I have some table and the following condition of query: if parameter A is null take all, if not, use it in the query. I know how to do that in 2 steps:
List<O> list = null;
if (A = null)
{
list = context.Obj.Select(o => o).ToList();
}
else
{
list = context.Obj.Where(o.A == A).ToList();
}
Is it possible to have the same as one query?
Thanks
How about:
list = context.Obj.Where(o => A == null || o.A == A)
.ToList();
You can do it in one query but still using a condition:
IEnumerable<O> query = context.Obj;
if (A != null)
{
query = query.Where(o => o.A == A);
}
var list = query.ToList();
Or you could use a conditional operator to put the query in a single statement:
var query = A is null ? context.Obj : context.Obj.Where(o => o.A == A);
var list = query.ToList();
I would personally suggest either of the latter options, as they don't require that the LINQ provider is able to optimise away the filter in the case where A is null. (I'd expect most good LINQ providers / databases to be able to do that, but I'd generally avoid specifying a filter when it's not needed.)
I opted for
var list = context.Obj.Where(o => A.HasValue ? o.a == A : true);
I would probably write the query like this:
IQueryable<O> query = context.Obj;
if (A != null)
query = query.Where(o => o.A == A);
var list = query.ToList()
It's not one expression, but I think it's quite readable.
Also, this code assumes that context.Obj is IQueryable<O> (e.g. you are using LINQ to SQL). If that's not the case, just use IEnumerable<O>.
Related
I am relatively new to LINQ but looking for some "best practice" advice on how to deal with the following. I know there are many ways to deal with this, but looking to see how more experienced people would write the code.
My LINQ at present:
var company = (from c in db.RPTINQUIRies
where c.CONCOM == concom && c.LOPER == engineer
orderby c.CREATION_DATE descending
select c);
Now the ActionResult parameter that is being passed in here (engineer) may or may not be empty. Where it is empty, I essentially want to remove the && C.LOPER == engineer clause all together.
What's the best way to deal with this?
It sounds like you just want:
where c.CONCOM == concom && (engineer == null || c.LOPER == engineer)
Alternatively, you could build up the query step by step:
var query = db.RPTINQUIRies.Where(c => c.CONCOM == concom);
if (engineer != null)
{
query = query.Where(c => c.LOPER == engineer);
}
query = query.OrderByDescending(c => c.CREATION_DATE);
I have a question regarding a Linq to SQL query.
I have following situation:
I have a search with lots of options, like location, availability, name, language etc ...
For this options i have to execute a query to retrieve the results according to options selected, how can i best do it, i cannot write a linq query like for each possibility and combination of options, but i cannot write one for all of them as it will not work, for example:
from p in context.people where p.location==model.location && p.availability==model.availability .... select p
In this case imagine availability is not selected and should not be searched for, but in this case it will be passed as false, or if location is not set and is null so it will only search for empty locations, although i just need all.
So my question is how do people handle this kind of behaviour with queries?
As you long as you do not execute the linq query immediately you can just add where clauses to it. You can do this for example:
var query = from p in context.people;
if(searchOnLocation)
{
query = query.where(p => p.location == model.location);
}
if(otherSearch)
{
query = query.where(p => p.someOtherProperty == someotherValue);
}
var result = query.ToList();
As long you don't call ToList() on your IQueryable, the linq will not be translated into SQL. It's only in the last call, that the linq will be translated and executed against the database
IQueryable<Person> query = context.people;
if(model.location != null)
query = query.Where(x => x.location == model.location);
if(model.availability != null)
query = query.Where(x => x.availability == model.availability);
// etc
Basically, you can compose more and more restrictions as you go.
