I am trying to bind distinct records to a dropdownlist. After I added distinct function of the linq query, it said "DataBinding: 'System.String' does not contain a property with the name 'Source'. " I can guarantee that that column name is 'Source'. Is that name lost when doing distinct search?
My backend code:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
var query = (from p in dc.Promotions
select p.Source).Distinct();
return query;
}
Frontend code:
PromotionDAL dal = new PromotionDAL();
ddl_Source.DataSource = dal.GetAllSource();
ddl_Source.DataTextField = "Source";
ddl_Source.DataValueField = "Source";
ddl_Source.DataBind();
Any one has a solution? Thank you in advance.
You're already selecting Source in the LINQ query, which is how the result is an IQueryable<string>. You're then also specifying Source as the property to find in each string in the databinding. Just take out the statements changing the DataTextField and DataValueField properties in databinding.
Alterantively you could remove the projection to p.Source from your query and return an IQueryable<Promotion> - but then you would get distinct promotions rather than distinct sources.
One other quick note - using query syntax isn't really helping you in your GetAllSources query. I'd just write this as:
public IQueryable<string> GetAllSource()
{
PromotionDataContext dc = new PromotionDataContext(_connString);
return dc.Promotions
.Select(p => p.Source)
.Distinct();
}
Query expressions are great for complicated queries, but when you've just got a single select or a where clause and a trivial projection, using the dot notation is simpler IMO.
You're trying to bind strings, not Promotion objects... and strings do not have Source property/field
Your method returns a set of strings, not a set of objects with properties.
If you really want to bind to a property name, you need a set of objects with properties (eg, by writing select new { Source = Source })
Related
//list is IEnumeable NOT IEnumerable<T>
var IEnumerable<object> = list.AsQueryable().Cast<object>().Select(x=> .........);
object actually has a POCO underlying Anonymous class e.g
AccountId,Name,SecretInfo
What I want in the select statement is
AccountId = x.GetType().GetProperty("AccountId").GetValue(x,null),
Name = x.GetType().GetProperty("Name").GetValue(x,null)
Also I want to hide the SecretInfo Column which I can pass as a hardcoded string "SecretInfo"
Basically the select list needs to be built up dynamically on the Anonymous type....
How can this be done....Any Linq punters out there who can help me?
The answer to your question relies on anonymous types. The following code is what you can use:
var result = list.AsQueryable().Cast<Info>().Select(x => new
{
AccountId = x.AccountId,
Name = x.Name
});
Between the brackets that follow the new keyword in the select statement, you are creating an anonymous type that will have two implicitly typed read-only fields (AccountId and Name). Hope this helps!
I would like to post this quote from the linked (no pun intended) article:
Anonymous types typically are used in the select clause of a query expression to return a subset of the properties from each object in the source sequence. For more information about queries, see LINQ Query Expressions (C# Programming Guide).
I am using Northwind Customers Table where I fill the dataset and get the datatable.
I am trying to use dynamic linq and want to select columnName dynamically
var qry = MyDataTable.AsEnumerable().AsQueryable().Select("new(Country)");
Right now I have hard coded country but even then I get this error
No property or field 'Country' exists in type 'datarow'
I would like to eventually change this query to take the column name dynamically.
Please help!!! thanks.
The important hint is here (in bold):
No property or field 'Country' exists
in type 'datarow'
The extension method AsEnumerable of the DataTable class returns an IEnumerable<T> where T has the type DataRow. Now the Select method of Dynamic LINQ wants to work with this type DataRow which hasn't a property Country of course.
You could try this instead:
var qry = MyDataTable.AsEnumerable().AsQueryable()
.Select("new(it[\"Country\"] as CountryAlias)");
it now represents a variable of type DataRow and you can use methods of this type and perhaps also the indexer in my example above. (Dynamic LINQ supports accessing array elements by an integer index, but I am not sure though if accessing an indexer with a string key will work.)
I've used Slauma's answer and it worked. In addition i was doing OrderBy with dynamic linq maybe this will help to someone. I'll just drop the code here.
string dynamicLinqText = $"it[\"{sortColumnName}\"] {sortDirection}"; //it["PERSON_NAME"] asc
result = result.AsEnumerable().OrderBy(dynamicLinqText).CopyToDataTable();
I want to create an Entity Object from a LinQ statement, but I don't want to load all its columns.
My ORDERS object has a lot of columns, but I just want to retrieve the REFERENCE and OPERATION columns so the SQL statement and result will be smaller.
This LinQ statement works properly and loads all my object attributes:
var orders = (from order in context.ORDERS
select order);
However the following statement fails to load only two properties of my object
var orders = (from order in context.ORDERS
select new ORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The error thrown is:
The entity or complex type
'ModelContextName.ORDERS' cannot be
constructed in a LINQ to Entities
query.
