Develop Reference

Using two Linq query in a single method

As shown in the below code, the API will hit the database two times to perform two Linq Query. Can't I perform the action which I shown below by hitting the database only once?
var IsMailIdAlreadyExist = _Context.UserProfile.Any(e => e.Email == myModelUserProfile.Email);
var IsUserNameAlreadyExist = _Context.UserProfile.Any(x => x.Username == myModelUserProfile.Username);

In order to make one request to database you could first filter for only relevant values and then check again for specific values in the query result:
var selection = _Context.UserProfile
.Where(e => e.Email == myModelUserProfile.Email || e.Username == myModelUserProfile.Username)
.ToList();
var IsMailIdAlreadyExist = selection.Any(x => x.Email == myModelUserProfile.Email);
var IsUserNameAlreadyExist = selection.Any(x => x.Username == myModelUserProfile.Username);
The .ToList() call here will execute the query on database once and return relevant values

Start with
var matches = _Context
.UserProfile
.Where(e => e.Email == myModelUserProfile.Email)
.Select(e => false)
.Take(1)
.Concat(
_Context
.UserProfile
.Where(x => x.Username == myModelUserProfile.Username)
.Select(e => true)
.Take(1)
).ToList();
This gets enough information to distinguish between the four possibilities (no match, email match, username match, both match) with a single query that doesn't return more than two rows at most, and doesn't retrieve unused information. Hence about as small as such a query can be.
With this done:
bool isMailIdAlreadyExist = matches.Any(m => !m);
bool isUserNameAlreadyExist = matches.LastOrDefault();

It's possible with a little hack, which is grouping by a constant:
var presenceData = _Context.UserProfile.GroupBy(x => 0)
.Select(g => new
{
IsMailIdAlreadyExist = g.Any(x => x.Email == myModelUserProfile.Email),
IsUserNameAlreadyExist = g.Any(x => x.Username == myModelUserProfile.Username),
}).First();
The grouping gives you access to 1 group containing all UserProfiles that you can access as often as you want in one query.
Not that I would recommend it just like that. The code is not self-explanatory and to me it seems a premature optimization.

You can do it all in one line, using ValueTuple and LINQ's .Aggregate() method:
(IsMailIdAlreadyExist, IsUserNameAlreadyExist) = _context.UserProfile.Aggregate((Email:false, Username:false), (n, o) => (n.Email || (o.Email == myModelUserProfile.Email ? true : false), n.Username || (o.Username == myModelUserProfile.Username ? true : false)));

Joining table to a list using Entity Framework

I have the following Entity Framework function that it joining a table to a list. Each item in serviceSuburbList contains two ints, ServiceId and SuburbId.
public List<SearchResults> GetSearchResultsList(List<ServiceSuburbPair> serviceSuburbList)
{
var srtList = new List<SearchResults>();
srtList = DataContext.Set<SearchResults>()
.AsEnumerable()
.Where(x => serviceSuburbList.Any(m => m.ServiceId == x.ServiceId &&
m.SuburbId == x.SuburbId))
.ToList();
return srtList;
}
Obviously that AsEnumerable is killing my performance. I'm unsure of another way to do this. Basically, I have my SearchResults table and I want to find records that match serviceSuburbList.

If serviceSuburbList's length is not big, you can make several Unions:
var table = DataContext.Set<SearchResults>();
IQuerable<SearchResults> query = null;
foreach(var y in serviceSuburbList)
{
var temp = table.Where(x => x.ServiceId == y.ServiceId && x.SuburbId == y.SuburbId);
query = query == null ? temp : query.Union(temp);
}
var srtList = query.ToList();
Another solution - to use Z.EntityFramework.Plus.EF6 library:
var srtList = serviceSuburbList.Select(y =>
ctx.Customer.DeferredFirstOrDefault(
x => x.ServiceId == y.ServiceId && x.SuburbId == y.SuburbId
).FutureValue()
).ToList().Select(x => x.Value).Where(x => x != null).ToList();
//all queries together as a batch will be sent to database
//when first time .Value property will be requested

Flattening Complex LINQ to SQL

I have a somewhat complex LINQ to SQL query that I'm trying to optimise (no, not prematurely, things are slow), that goes a little bit like this;
IQueryable<SearchListItem> query = DbContext.EquipmentLives
.Where(...)
.Select(e => new SearchListItem {
EquipmentStatusId = e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null).Id,
StatusStartDate = e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null).DateFrom,
...
});
The where clauses aren't important, they don't filter EquipmentStatuses, happy to include if someone thinks they're required.
This is on quite a large set of tables and returns a fairly details object, there's more references to EquipmentStatuses, but I'm sure you get the idea. The problem is that there's quite obviously two sub-queries and I'm sure that (among some other things) is not ideal, especially since they are exactly the same sub-query each time.
Is it possible to flatten this out a bit? Perhaps it's easier to do a few smaller queries to the database and create the SearchListItem in a foreach loop?

