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  • c# Delegates, Events and Lambda Expr for new students

    - by MarkP
    I've been asked by my pointy haired boss to educate our new co-ops (interns) in the ways of C#. I have roughly ~30mins to cover the topics of Delegates, Events and Lambda Expressions. The time restriction is rather tight and the topics are broad. Since I'm not a C# guru, I would like some hints and pointers. Since my time is short, what points should I cover with respect to the three topics listed above? What are some good Do's and Dont's when using those three things? I might have time for a short Lambda Expr demo. What is the most common use of LExpr (probably a Select().Where() statement on an enumerable??) that I could demo? Thanks. EDIT: The students have working knowledge of C++ and Java.

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  • Lambda "if" statement?

    - by AndyC
    I have 2 objects, both of which I want to convert to dictionarys. I use toDictionary<(). The lambda expression for one object to get the key is (i = i.name). For the other, it's (i = i.inner.name). In the second one, i.name doesn't exist. i.inner.name ALWAYS exists if i.name doesn't. Is there a lambda expression I can use to combine these two? Basically to read as: "if i.name exists then set id to i.name, else set id to i.inner.name". Many thanks.

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  • Multiple Conditions in Lambda Expressions at runtime C#

    - by Ryan
    Hi, I would like to know how to be able to make an Expression tree by inputting more than one parameter Example: dataContext.Users.Where(u => u.username == "Username" && u.password == "Password") At the moment the code that I did was the following but would like to make more general in regards whether the condition is OR or AND public Func<TLinqEntity, bool> ANDOnlyParams(string[] paramNames, object[] values) { List<ParameterExpression> paramList = new List<ParameterExpression>(); foreach (string param in paramNames) { paramList.Add(Expression.Parameter(typeof(TLinqEntity), param)); } List<LambdaExpression> lexList = new List<LambdaExpression>(); for (int i = 0; i < paramNames.Length; i++) { if (i == 0) { Expression bodyInner = Expression.Equal( Expression.Property( paramList[i], paramNames[i]), Expression.Constant(values[i])); lexList.Add(Expression.Lambda(bodyInner, paramList[i])); } else { Expression bodyOuter = Expression.And( Expression.Equal( Expression.Property( paramList[i], paramNames[i]), Expression.Constant(values[i])), Expression.Invoke(lexList[i - 1], paramList[i])); lexList.Add(Expression.Lambda(bodyOuter, paramList[i])); } } return ((Expression<Func<TLinqEntity, bool>>)lexList[lexList.Count - 1]).Compile(); } Thanks

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  • C# and F# lambda expressions code generation

    - by ControlFlow
    Let's look at the code, generated by F# for simple function: let map_add valueToAdd xs = xs |> Seq.map (fun x -> x + valueToAdd) The generated code for lambda expression (instance of F# functional value) will looks like this: [Serializable] internal class map_add@3 : FSharpFunc<int, int> { public int valueToAdd; internal map_add@3(int valueToAdd) { this.valueToAdd = valueToAdd; } public override int Invoke(int x) { return (x + this.valueToAdd); } } And look at nearly the same C# code: using System.Collections.Generic; using System.Linq; static class Program { static IEnumerable<int> SelectAdd(IEnumerable<int> source, int valueToAdd) { return source.Select(x => x + valueToAdd); } } And the generated code for the C# lambda expression: [CompilerGenerated] private sealed class <>c__DisplayClass1 { public int valueToAdd; public int <SelectAdd>b__0(int x) { return (x + this.valueToAdd); } } So I have some questions: Why does F#-generated class is not marked as sealed? Why does F#-generated class contains public fields since F# doesn't allows mutable closures? Why does F# generated class has the constructor? It may be perfectly initialized with the public fields... Why does C#-generated class is not marked as [Serializable]? Also classes generated for F# sequence expressions are also became [Serializable] and classes for C# iterators are not.

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  • Trouble move-capturing std::unique_ptr in a lambda using std::bind

    - by user2478832
    I'd like to capture a variable of type std::vector<std::unique_ptr<MyClass>> in a lambda expression (in other words, "capture by move"). I found a solution which uses std::bind to capture unique_ptr (http://stackoverflow.com/a/12744730/2478832) and decided to use it as a starting point. However, the most simplified version of the proposed code I could get doesn't compile (lots of template mistakes, it seems to try to call unique_ptr's copy constructor). #include <functional> #include <memory> std::function<void ()> a(std::unique_ptr<int>&& param) { return std::bind( [] (int* p) {}, std::move(param)); } int main() { a(std::unique_ptr<int>(new int())); } Can anybody point out what is wrong with this code? EDIT: tried changing the lambda to take a reference to unique_ptr, it still doesn't compile. #include <functional> #include <memory> std::function<void ()> a(std::unique_ptr<int>&& param) { return std::bind( [] (std::unique_ptr<int>& p) {}, // also as a const reference std::move(param)); } int main() { a(std::unique_ptr<int>(new int())); }

