Expression Tree Tutorial - Knowledge Base (KB)
66 results in tag: entity-framework
What is the best way to create a Linq Expression Tree that compares to a generic object?
I have an IQueryable and an object of type T....I want to do IQueryable().Where(o => o.GetProperty(fieldName) == objectOfTypeT.GetProperty(fieldName))...so ......public IQueryable<T> DoWork<T>(string fieldName) where T : EntityObject { ... T...
asked by JTew
Within a generic extension method, how do I apply OrderBy to an IQueryable using a string column name?
public static IQueryable<TResult> ApplySortFilter<T, TResult>(this IQueryable<T> query, string columnName) where T : EntityObject { var param = Expression.Parameter(typeof(T), "o"); var body = Expression.PropertyOrField(param,columnName); var sor...
asked by JTew
Where Predicate Type Arguments in C#
I have an XElement with values for mock data. ...I have an expression to query the xml:...Expression<Func<XElement, bool>> simpleXmlFunction = b => int.Parse(b.Element("FooId").Value) == 12; ...used in:...var simpleXml = xml.Elements("Foo").Where(si...
asked by blu
To pick an anonymous type, establish a LINQ Expression Tree.
I would like to generate the following select statement dynamically using expression trees:...var v = from c in Countries where c.City == "London" select new {c.Name, c.Population}; ...I have worked out how to generate...var v = from c in ...
asked by Tom Deloford
How may Entity Framework be wrapped to intercept LINQ expressions right before they are executed?
I want to rewrite certain parts of the LINQ expression just before execution. And I'm having problems injecting my rewriter in the correct place (at all actually)....Looking at the Entity Framework source (in reflector) it in the end comes down to the ...
asked by Davy Landman
When invoking a method-provided expression on a member property, LinqKit throws an InvalidCastException.
Given a simple parent/child class structure. I want to use linqkit to apply a child lambda expression on the parent. I also want the Lambda expression to be provided by a utility method....public class Foo { public Bar Bar { get; set; } } public cl...
asked by user293499
Expression Tree's like operator
I have a Linq extension method to dynamically filter Linq queries using string values. For example: ...query.WhereHelper("columName", ">", 1).... I could use many different filter operators like GreaterThan or NotEqual etc. but not "Like". There is no Exp...
asked by dstr
Unable to construct a constant variable of type †System.Objectâ€TM in Entity Framework - Linq. There are only primitive kinds.
I have a method to build an expression for a linq query for a given type, property, and value. This works wonderfully as long as the property on the type is NOT nullable. Here is the example I am working from (...http://www.marcuswhitworth.com/2009/12/dyn...
asked by Shane
I'm dialing the number. Any Entity Framework Extension Method that uses Expression Trees
I have looked at few examples here ...Calling a Method from an Expression... and on ...MSDN... but I have not been able to get the right method call/object type for Any() for the query below. I seem to be able to get the property call but not IEnumerable ...
asked by RCrabtree
How can I generate an expression tree for a query by reflecting over T?
I'm trying to build a generic class to work with entities from EF. This class talks to repositories, but it's this class that creates the expressions sent to the repositories. Anyway, I'm just trying to implement one virtual method that will act as a base...
asked by Gup3rSuR4c
For the maximum date value, an expression tree must be built.
I'm trying to build an expression tree for this linq query: so I can pass in a generic Entity:...this.EntityCollection.Select((ent) => ent.TimeStamp).Max() ...I am wanting to create a class that takes a generic Entity and finds the max of its TimeStamp pr...
asked by Adam
How do you put this together: selector + predicate?
Assume that we have two classes...public class EntityA { public EntityB EntityB { get; set; } } public class EntityB { public string Name { get; set; } public bool IsDeleted { get; set; } } ...And two expressions for selector and predicator..
asked by qmicron
How to use Expression Tree to build dynamic conditions
Please consider this code:...System.Linq.Expressions.Expression<Func<tbl, bool>> exp_details = r => r.ID_Master == Id && r.Year == Year && r.Month == Month ; ...I want to write a function that expect some argument, and then retrieve some data from my data...
asked by Arian
Complex mappings are used to translate an expression tree from one type to another.
inspired by ...this answer... I'm trying to map a property on a model class to an expression based on the actual entity. These are the two classes involved:...public class Customer { public string FirstName { get; set; } public string LastName { g...
asked by Kralizek
Generic expression for where clause - "The LINQ expression node type 'Invoke' is not supported in LINQ to Entities."
I am trying to write a really generic way to load EF entities in batches, using the Contains method to generate a SQL IN statement. I've got it working if I pass the entire expression in, but when I try to build the expression dynamically, I am getting a...
asked by Randar Puust
Build a nested lambda Expression Tree for an Entity Framework Many to Many relationship?
I am trying to build an Entity Framework 4 Many to Many relationship filter, this is for a Dynamic Data project. I know that I need to build an Expression Tree at run time, and I am familiar with doing this for something like a simple where expression li...
asked by Brian Tax
How to convert from Expression<Func<DomainType>>predicate to Expression<Func<DTOtype>predicate
Some background: I have in my application multiple layers, two of which are a domain layer and an infrastructure layer which serves as my DAL. In the domain layer, I have implemented a generic repository pattern as such:... public interface IRepository...
asked by John Pool
Generate dynamic select lambda expressions
I am somewhat new to expression trees and I just don't quite understand some things....What I need to do is send in a list of values and select the columns for an entity from those values. So I would make a call something like this:...DATASTORE<Contact> d...
asked by cjohns
List of Expression<Func<T, TProperty>>
I'm searching a way to store a collection of ...Expression<Func<T, TProperty>>... used to order elements, and then to execute the stored list against a ...IQueryable<T>... object (the underlying provider is Entity Framework). ...For example, I would like...
asked by Bidou
Best way to concat strings and numbers in SQL server using Entity Framework 5?
For some reason Microsoft decided to not support simple concat in EF5....e.g....Select(foo => new { someProp = "hello" + foo.id + "/" + foo.bar } ...This will throw if foo.id or foo.bar are numbers....The workaround I'v...
asked by Roger Johansson
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