Here's my take given your comments, and with some assumptions I've made
It may look scary, but give it a try, with and without the ToList() before the GroupBy()
If you have LinqPad, check the SQL produced, and the number of queries, or just plug in the SQL Server Profiler
With LinqPad you could even put a Stopwatch to measure things precisely
Enjoy ;)
var query = DbContext.EquipmentLives
.AsNoTracking() // Notice this!!!
.Where(...)
// WARNING: SelectMany is an INNER JOIN
// You won't get EquipmentLive records that don't have EquipmentStatuses
// But your original code would break if such a case existed
.SelectMany(e => e.EquipmentStatuses, (live, status) => new
{
EquipmentLiveId = live.Id, // We'll need this one for grouping
EquipmentStatusId = status.Id,
EquipmentStatusDateTo = status.DateTo,
StatusStartDate = status.DateFrom
//...
})
// WARNING: Again, you won't get EquipmentLive records for which none of their EquipmentStatuses have a DateTo == null
// But your original code would break if such a case existed
.Where(x => x.EquipmentStatusDateTo == null)
// Now You can do a ToList() before the following GroupBy(). It depends on a lot of factors...
// If you only expect one or two EquipmentStatus.DateTo == null per EquipmentLive, doing ToList() before GroupBy may give you a performance boost
// Why? GroupBy sometimes confuses the EF SQL generator and the SQL Optimizer
.GroupBy(x => x.EquipmentLiveId, x => new SearchListItem
{
EquipmentLiveId = x.EquipmentLiveId, // You may or may not need this?
EquipmentStatusId = x.EquipmentStatusId,
StatusStartDate = x.StatusStartDate,
//...
})
// Now you have one group of SearchListItem per EquipmentLive
// Each group has a list of EquipmenStatuses with DateTo == null
// Just select the first one (you could do g.OrderBy... as well)
.Select(g => g.FirstOrDefault())
// Materialize
.ToList();

You don't need to repeat the FirstOrDefault. You can add an intermediate Select to select it once and then reuse it:
IQueryable<SearchListItem> query = DbContext.EquipmentLives
.Where(...)
.Select(e => e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null))
.Select(s => new SearchListItem {
EquipmentStatusId = s.Id,
StatusStartDate = s.DateFrom,
...
});
In query syntax (which I find more readable) it would look like this:
var query =
from e in DbContext.EquipmentLives
where ...
let s = e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null)
select new SearchListItem {
EquipmentStatusId = s.Id,
StatusStartDate = s.DateFrom,
...
});
There is another problem in your query though. If there is no matching EquipmentStatus in your EquipmentLive, FirstOrDefault will return null, which will cause an exception in the last select. So you might need an additional Where:
IQueryable<SearchListItem> query = DbContext.EquipmentLives
.Where(...)
.Select(e => e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null))
.Where(s => s != null)
.Select(s => new SearchListItem {
EquipmentStatusId = s.Id,
StatusStartDate = s.DateFrom,
...
});
or
var query =
from e in DbContext.EquipmentLives
where ...
let s = e.EquipmentStatuses.FirstOrDefault(s => s.DateTo == null)
where s != null
select new SearchListItem {
EquipmentStatusId = s.Id,
StatusStartDate = s.DateFrom,
...
});

Given that you don't test for null after calling FirstOrDefault(s => s.DateTo == null) I assume that:
either for each device there is always a status with DateTo == null or
you need to see only devices which have such status
In order to do so you need to join EquipmentLives with EquipmentStatuses to avoid subqueries:
var query = DbContext.EquipmentLives
.Where(l => true)
.Join(DbContext.EquipmentStatuses.Where(s => s.DateTo == null),
eq => eq.Id,
status => status.EquipmentId,
(eq, status) => new SelectListItem
{
EquipmentStatusId = status.Id,
StatusStartDate = status.DateFrom
});
However, if you do want to perform a left join replace DbContext.EquipmentStatuses.Where(s => s.DateTo == null) with DbContext.EquipmentStatuses.Where(s => s.DateTo == null).DefaultIfEmpty().