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  • Use Lambda in Attribute constructor to get method's parameters

    - by Anthony Shaw
    I'm not even sure if this is possible, but I've exhausted all of my ideas trying to figure this out, so I figured I'd send this out to the community and see what you thought. And, if it's not possible, maybe you'd have some ideas as well. I'm trying to make an Attribute class that I can add to a method that would allow me to use a lambda expression to get each parameter of the method public ExampleAttribute : Attribute { public object Value { get; set; } public ExampleAttribute(--something here to make the lambda work--, object value) { Value = value; } } I'd like to be able to something like this below: [Example(x=x.Id, 4)] [Example(x=x.filter, "string value")] public ActionResult Index(int Id, string filter) { return View(); } I understand that I might be completely dreaming with this idea. I'm basically trying to write a model to allow for self-documenting REST API Documentation. In a recent project here at work we wrote a dozen or so services with 5 to 15 methods on each, I figure it's easier to write something to do this, than to hand code a documentation page for each. I would plan to eventually release this as an open-source project once I have it in a place that I feel it's releasable.

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  • Boost lambda: Invoke method on object

    - by ckarras
    I'm looking at boost::lambda as a way to to make a generic algorithm that can work with any "getter" method of any class. The algorithm is used to detect duplicate values of a property, and I would like for it to work for any property of any class. In C#, I would do something like this: class Dummy { public String GetId() ... public String GetName() ... } IEnumerable<String> FindNonUniqueValues<ClassT> (Func<ClassT,String> propertyGetter) { ... } Example use of the method: var duplicateIds = FindNonUniqueValues<Dummy>(d => d.GetId()); var duplicateNames = FindNonUniqueValues<Dummy>(d => d.GetName()); I can get the for "any class" part to work, using either interfaces or template methods, but have not found yet how to make the "for any method" part work. Is there a way to do something similar to the "d = d.GetId()" lambda in C++ (either with or without Boost)? Alternative, more C++ian solutions to make the algorithm generic are welcome too. I'm using C++/CLI with VS2008, so I can't use C++0x lambdas.

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  • Passing in a lambda to a Where statement

    - by sonicblis
    I noticed today that if I do this: var items = context.items.Where(i => i.Property < 2); items = items.Where(i => i.Property > 4); Once I access the items var, it executes only the first line as the data call and then does the second call in memory. However, if I do this: var items = context.items.Where(i => i.Property < 2).Where(i => i.Property > 4); I get only one expression executed against the context that includes both where statements. I have a host of variables that I want to use to build the expression for the linq lambda, but their presence or absence changes the expression such that I'd have to have a rediculous number of conditionals to satisfy all cases. I thought I could just add the Where() statements as in my first example above, but that doesn't end up in a single expression that contains all of the criteria. Therefore, I'm trying to create just the lambda itself as such: //bogus syntax if (var1 == "something") var expression = Expression<Func<item, bool>>(i => i.Property == "Something); if (var2 == "somethingElse") expression = expression.Where(i => i.Property2 == "SomethingElse"); And then pass that in to the where of my context.Items to evaluate. A) is this right, and B) if so, how do you do it?

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  • Lambda Functions in PHP aren't Logical

    - by Chacha102
    Note: I have condensed this article into my person wiki: http://wiki.chacha102.com/Lambda - Enjoy I am having some troubles with Lambda style functions in PHP. First, This Works: $foo = function(){ echo "bar"; }; $foo(); Second, This Works: class Bar{ public function foo(){ echo "Bar"; } Third, This works: $foo = new stdClass; $foo->bar = function(){ echo "bar"; }; $test = $foo->bar; $test(); But, this does not work: $foo = new stdClass; $foo->bar = function(){ echo "bar"; }; $foo->bar(); And, this does not work class Bar{ public function foo(){ echo "Bar"; } $foo = new Bar; $foo->foo = function(){ echo "foo"; }; $foo->foo(); // echo's bar instead of Foo. My Question is Why?, and how can I assure that both this: $foo->bar = function(){ echo "test"; }; $foo->bar(); and this $foo = new Bar; $foo->bar(); are called properly? Extra Points if you can point to documentation stating why this problem occurs.