What is the problem? Isn't it possible to partially load an object this way?
Thank you in advance for your answers.
ANSWER
Ok I should thank you both Yakimych and Dean because I use both of your answers, and now I have:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
})
.AsEnumerable()
.Select(o =>
(ORDERS)new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
}
).ToList().AsQueryable();
And I get exactly what I want, the SQL Statement is not perfect but it returns only the 2 columns I need (and another column which contains for every row "1" but I don't know why for the moment) –
I also tried to construct sub objects with this method and it works well.
No, you can't project onto a mapped object. You can use an anonymous type instead:
var orders = (from order in context.ORDERS
select new
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION
});
The problem with the above solution is that from the moment you call AsEnumerable(), the query will get executed on the database. In most of the cases, it will be fine. But if you work with some large database, fetching the whole table(or view) is probably not what you want. So, if we remove the AsEnumerable, we are back to square 1 with the following error:
The entity or complex type 'ModelContextName.ORDERS' cannot be constructed in a LINQ to Entities query.
I have been struggling with this problem for a whole day and here is what I found. I created an empty class inheriting from my entity class and performed the projection using this class.
public sealed class ProjectedORDERS : ORDERS {}
The projected query (using covariance feature):
IQueryable<ORDERS> orders = (from order in context.ORDERS
select new ProjectedORDERS
{
REFERENCE = order.REFERENCE,
OPERATION = order.OPERATION,
});
Voilà! You now have a projected query that will map to an entity and that will get executed only when you want to.
I think the issue is creating new entities within the query itself, so how about trying this:
context.ORDERS.ToList().Select(o => new ORDERS
{
REFERENCE = o.REFERENCE,
OPERATION = o.OPERATION
});
I am trying to create a LINQ query to return genres by movieid. The LINQ works in LINQPAD4. Can someone help me with the proper syntax? I am getting the following errors:
Cannot implicitly convert type 'System.Linq.IQueryable' to 'System.Linq.IQueryable'. An explicit conversion exists (are you missing a cast?)
and
Cannot implicitly convert type 'System.Collections.Generic.List' to 'System.Collections.Generic.List'
Code:(note I have wrapped Title in the following line with parenthesis, but are actually brackets in my code.
public List(Genre) GetGenresByMovieId(string movieid)
{
var genres = from t in MovieCatalog.Titles
where t.Id == "BVlLx"
select t.Genres;
return genres.ToList();
}
The right query would look like
public IEnumerable<Genre> GetGenresByMovieId(string movieId)
{
return from title in ctx.Titles
from genre in title.Genres
where title.Id == "BVlLx"
select genre;
}
In the method call syntax, you need to use SelectMany, not Select, since the filter on titles returns a list of titles (which will always contain just one title, but the compiler doesn't know that) and so you want to "concatenate" all genres for each title in the results.
The return type is actually IQueryable, but if you only plan to enumerate over it, you can use IEnumerable, or call ToList() to force execution right there in the method (the way I wrote it the query would actually execute only once you try to enumerate it).
Your problem is your projection:
select new { Name = g.Name }
That is projecting the query into an anonymous type.
You need to project into the IQueryable you have declared (IQueryable<Genre>)
When working with LINQ queries, it's preferable to use implicitly-typed variables (var).
Also, not sure why you have that extra "from" in your query, you don't need that.
Something like this should work:
var genres = from t in MovieCatalog.Titles
where t.Id = "BVlLx"
select t.Genres;
return genres.ToList();
var genres should be typed to an IQueryable<Genre>.
That is assuming you want to return a collection of Genre objects.
If you just want the name, do this:
select t.Genres.Name
But that will return a collection of string objects (and var genres should be typed to an IQueryable<string>).
However, i have no idea about the NetFlix OData API, but that should get you on the right track.
I am trying to figure out how to dynamically specify the properties for my select clause in a linq query.
Lets say I have a collection of employee objects. At run time, the end user will be specifying which properties they would like to see for those employees, so I need to be able to dynamically construct my Linq select clause.
I have used the dynamic Linq library, but I prefer not to use that, because it requires me to build a string to pass to the select method. I'd like to understand how to do this via Expressions.
This looks like something that fits more with your requirements of not using dynamic linq.
Use Reflection to get the dynamic Column Values
//columns variable has column name as comma separated String which you
can save in DB //example string columns ="Name,Id,Age";
var strColumns =columns.split(,);
foreach(var myObject in MyObjectcollection)
{
for(int index =0;index<strColumns.count();index++)
{
//Create a collection of objects
mycollection.add(myObject.GetType().GetProperty(strColumns[index]).GetValue(myObject, null));
}
}