Is that possible in LINQ to write a nice one-liner to get a first matched element or if there's no match than get first element in the collection?
E.g. you have a collection of parrots and you want yellow parrot but if there's no yellow parrots - then any will do, something like this:
Parrots.MatchedOrFirst(x => x.Yellow == true)
I'm trying to avoid double-go to SQL Server and the ORM we use in this particular case is Dapper.
What about:
var matchedOrFirst = Parrots.FirstOrDefault(x => x.Yellow == true)
?? Parrots.FirstOrDefault();
Edit
For structs, this should work:
var matchedOrFirst = Parrots.Any(x => x.Yellow == true)
? Parrots.First(x => x.Yellow == true)
: Parrots.FirstOrDefault();
Edit: It was a linq to SQL solution
First building a handy extension
public static T MatchedOrFirstOrDefault<T>(this IQueryable<T> collection, System.Linq.Expressions.Expression<Func<T, Boolean>> predicate)
{
return (from item in collection.Where(predicate) select item)
.Concat((from item in collection select item).Take(1))
.ToList() // Convert to query result
.FirstOrDefault();
}
Using the code
var matchedOrFirst = Parrots.MatchedOrFirstOrDefault(x => x.Yellow);
If you want to avoid a 2nd SQL call and since requires branching logic, its unlikely that Dapper will know how to convert a LINQ query you come up with into appropriate SQL IIF, CASE, or whatever other SQL-specific functions you end up using.
I recommend you write a simple stored procedure to do that and call it from Dapper.
Depending on its usage though, if this page only has one or two queries on it already, and is located reasonably close (latency wise) to the server, a 2nd simple SELECT won't hurt the overall application that much. Unless it is in a loop or something, or your example is trivial compared to the actual query regarding the cost of the first SELECT.
Here is the code:
public ActionResult Index()
{
var crView = db.CRCases.ToList();
return View(crView);
}
Would like to filter like we have in sql statement using WHERE and ORDER BY.
Is there any good reference on how to apply it?
linq is your friend here.
you can do
crView = crView.Where(x => x.yourPredicate).OrderBy( y => y.OrderClause);
CRCases should be a DBSet<T>. You can use LinQ like this :
db.CRCases.Where(x => x.TheProperty =xxx).ToList()
You can use LINQ = Language Integrated Query.
This have two different notations:
Functional: you can change several function calls beginning on an IEnumerable.
db.CrCases.Where(predicate).OrderBy(sortExpression)
"SQL like": it really looks like an "unordered SQL".
from crCase in db.CrCases where condition orderby sortExpression select crCase
The condition and sort expression in the first notation require lambda expressions (you can see them in action here). In the second case the condition is expressed.
Most of the queries can be written either way.
You'll usually use var to store the query, and materialize it later.
var query = from crCase in db.CrCases
where condition orderby sortExpression
select crCase
CrCase firstCase = query.Firts();
List<CrCase> cases = query.ToList();
There are several flavors of LINQ. They all look equal on the surface, but are translated into another thing. For example:
LINQ to objects: it's the more general, an allows to make queries on collections. Tis is what you'll use for this particular case.
LINQ to SQL: this allows to write queries in C# which are transalated to SQL queries in the underlying database
LINQ to Entities: similar to the previous, but you make your queries to an Entity Framework model, and the queries are finally transalated to the DB
There are some others flavors of LINQ. And, in fact, you can create your own, but that's not easy.
Finally, you have to take into account that a LINQ query isn't executed until you materialize it. I.e. it's not executed until you try to get data from it (converting it ToList(), ToArray() or whatever, enumerating it, or accesing any of it's data First(), Sum(), Count()). Depending on the flavor, the executing can be running C# code (LINQ to objects) or run a SQL Query in a DB (LINQ to SQL) or whatever.
Here you have a great place to learn how to use LINQ:
LINQ 101
What is best way to write query performance wise. e.g. i can write query to get all employes on specific last name as follow
ObjectQuery<Employee> queryEmp =
context.CreateQuery<Employee>(
"Select value e from Employees AS e Where e.LastName = #lastName",
new ObjectParameter("lastName", typeof(String)) { Value = "Farooqi" });
ObjectResult<Employee> results = query.Execute(MergeOption.AppendOnly);
OR
var v = from e in context.Employees where e.LastName == "Farooqi" select e;
OR
context.Employees.Where(e => e.LastName == "Farooqi");
OR
context.Employees.Where("LastName = #lastName", new ObjectParameter("lastName", typeof(String)) { Value = "Farooqi" });
My question is this that which method for querying is best.
