Expression Tree Tutorial - Knowledge Base (KB)
20 results for: in tag: c#-4.0
Getting run-time value of a ParameterExpression in a 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 does ExpressionVisitor.Visit<T> Do?
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
Why do I get a null reference exception in this expression tree?
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
C# 4 "dynamic" in expression trees
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
Expression.Call and "Ambiguous Match Found"
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
Convert Linq to Sql Expression to Expression Tree
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
Custom 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 to get the value of a ConstantExpression which uses a local variable?
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
ERROR Static method requires null instance, non-static method requires non-null instance
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
how to evaluate an Expression inside ExpressionVisitor?
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
Func<string, bool> to bool for use in expression trees
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 create dynamic condition using Expression Tree
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 an expression tree contain a named argument specification?
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 javad amiry
Create a Lambda Expression With 3 conditions
I want to create a lambda expression dynamically for this:...(o => o.Year == year && o.CityCode == cityCode && o.Status == status) ...and I write this:...var body = Expression.AndAlso( Expression.Equal( Expression.Proper...
asked by Arian
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