration, which wastes time and memory space... so tell me which area need to change in query?

[Mou_kol]

Sir Morning, you said "And why are you converting things to List so often? All that does is "collapse" the iteration, which wastes time and memory space..." which still not clear that what you try to say? are pointing why i use tolist()...what problem may occur for this. please make me aware in details so i may avoid this kind of mistake in future. thanks

[Mou_kol]

sir, you use .AsEnumerable() where as i used .ToList()....what would be the difference in term of performance of using .AsEnumerable(). if possible please explain this one with a easy example that what will happen internally to use .ToList() which causes performance issue. tell me if i write this way then what will be the performance

var usersWithRoles = (from user in context.Users
select new
{
UserId = user.Id,
Username = user.UserName,
Email = user.Email,
RoleNames = (from userRole in user.Roles
join role in context.Roles on userRole.RoleId
equals role.Id
select role.Name)
}).ToList().Select(p => new Users_in_Role_ViewModel()

                              {  
                                  UserId = p.UserId,  
                                  Username = p.Username,  
                                  Email = p.Email,  
                                  Role = string.Join(",", p.RoleNames)  
                              });

just inner tolist function......does it increase performance ? you code as follows

var usersWithRoles = context.Users
.Select(user => new
{
UserId = user.Id,
Username = user.UserName,
Email = user.Email,
RoleNames = user.Roles.Join(context.Roles, userRole => userRole.RoleId, role => role.Id, (userRole, role) => role.Name)
})
.AsEnumerable()
.Select(p => new Users_in_Role_ViewModel
{
UserId = p.UserId,
Username = p.Username,
Email = p.Email,
Role = string.Join(",", p.RoleNames)
});

this is your join which not clear to me.

RoleNames = user.Roles.Join(context.Roles, userRole => userRole.RoleId, role => role.Id, (userRole, role) => role.Name)

the join is happening between which tables ? if re-write this way then what will be performance

List usersWithRoles= db.Users.Include(a => a.Roles).Select(p =>
new Users_inRole_ViewModel {
UserId = p.

[OriginalGriff]

Linq queries and Linq methods aren't executed immediately, they use something called "Deferred execution" which (basically) means that nothing is done until you actually need the final results. There is a basic example of this here: Deferred Execution Example (C#) | Microsoft Docs[^] Deferred execution means better efficiency (sometimes) because you can "combine operations" instead of having to loop each time to produce the intermediate result which you then pass to the next section of the evaluation. Remember: Linq isn't "no loops" it's about "hiding loops" where you can't see them and evaluating them only when it has to. But ... when you say "ToList" you are explicitly requesting an object be created holding the whole results - so the whole operation has to be performed immediately to generate the collection you asked for. So when you then continue to process it, you need to do another loop to process the next operation. Every time you call ToList, you make Linq work harder, and that takes both more time, and uses more memory. Only ever "Collapse the operation" when you actually need the results!

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