What i am confused with is that i think (but not sure) that using last three methods other than the first one will fetch all records of employee from database then by enumerating all records those will be returened which fulfill the specified where condition, so i think last three ways will take more time than the first one so i used only first one.
But what is the best one in actual i dont know.
Do all method only fetch records form database which fulfill the were condition or fetch all records from db and then enumrated to return filtered records at applicaion end ?
There is little difference; the 2nd/3rd have the advantage of enforcing type correctness at compile-time, since the expression-tree won't compile if LastName doesn't exist or is (say) an int - avoids typos and the issue of "magic strings".
Re your supposition about bringing back everything; no. The 2nd/3rd compile to expression trees, which can be de-constructed and used to create the correct SQL (performing the filter at the server). Very clever, but often misunderstood. The key point is that this is Queryable.Where (taking an Expression<Func<T, bool>>), not Enumerable.Where (taking a Func<T, bool>). Feel free to perform a trace.
Likewise, there is little difference between the 1st/4th - both will filter at the server.
I know a typical way is like this:
IQueryable query = from staff in dataContext.Staffs;
if(name1 != null)
{
query = from staff in query where (staff.name == name1);
}
However, from a program we took over from other developers, we saw code like this:
IQueryable query = from staff in dataContext.Staffs;
query = from staff in query where (name1 == null || staff.name == name1);
If this is a normal SQL statement, I would definitely say that the 2nd one is a bad practice. Because it adds a meaningless where clause to the query when name1 is null.
But I am new to LINQ, so I am not sure if LINQ is different?
you can write it like
IQueryable query = from staff in dataContext.Staffs;
query = from staff in query where (name1 != null && staff.name == name1);
This way second part of your condition will not be evaluated if your first condition evaluates to false
Update:
if you write
IQueryable query = from staff in dataContext.Staffs;
query = from staff in query where (name1 == null || staff.name == name1);
and name1 is null second part of your condition will not be evaluated since or condition only requires one condition to return true
plz see this link for further detail
Often this sort of thing feels smoother to write using the fluent syntax, rather than the query syntax.
e.g.
IQueryable query = dataContext.Staffs;
if(name1 != null)
{
query = query.Where(x => x.name == name1);
}
So if name1 is null, you just don't do any Where() call. If you have multiple different filters, all of which may or may not be required, and perhaps various different sort orders, I find this becomes a lot more manageable.
Edit for alex: OK, I was answering the question about adding a where clause only when a value is not null. In response to the other part of the question, I tried this out with Entity Framework 4 to see what SQL that LINQ produced. You do this by casting query to an ObjectQuery and calling .ToTraceString(). The results were that the WHERE clause came out as follows:
WHERE #p__linq__0 IS NULL OR [Extent1].[name] = #p__linq__1
So, yes, it's classic bad SQL, if you have an index on the name column, don't expect it to be used.
Edit #2: Tried this again using LINQ to SQL rather than Entity Framework, with rather different results. This time, trying the query with name1 being null results in no WHERE clause at all, as you'd hope; trying it with name1 being "a" resulted in a simple WHERE [t0].[name] = #p0 and #p0 sent as "a". Entity Framework does not seem to optimize thus. That's a bit worrying.
The best way to do this is to create yourself an extension method that will take in a conditional statement and a where expression. If the condition is true then it will use the where expression else it will not use it. This can dramatically clean up your code, eliminating the need for if statements.
public static class LinqExtensions
{
public static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition, Expression<Func<T, bool>> whereClause)
{
if (condition)
{
return query.Where(whereClause);
}
return query;
}
}
Now you can write your code like this:
IQueryable<Staffs> query = dataContext.Staffs.AsQueryable().WhereIf(name1 != null, x => x.Name == name1);
So I tried the .Where(..., x => ...) extension method listed here as an answer but it doesn't work against Entity Framework as Linq To Entities doesn't know how to translate that into TSQL.
So here's my solution getting my Func on:
Expression<Func<SomeEfPoco, bool>> columnBeingFilteredPredicate = x => true; // Default expression to just say yes
if (!string.IsNullOrWhiteSpace(someColumnBeingFilteredValue))
{
columnBeingFilteredPredicate = x => x.someColumnBeingFiltered == someColumnBeingFilteredValue;
}
_context.SomeEfPocos.Where(x => ..... &&
..... &&
..... &&)
.Where(columnBeingFilteredPredicate);
someColumnBeingFilteredValue in my case is a string parameter on the encapsulating method with a default value of NULL.
