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();
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 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 })
I have several entity objects for eg. Customer, Orders which derive from IComparable
and all all mapped to database fields.
I bind the grid at runtime as a List<Customer>, List<Orders> etc.
I am writing a custom column class
where I can get Parent.DataSource (it would always be List<>) but the actual type is unknown. I need to convert that to a list type (maybe IList) so I could write linq queries against the datasource.
something like
IList t = Parent.DataSource as IList
var qry = from cl in t
You should be able to convert your Parent.DataSource into the appropriate type via LINQ's Cast() method, and query against it:
var query = from customer in Parent.DataSource.Cast<Customer>()
where customer.Foo == "Bar"
select customer;
you can use Cast in Linq .
var query = from customers in Parent.DataSource.Cast<Customer>()
select customers;
Cast<Customer> will convert your Parent.DataSource to your corresponding Customer entity
Is this linq query efficient?
var qry = ((from member in this.ObjectContext.TreeMembers.Where(m => m.UserId == userId && m.Birthdate == null)
select member.TreeMemberId).Except(from item in this.ObjectContext.FamilyEvents select item.TreeMemberId));
var mainQry = from mainMember in this.ObjectContext.TreeMembers
where qry.Contains(mainMember.TreeMemberId)
select mainMember;
Will this be translated into multiple sql calls or just one? Can it be optimised? Basically I have 2 tables, I want to select those records from table1 where datetime is null and that record should not exist in table2.
The easiest way to find out if the query will make multiple calls is to set the .Log property of the data context. I typically set it to write to a DebugOutputWriter. A good example for this kind of class can be found here.
For a general way of thinking about it however, if you use a property of your class that does not directly map to a database field in a where clause or a join clause, it will typically make multiple calls. From what you have provided, it looks like this is not the case for your scenario, but I can't absolutely certain and suggest using the method listed above.
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));
}
}