If you want to implement query without if condition than you can use following syntax:
var query = context.people.
where(p => p.location == (model.location ?? p.location)
&& p.availability == (model.availability ?? p.availability))
.ToList();
I am trying to build a method in my asp.net WebAPI to grab data based on the arguments passed on the method. The method is used to perform a search on restaurant data. I have a variable called 'type' that determines the type of data search performed. The second variable 'keyword' is the keyword searched by the user. The WHERE condition in my LINQ query depends on the type and needs to be dynamic, so I have used a separate variable outside the LINQ query to define the condition. I have tried assigning this variable to my WHERE statement on the LINQ query but it doesn't seem to work. Can someone help with it please? I have been stuck on this for a few days now
public IQueryable<RestaurantView> GetRestaurantsForSearch(string keyword, int type, string location)
{
//
var condition = "";
if(type == 1)
{
condition = "x.RestaurantName.Contains(keyword)";
} else if(type == 2){
condition = "x.Cuisine.Equals(keyword)";
}
else {
condition = "x.Rating.Equals(keyword)";
}
var query = from x in db.Restaurants
join y in db.Cuisine on x.RestaurantCuisine equals y.CuisineID
where condition
select new RestaurantView
{
RestaurantID = x.RestaurantID,
RestaurantName = x.RestaurantName,
RestaurantCuisine = y.CuisineName,
RestaurantDecription = x.RestaurantDecription
};
return query;
}
Try this:
Predicate<Restaurant> pred;
if (type == 1) pred = x => x.RestaurantName.Contains(keyword);
else if (type == 2) pred = x => x.Cuisine.Equals(keyword);
else pred = x => x.Rating.Equals(keyword);
var query = from x in db.Restaurants
join y in db.Cuisine on x.RestaurantCuisine equals y.CuisineID
where pred(x)
select new RestaurantView
{
RestaurantID = x.RestaurantID,
RestaurantName = x.RestaurantName,
RestaurantCuisine = y.CuisineName,
RestaurantDecription = x.RestaurantDecription
};
return query;
You need to look a dynamic linq library i think then you can execute string statements inside your linq
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
or you can execute direct query
http://msdn.microsoft.com/en-us/library/system.data.linq.datacontext.executequery.aspx
If you are ok with dropping your comprehensive LINQ query in favour of the extension method syntax, it's pretty simple (I'm on a netbook without VS, so I apologize that this is untested but should give you the idea):
var query = db.Restaurants
.Include("Cuisine")
if(type == 1)
{
query= query.Where(x => x.RestaurantName.Contains(keyword));
}
else if(type == 2)
{
query = query.Where(x => x.Cuisine == keyword);
}
else {
query = query.Where(x => x.Rating == keyword);
}
This builds out your expression tree differently based on your logic checks, which will result in a different SQL query being generated based on the value of type.
I notice that in your join, Cuisine appears to be an Entity, but in your logic checks, you attempt to filter by comparing Cuisine to a string so I think there is some disconnect.
var query = from x in db.Restaurants
join y in db.Cuisine on x.RestaurantCuisine equals y.CuisineID
where condition
select new RestaurantView
{
RestaurantID = x.RestaurantID,
RestaurantName = x.RestaurantName,
RestaurantCuisine = y.CuisineName,
RestaurantDecription = x.RestaurantDecription
};
return query;
}
how to get the return query value in client side to assign for grid view binding
If I want to append a AND statement to my query, I can do:
query = query.Where(obj=>obj.Id == id);
if(name.HasValue)
query = query.Where(obj=>obj.Name == name);
and it will give me:
query.Where(obj=>obj.Id == id && obj.Name == name)
How can I append a OR statement that will result in:
query.Where(obj=>obj.Id == id || obj.Name == name)
You can't do it natively. However, you can use PredicateBuilder to compose the query before you run it, and it supports ORs.
var predicate = PredicateBuilder.False<Product>();
predicate = predicate.Or (obj=>obj.Id == id);
if(name.HasValue) predicate = predicate.Or (obj=>obj.Name == name);
return query.Where(predicate);
Simply this if I'm not missing something:
query.Where(obj=>obj.Id == id || (obj.Name == name && name.HasValue))
You might want to read this question (my question...) and answer for more complicated scenarios:
How to filter IEnumerable based on an entity input parameter
I would just build this into a single condition:
if (name.HasValue)
query = query.Where(obj=>obj.Id == id && obj.Name == name);
else
query = query.Where(obj=>obj.Id == id);
I would use gdoron's solution, but if it seems unpractical for larger sets of queries, a slightly more complicated solution containing set operations might help you:
var queryById = query.Where(obj => obj.Id == id);
var queryByName = query.Where(obj => obj.Name == name);
query = queryById.Union(queryByName);
It gets much more difficult if your original query contains duplicate items.