LINQ is diffrent in some other causes (not in this causes),
LINQ is the way to get data in the "Faster way" with a littel code and clear cod as possible, there a many benefits of LINQ:
Makes it easier to transform data into objects. I'm sure you've heard the term "Impedence Mismatch" being used quite often, meaning that LINQ reduces the amount of work you must do to translate between object-oriented code and data paradigms such as hierarchical, flat-file, messages, relational, and more. It doesn't eliminate the "Impedence Mismatch" because you must still reason about your data in its native form, but the bridge from here to there is (IMO) much shorter.
A common syntax for all data. Once you learn query syntax, you can use it with any LINQ provider. I think this is a much better development paradigm than the Tower of Babel that has grown over the years with data access technologies. Of course, each LINQ provider has unique nuances that are necessary, but the basic approach and query syntax is the same.
Strongly typed code. The C# (or VB.NET) query syntax is part of the language and you code with C# types, which are translated into something a provider understands. This means that you gain the productivity of having your compiler find errors earlier in the development lifecycle than elsewhere. Granted, many errors in stored proc syntax will generate errors when you save, but LINQ is more general than SQL Server. You have to think of all the other types of data sources that generate runtime errors because their queries are formed with strings or some other loosely typed mechanism.
Provider integration. Pulling together data sources is very easy. For example, you can use LINQ to Objects, LINQ to SQL, and LINQ to XML together for some very sophisticated scenarios. I think it's very elegant.
Reduction in work. Before LINQ, I spent a lot of time building DALs, but now my DataContext is the DAL. I've used OPFs too, but now I have LINQ that ships with multiple providers in the box and many other 3rd party providers, giving me the benefits from my previous points. I can set up a LINQ to SQL DataContext in a minute (as fast as my computer and IDE can keep up).
Performance in the general case doesn't become an issue. SQL Server optimizes queries quite well these days, just like stored procs. Of course, there are still cases where stored procs are necessary for performance reasons. For example, I've found it smarter to use a stored proc when I had multiple interactions between tables with additional logic inside of a transaction. The communications overhead of trying to do the same task in code, in addition to getting the DTC involved in a distributed transaction made the choice for a stored proc more compelling. However, for a query that executes in a single statement, LINQ is my preferred choice because even if there was a small performance gain from a stored proc, the benefits in previous points (IMO) carry more weight.
Built-in security. One reason I preferred stored procs before LINQ was that they forced the use of parameters, helping to reduce SQL injection attacks. LINQ to SQL already parameterizes input, which is just as secure.
LINQ is declarative. A lot of attention is paid to working with LINQ to XML or LINQ to SQL, but LINQ to Objects is incredibly powerful. A typical example of LINQ to Objects is reading items from a string[]. However, that's just a small example. If you think about all of the IEnumerable collections (you can also query IEnumerable) that you work with every day, the opportunities are plentiful. i.e. Searching an ASP.NET ListBox control for selected items, performing set operations (such as Union) on two collections, or iterating through a List and running a lambda in a ForEach of each item. Once you begin to think in LINQ, which is declarative in nature, you can find many of your tasks to be simpler and more intuitive than the imperative techniques you use today.
I could probably go on, but I'd better stop there. Hopefully, this will provide a more positive view of how you could be more productive with LINQ and perhaps see it as a useful technology from a broader perspective.
I've seen this pattern in standard SQL, and it seems useful if you have several parameters that may be NULL. For example:
SELECT * FROM People WHERE ( #FirstName IS NULL OR FirstName = #FirstName )
AND ( #LastName IS NULL OR LastName = #LastName )
If you see this in LINQ, it's possible they just blindly translated their old SQL-queries.
I like use the Expression
e.g.
Expression<Func<Persons, bool>> expresionFinal = c => c.Active == true;
if (DateBirth.HasValue)
{
Expression<Func<Persons, bool>> expresionDate = c => (EntityFunctions.TruncateTime(c.DateBirth) == DateBirth);
expresionFinal = PredicateBuilder.And(expresionFinal, expresionDate);
}
IQueryable query = dataContext.Persons;
query = query.Where(expresionFinal);
For EF Core I broke it up like this:
IQueryable<Partners> recs = contextApi.Partners;
if (status != -1)
{
recs = recs.Where(i => i.Status == status);
}
recs = recs.OrderBy(i => i.Status).ThenBy(i => i.CompanyName);
foreach (var rec in recs)
{
}
I had to be explicit with my typing instead of relying on var.