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  • How with lambda function in MVC3

    - by doogdeb
    I have a model which contains view models for each view. This model is held in session and is initialised when application starts. I need to be able to populate a field from one view model with the value from another so have used a lambda function. Below is my model. I am using a lambda so that when I get Test2.MyProperty it will use the FunctionTestProperty to retrieve the value from Test1.TestProperty. public class Model { public Model() { Test1 = new Test1() Test2 = new Test2(FunctionTestProperty () => Test1.TestProperty) } } public class Test1 { public string TestProperty { get; set; } } public class Test2 { public Test2() : this (() => string.Empty) {} public Test2(Func<string> functionTestProperty) { FunctionTestProperty = functionTestProperty; } public Func<string> FunctionTestProperty { get; set; } public string MyProperty { get{ return FunctionTestProperty() ?? string.Empty; } } } This works perfectly when I first run the application and navigate from Test1 to Test2; I can see that when I get the value for MyProperty it calls back to Model constructor and retrieves the Test1.TestProperty value. However when I then submit the form (Test2) it calls the default constructor which sets it to string.Empty. So if I go back to Test1 and back to Test2 again it always then calls the Test2 default constructor. Does anyone know why this works when first running the application but not after the view is submitted, or if I have made an obvious mistake?

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  • Lambda expressions as CLR (.NET) delegates / event handlers in Visual C++ 2010

    - by absence
    Is it possible to use the new lambda expressions in Visual C++ 2010 as CLR event handlers? I've tried the following code: SomeEvent += gcnew EventHandler( [] (Object^ sender, EventArgs^ e) { // code here } ); It results in the following error message: error C3364: 'System::EventHandler' : invalid argument for delegate constructor; delegate target needs to be a pointer to a member function Am I attempting the impossible, or is simply my syntax wrong?

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  • C++ boost::lambda::ret equivalent in phoenix

    - by aaa
    hello. Boost lambda allows to overwrite deduced return type using ret<T> template. I have tried searching for equivalent in phoenix but could not find one. Is there an equivalent in phoenix? I know how to make my own Replacement but I would rather not. thank you

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  • Convert this Linq query from query syntax to lambda expression

    - by Jinkinz
    I'm not sure I like linq query syntax...its just not my preference. But I don't know what this query would look like using lambda expressions, can someone help? from securityRoles in user.SecurityRoles from permissions in securityRoles.Permissions where permissions.SecurableEntity.Name == "Unit" && permissions.PermissionType.Name == "Read" orderby permissions.PermissionLevel.Value descending select permissions There is a many-to-many relationship between users and security roles that makes this extra confusing. Thanks! Kelly

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  • To Reference A Generic Method With A Lambda Expression

    - by SDReyes
    It is possible to reference a generic method using a Lambda Expression Object? For example, having: TheObject: public abstract class LambdaExpression : Expression TheMethod (an extension method of LINQ): public static TSource Last<TSource>( this IEnumerable<TSource> source ) I'm trying to create an instance of TheObject, that references to TheMethod. How do you do such thing?

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  • Rewriting a statement using LAMBDA(C#)

    - by Thinking
    Is it possible to write the folowing using lambda(C#) private static void GetRecordList(List<CustomerInfo> lstCustinfo) { for (int i = 1; i <= 5; i++) { if (i % 2 == 0) lstCustinfo.Add(new CustomerInfo { CountryCode = "USA", CustomerAddress = "US Address" + i.ToString(), CustomerName = "US Customer Name" + i.ToString(), ForeignAmount = i * 50 }); else lstCustinfo.Add(new CustomerInfo { CountryCode = "UK", CustomerAddress = "UK Address" + i.ToString(), CustomerName = "UK Customer Name" + i.ToString(), ForeignAmount = i * 80 }); } }

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  • Lambda Expressions and Memory Management

    - by Surya
    How do the Lambda Expressions / Closures in C++0x complicate the memory management in C++? Why do some people say that closures have no place in languages with manual memory management? Is there claim valid and if yes, what are the reasons behind it?

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  • Correct use of boost lambda

    - by Niels P.
    Consider the following piece of C++0x code: a_signal.connect([](int i) { if(boost::any_cast<std::string>(_buffer[i]) == "foo") { base_class<>* an_object = new derived_class(); an_object->a_method(_buffer[i]); }}); How would it correctly look in Boost Lambda (since this C++0x feature can't be used yet)?

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  • using lambda instead of let in scheme

    - by Radagaisus
    Hey, In SICP 1.2.1 there is a function that makes a rational number, as follow: (define (make-rat n d) (let ((g (gcd n d))) (cons (/ n g) (/ d g)))) I'm just curious how you can implement the same thing using lambda instead of let, without calling GCD twice. I couldn't figure it out myself.

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