Expression Tree Tutorial - Knowledge Base (KB)
21 results in tag: c#-4.0
Obtaining a parameterExpression's run-time value in an expression tree
I am missing the obvious: How do I access the value of a parameter inside a lambda expression expression tree? ...Scenario: For a delegate x I dynamically create a lambda expression with an expression tree body which has the same signature as the delegate...
asked by dalo
What is the purpose of ExpressionVisitor.VisitT>?
Before someone shouts out the answer, please read the question through....What is the purpose of the method in .NET 4.0's ExpressionVisitor:...public static ReadOnlyCollection<T> Visit<T>(ReadOnlyCollection<T> nodes, Func<T, T> elementVisitor) ...My first...
asked by Jeff
In this expression tree, why am I getting a null reference exception?
I have a tree expression that looks like this:....Block( System.Object $instance, MyType2 $result) { $result = (MyType2)((MyType1)$instance).Property1; .Goto return { }; .Label .LabelTarget useDefault:; $result = .Default(MyTyp...
asked by Sandor Drieënhuizen
Expression trees in C# 4 are "dynamic."
I'm trying to figure out how to put all the pieces together, and would appreciate a concrete source code sample for a simple case to start with....Consider the following C# code:...Func<int, int, int> f = (x, y) => x + y; ...I can produce an equivalent fu...
asked by Pavel Minaev
"Ambiguous Match Found" with Expression.Call
I'm trying to write an expression that will call ToString on a property and assign it's value to a local variable. However, calling ToString on a object instance w/ an overload of ToString, causes an exception of "Ambigous Match Found" to be thrown. Here'...
asked by James Alexander
Linq to SQL Expression to Expression Tree Conversion
Can anyone convert this simple LINQ-to-SQL to an Expression Tree:...List<Region> lst = (from r in dc.Regions where r.RegionID > 2 && r.RegionDescription.Contains("ern") select r).ToList();
asked by Arian
Personalized expression
I found this example code:...public IQueryable<T> Get<T>(ObjectSet<T> obj) where T : class { Type type = typeof(T); var x = type.GetInterface("IMyInterface"); if (x != null) { var property = type.GetProperty("MyStringField"); ...
asked by bit
How can I acquire the value of a local variable-based ConstantExpression?
I created an ExpressionVisitor implementation that overrides VisitConstant. However, when I create an expression that utilizes a local variable I can't seem to get the actual value of the variable. ...public class Person { public string FirstName { ge...
asked by devlife
Interval grouping using dynamic linq (DateTime, Numeric)
I search everywhere and didn`t find anwser for this question. I want to group by intervals (DateTime, Numeric) in Dynamic linq (the data will be crated dynamically so i must use dynamic linq)...Lets assume that we have such data:...ID|Date|Price 1|2010-1...
asked by WebDeveloper
ERROR A static method needs a null instance, while a non-static method does not.
I am trying to create an expression tree. I need to read data from a data table and check its columns. The columns to be checked and also the number of columns to be checked are known at run time only. The column names are given to me as a string array a...
asked by Jani5e
In ExpressionVisitor, how do you evaluate an expression?
I need to use ExpressionVisitor to analyse an Expression before executing it. For my needs, i need to evaluate the right part of a Divide expression but i don't know how to do it. Here's a sample code that i have:...internal class RulesChecker : Expressio...
asked by Alexandre Jobin
To use in expression trees, convert Funcstring, bool> to bool.
I've got an overly complicated binary expression tree building system It takes in a string and a pair of objects (Player and World)...Each node on the tree represents an external function that takes a string, player and world, and either returns a bool (...
asked by bigjokerfish
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
Why can't a named argument specification be found in an expression tree?
Using AutoMapper, I hit a place where a named argument would've fit very nicely:....ForMember(s => s.MyProperty, opt => opt.MapFrom(s => BuildMyProperty(s, isAdvanced: false))) ...But the compiler yelled at me:...An expression tree may not contain a named...
asked by Brandon Linton
dynamic sort in linq
please consider this scenario:...I have a list of a class with about 50 fields.I want to have a Combobox that user can select according to what field list will sort.For example if user select "F1" list sort according to "F1"....I don't want to sort with ....
asked by Arian
Trying to filter on a Nullable type using Expression Trees
I have pasted my entire test app below. It's fairly compact so I am hoping that it's not a problem. You should be able to simply cut and paste it into a console app and run it. ...I need to be able to filter on any one or more of the Person objects' prope...
asked by Gary O. Stenstrom
How to convert Func<T,bool> to Expression<Func<T,bool>>
I have a Func like this :... Func<MyClass, bool> func = x=>Id == 5; ...How I can convert it to :... Expression<Func<MyClass, bool>>
asked by Arian
Expression Trees with dynamic parameter
I want to convert this:...Func<dynamic, object> myFunc = t => return t.Name + " " + t.Surname; ...Into an Expression Tree....What I have came up with, is this:...ParameterExpression target = ExpressionParameter(typeof(dynamic), "target"); ParameterExpress...
asked by Matias Cicero
How to Convert Predicate to String and String To Predicate
I work on a special project that have to convert Predicates (or Expressions) to string and store on the database and retrieve it and convert it to Predicates and evaluate it because I want to change it in run-time. Please help me to imeplement ConvertStr...
asked by ArMaN
Making a lambda expression to call a method from a generic class
I have these simple interface:...public interface IQuery<TResult> { } public interface IQueryHandler<in TQuery, out TResult> where TQuery : IQuery<TResult> { TResult Handle(TQuery query); } ...And there are some implementation of them. I'm tryin...
asked by ravy amiry
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