Is it possible to use custom method In query for example:
var result = from u in context.MyTable where MyMethod(u) == 10 select u;
As Pranay explains, you cannot have a custom (C#) method as part of the LINQ to SQL query, because LINQ to SQL wouldn't be able to look at the expression tree of the method and so it cannot translate it to SQL.
One option that you have is to write your function in SQL and store it as a SQL function on the SQL Server (possibly, you could also use SQL CLR, but I have not tried that). Then you can add the function to your DataContext type and LINQ to SQL will translate it to calls to the function on SQL server. Something like:
var result = from u in context.MyTable
where context.MyMethod(u) == 10 select u;
The problem, of course, is that you'll need to write the function in SQL (I think SQL CLR could also work - not sure about the performance and other possible complications though)
I also wrote an article (some time ago) that shows how to do this when you write the "method" as an expression tree way (as a value of type Expression<Func<...>>), which is possible, because in this case, the code is compiled as an expression tree. However, there is some postprocessing that has to be done and you can still write just a single expression that can be easily inlined in the LINQ query.
Check this full article : What is and what isn't possible with linq
Following is not possible
// function used in filter
static bool MyFunc(Nwind.Product p)
{
return p.ProductName.StartsWith("B");
}
// query that uses MyFunc
var q =
from p in db.Products
where MyPriceFunc(p.UnitPrice) > 30m
select p
It compiles with no errors, but when you execute it LINQ to SQL throws an exception saying: "Static method System.Boolean MyTest(LINQTest.Nwind.Product) has no supported translation to SQL."
The exception is actually thrown when you try to fetch results from q (for example using the foreach statement), because LINQ to SQL attempts to convert the expression trees to T-SQL only when the results are needed and the query must be executed.
To fix the example you can simply copy the code that checks whether product name starts with "B" to the where clause of the query and it would work fine.
Yes, but if you are using Linq-to-Sql - your method has to have special code to handle to SQL conversion.
I built a Linq query on a MongoDB collection, which retrieves properly my docs:
var parts = mongoParts.AsQueryable()
.Where(mongo => mongo.Prop1 == bmbMatch.Prop1 && mongo.Prop2 == bmbMatch.Prop2)
.ToList();
This works fine, but to make this less code I wrote a Generic function taking a
Func<T, T, bool> selector
Which take the exact same lamdba expression as in Where clause above, which makes it:
var mongo = mongoQuery.AsQueryable()
.Where(tmongo => selector(tmongo, localMatch))
.ToList();
with selector as :
(mongo, local) => mongo.Prop1== local.Prop1 && mongo.Prop2 == local.Prop2
This gives me a "Unsupported where clause: "
Any ideas?
From the internet
Not all LINQ queries can be turned into equivalent MongoDB queries,
because C# allows you to write expressions that don't have any
equivalent in the MongoDB query language.
That being said, LINQ to MongoDB probably can't translate delegates.
You can find all supported LINQ queries on this address.
Conclusion: If you need to query very large data or make queries as fast as possible, you will have to stick with the former solution. Otherwise, pull all data from your db and query them in-memory.
This is my code:
string queryString = "Marco".ToLower();
utenti = db.User.Where(p =>
queryString.Contains(p.Nickname.ToLower()) ||
queryString.Contains(p.Nome.ToLower()) ||
queryString.Contains(p.Cognome.ToLower())).ToList();
but I get:
Only arguments that can be evaluated on the client are supported for the String.Contains method.
Why? Can't I use .Contains()?
Try .IndexOf. It is not LINQ that can't do Contains, it's LINQ to Entities and LINQ to SQL that can't.
string queryString = "Marco";
utenti = db.User.Where(p =>
queryString.IndexOf(p.Nickname, StringComparison.OrdinalIgnoreCase) >= 0 ||
queryString.IndexOf(p.Nome, StringComparison.OrdinalIgnoreCase) >= 0 ||
queryString.IndexOf(p.Cognom, StringComparison.OrdinalIgnoreCasee) >= 0)
.ToList();
Why?
LINQ uses deferred execution. This means it waits until you want to iterate over your query results before it does anything. There are 3 main types of LINQ:
LINQ to Objects - when your IEnumerable is already on the heap.
LINQ to Entities - when you want to query a database using Entity Framework.
LINQ to SQL - when you want to query a database using LINQ to SQL.
Deferred execution in the context of the second 2 means that your query is not executed on the database until you enumerate the results in a foreach block, or invoke an enumeration method like .ToList, .ToArray, etc. Until then, your query is just stored as expression trees in memory.
Your query would work just peachy if db.User was a collection in memory. However when the data is in a database, LINQ to Entities (or LINQ to SQL) must translate your expression trees to what it calls a "store expression" -- which is just fancy talk for "convert my LINQ expressions to SQL".
Now imagine you had a custom C# algorithm you wanted to use for your query, and you did something like this:
var result = db.User.Where(x => MyCustomMethod(x));
There is no way today that LINQ to Entities can convert your C# code into a SQL query (store expression). It is the same with a lot of other C# methods you rely on daily. It also does not support .ToLower, .ToUpper, .StartsWith, .EndsWith, etc. There is a limited number of C# methods that can be converted to store expressions, and .IndexOf just happens to be one of them.
However keep in mind that it is only the string object's Contains method that we are talking about here that is not supported for store expressions. LINQ to Entities does support .Contains on IEnumerables. The following is valid and will work with LINQ to Entities (not sure about LINQ to SQL):
var idsIWantToFind = new[] { 1, 2, 3 };
var users = db.Where(x => idsIWantToFind.Contains(x.UserId));
The above is the equivalent of doing a SQL WHERE UserId IN (1, 2, 3) predicate.
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);
Here's a little LinqToSql GOTCHA:
// Returns the number of counties in a state,
// or all counties in the USA if the state is null
public static int CountCounties(State s) {
var q =
from cy in County.GetTable() // my method to get the ITable
where (s == null || s.Code == cy.StateCode) // shortcut OR operator, right...?
select cy;
return q.Count();
}
Guess what - if you pass a null State object to this method, you get a null reference exception! It seems that LinqToSql doesn't use the || shortcut operator as a shortcut!
Answer credit goes to whoever proposes the best explanation & workaround for this.
If it's linq to sql then remeber that Linq is just parsing your query into SQL.
It is therefore sending both of your where clauses to the database, hence the exception. I dont find this surprising really, though it is arguably wrong.
You will just have to do an independant check.
if (!string.isNullOrEmpty(state.statecode)
q = q.where( s => s.code == state.statecode
This is not related to LINQ in general. In this case, the LINQ-to-SQL provider tries to parse the your lambda expression and make it a TSQL query. It cannot make too many assumptions based on your expression since it's trying to delegate most of the work to the database.
Long story short, the provider simply cannot translate it to SQL.