Get Method is null off IQueryable (Entity Framework)

I'm trying to pass lambda expressions and a type to my DAL. I have this statement:
(entities).GetType().GetMethod("Where")
"entities" is the Table of entities on the DataContext.
When I run the statement I get a null even though Linq.Table inherits IQueryable.
Anyone have an idea?
Here is the entire method:
public object GetResultSet(Dictionary<Type, Func<object, bool>> values)
{
using (ICSDataContext db = DataContextFactory.CreateDataContext<ICSDataContext>(DataContexts.ICS))
{
foreach (var entry in values)
{
var property = db.GetType().GetProperty(entry.Key.Name + "s");
IQueryable entities = (IQueryable)property.GetValue(db, null);
var whereMethod = (entities).GetType().GetMethod("Where")
.MakeGenericMethod(Type.GetType(entry.Key.AssemblyQualifiedName));
return whereMethod.Invoke(entities, new object[] { entry.Value });
}
}
return null;
}
Thanks

As an alternative you could do something like
db.Set<Type>()
which will return you the DBSet of the appropriate type, with Where accessible without reflection. Also you may want to use Expression> rather than Func, expressions work on queryables where as funcs work on enumerables. If you pass a func into a Where clause it pulls the entire dbset down and processes it in memory.
Typed expressions are also a little easier to work with (intellesence, type checking).
Expression<Func<User,bool>> filter = c=>c.FirstName == "Bob";
As another alternative you can look into System.Linq.Dynamic, ScottGu has a write up on it here. The article and the code are old, but it works with EF 6. It allows things like
.Where("CategoryId=2 and UnitPrice>3")

From answer by LukeH under here:
var where1 = typeof(Queryable).GetMethods()
.Where(x => x.Name == "Where")
.Select(x => new { M = x, P = x.GetParameters() })
.Where(x => x.P.Length == 2
&& x.P[0].ParameterType.IsGenericType
&& x.P[0].ParameterType.GetGenericTypeDefinition() == typeof(IQueryable<>)
&& x.P[1].ParameterType.IsGenericType
&& x.P[1].ParameterType.GetGenericTypeDefinition() == typeof(Expression<>))
.Select(x => new { x.M, A = x.P[1].ParameterType.GetGenericArguments() })
.Where(x => x.A[0].IsGenericType
&& x.A[0].GetGenericTypeDefinition() == typeof(Func<,>))
.Select(x => new { x.M, A = x.A[0].GetGenericArguments() })
.Where(x => x.A[0].IsGenericParameter
&& x.A[1] == typeof(bool))
.Select(x => x.M)
.SingleOrDefault();
Then this:
var gmi = where1.MakeGenericMethod(typeof(T));

Conditionally Insert Where Clause in LINQ Method Syntax with Select Many

Not sure if this possible using the LINQ method chain syntax or at all, but I would like to conditionally insert a where class in the chain if a parameter passed to the method is not null.
Here is the redundant code I would like to simplify:
public ICollection<Organization> getNetworkServiceRecipients(string serviceId = null)
{
ICollection<Organization> children = get_all_children();
if (serviceId != null)
{
return children.SelectMany(o => o.receives_these_services)
.Where(s => s.serviceId == serviceId)
.Select(o => o.serviceRecipient)
.Distinct()
.ToList();
}
else
{
return (children.SelectMany(o => o.receives_these_services)
.Select(o => o.serviceRecipient)
.Distinct()
.ToList());
}
}
I have been trying to insert the where clause programmatically based on whether serviceId is null or not. All of the answers I have found where based on the query syntax, but I couldn't translate. Any suggestions?

If you don't want to have it in the actual where query as dotnetom mentioned, you can do something like this:
public ICollection<Organization> getNetworkServiceRecipients(string serviceId = null)
{
var services = get_all_children().SelectMany(o => o.receives_these_services);
if (serviceId != null)
services = services.Where(s => s.serviceId == serviceId);
return services.Select(o => o.serviceRecipient)
.Distinct()
.ToList();
}

You can try this approach:
public ICollection<Organization> getNetworkServiceRecipients(string serviceId = null)
{
ICollection<Organization> children = get_all_children();
return children.SelectMany(o => o.receives_these_services)
.Where(s => serviceId == null || s.serviceId == serviceId)
.Select(o => o.serviceRecipient)
.Distinct()
.ToList();
}
In this case if your variable serviceId is null then only the first part of where condition would be executed, otherwise first part would be true and only second condition would matter.

Similar answer as dotnetom, but uses a ternary to determine which lambda to use
so that serviceId == null doesn't get executed on a per-item basis.
return children.SelectMany(o => o.receives_these_services)
.Where( serviceId == null ? (_ => true) : (s => s.serviceId == serviceId))
.Select(o => o.serviceRecipient)
.Distinct()
.ToList();