Another way may be using Expression to formulate your queries. You can modify the expression tree before executing it, so you can add more conditions to the Where sub-tree. That is a whole lot of work and it's an overkill on 99.9% (rough estimate :) ) of cases.
Part of the problem is that you over write your original query. Your OR operation would be equivalent to:
subquery1 = query.Where(obj=>obj.Id == id);
subquery2 = query.Where(obj=>obj.Name == name);
query = subquery1.Concat(subquery2).Distinct();
As you can see, that becomes pretty unwieldy as well, however if You are going to do this form, then you must maintain the original sequence so that you can get both the right and left sides of the equation processed then you need to be sure duplicates get removed.
Instead of all that, I would try to figure a way to build up that conditional statement dynamically using lambdas e.g.
I haven't actually run this, but something like this should work.
var cond = obj=>obj.Id == id;
...
// need an or
cond = obj=>cond(obj) || obj.Name == name;
query = query.Where(obj=>cond(obj));
Hope this gives you an idea.
I have a scenario where I have to use a dynamic where condition in LINQ.
I want something like this:
public void test(bool flag)
{
from e in employee
where e.Field<string>("EmployeeName") == "Jhom"
If (flag == true)
{
e.Field<string>("EmployeeDepartment") == "IT"
}
select e.Field<string>("EmployeeID")
}
I know we can't use the 'If' in the middle of the Linq query but what is the solution for this?
Please help...
Please check out the full blog post: Dynamic query with Linq
There are two options you can use:
Dynamic LINQ library
string condition = string.Empty;
if (!string.IsNullOrEmpty(txtName.Text))
condition = string.Format("Name.StartsWith(\"{0}\")", txtName.Text);
EmployeeDataContext edb = new EmployeeDataContext();
if(condition != string.empty)
{
var emp = edb.Employees.Where(condition);
///do the task you wnat
}
else
{
//do the task you want
}
Predicate Builder
Predicate builder works similar to Dynamic LINQ library but it is type safe:
var predicate = PredicateBuilder.True<Employee>();
if(!string.IsNullOrEmpty(txtAddress.Text))
predicate = predicate.And(e1 => e1.Address.Contains(txtAddress.Text));
EmployeeDataContext edb= new EmployeeDataContext();
var emp = edb.Employees.Where(predicate);
difference between above library:
PredicateBuilder allows to build typesafe dynamic queries.
Dynamic LINQ library allows to build queries with dynamic Where and OrderBy clauses specified using strings.
So, if flag is false you need all Jhoms, and if flag is true you need only the Jhoms in the IT department
This condition
!flag || (e.Field<string>("EmployeeDepartment") == "IT"
satisfies that criterion (it's always true if flag is false, etc..), so the query will become:
from e in employee
where e.Field<string>("EmployeeName") == "Jhom"
&& (!flag || (e.Field<string>("EmployeeDepartment") == "IT")
select e.Field<string>("EmployeeID")
also, this e.Field<string>("EmployeeID") business, smells like softcoding, might take a look into that. I guess
from e in employee
where e.EmployeeName == "Jhom"
&& (!flag || (e.EmployeeDepartment == "IT")
select e.EmployeeID
would be more compact and less prone to typing errors.
EDIT: This answer works for this particular scenario. If you have lots of this kinds of queries, by all means investingate the patterns proposed in the other answers.
You can chain methods :
public void test(bool flag)
{
var res = employee.Where( x => x.EmployeeName = "Jhom" );
if (flag)
{
res = res.Where( x => x.EmployeeDepartment == "IT")
}
var id = res.Select(x => x.EmployeeID );
}
from e in employee
where e.Field<string>("EmployeeName") == "Jhom" &&
(!flag || e.Field<string>("EmployeeDepartment") == "IT")
select e.Field<string>("EmployeeID")
You can call LINQ methods explicitly and chain them conditionally.
public IEnumerable<string> FilterEmployees (IEnumerable<Employee> source, bool restrictDepartment)
{
var query = source.Where (e => e.Field<string>("EmployeeName") == "Jhom");
if (restrictDepartment) // btw, there's no need for "== true"
query = query.Where (e => e.Field<string>("EmployeeDepartment") == "IT");
return query.Select (e => e.Field<string>("EmployeeID"));
}