I like the idea with Extension
public static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition, Expression<Func<T, bool>> whereClause)
=> condition ? query.Where(whereClause) : query;
No, I am not strongly agree with you.
here you just gave a simple logic
if(name1 != null)
// do your stuff
but what will happen if you do something different with the name1 that have null value..!!
Ok, now consider this situation.
In this example you shows how to handle possible null values in source collections.
An object collection such as an IEnumerable<T> can contain elements whose value is null.
If a source collection is null or contains an element whose value is null,
and your query does not handle null values, a NullReferenceException will be thrown when you execute the query.
Probably this could be a issue...
I use the extension method below. It's less flexible than the WhereIf extension from the other answers, but it's shorter to use.
public static IQueryable<T1> FilterBy<T1, T2>(this IQueryable<T1> query, T2 expectedValue, Expression<Func<T1, T2>> propertyAccessor)
{
if (propertyAccessor == null) throw new ArgumentNullException(nameof(propertyAccessor));
if (expectedValue == null) return query;
var equalExpr = Expression.Equal(propertyAccessor.Body, Expression.Constant(expectedValue, typeof(T2)));
var lambda = Expression.Lambda<Func<T1, bool>>(equalExpr, propertyAccessor.Parameters);
return query.Where(lambda);
}
It can be used like:
var query = dataContext.Staffs.FilterBy(name, s => s.Name);
I’m looking for a tool that will dynamically generate Linq to Entity queries from a given entity, a Query By Entity (Example), if you will. Given an entity and the object context it belongs to, the generator returns an ObectQuery or IQueryable that could be further modified or executed. Ideally, the query builder would not directly reference Entity Model, rather it would use the object context to build the query from the model. I imagine the code looking something like this:
QueryBuilder qb = new QueryBuilder(new EntitiesContext());
Customer c = new Customer();
qb.Add(c);
c.FirstName = "Jim";
var qry = qb.BuildQuery();
int total = qry.Count();
The underlying query would look something like this:
var query = from c in ctx.Customers
where c.FirstName == "Jim"
select c;
Does such a thing already exist somewhere? I can imagine coding something like this myself, but I would rather start using something that already exists.
I'm not exactly sure what your goals are, if you would post some more information on your context (an actual problem), but Dynamic Linq might help you constructing linq queries dynamically:
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
Wiebe
I don't really see the value in a QueryBuilder. If it's as simple as in your sample you could only use it to find exact matches. If you need more complex queries you'd probably be better of using Linq directly.
You know that you can chain your queries? That's probably not exactly what you are looking for but you could do
IQueryable<Customers> source=context.Customers;
if (...)
{
source = from c in source
where c.FirstName.StartsWith("Jim")
select c;
}
if (...)
{
source = from c in source
where contries.Contains(c.Country)
select c;
}
// ...
You can use dynamic Linq to build up your Linq expressions at run time - simply appending methods to an existing Linq query (before you execute it).
Having said that, the notion of a "QueryBuilder" class is, as Chris mentioned, of questionable merit.
Strictly speaking, Linq is itself a query builder, since it takes a Linq statement and constructs a "graph" which a query provider is then able to use to do the underlying work (compiling a SQL statement for Linq-To-SQL, or enumerating objects of Linq-For-Objects).
If you want to offer end-users the ability to build queries dynamically, consider creating a UI that builds the Linq statement by adding query methods like Chris pointed out. Remember that with a Linq query, you can add as many expressions as you like - the query only executes when the enumerator is tripped.
i.e. To select the oldest Customer under 65 named Jim:
Dim qry = From customer in DataContext.Customers
qry = qry.Where( Function (c as Customer) c.Name="Jim" )
qry = qry.Where( Function (c as Customer) c.Age < 65 )
qry = qry.OrderByDescending( Function (c as Customer) c.Age )
qry = qry.Take(1)
Dim oldJim as Customer = qry.FirstOrDefault()
The FirstOrDefault method, like using ToArray or For Each, trips the enumerator and thus processes the query.
Hope this helps!