I'm building a hierarchical collection class that orders magnetic resonance images spatially and arranges them into groupings based on the various acquisition parameters that were used to generate them. The specific method used to perform the grouping is provided by the user of the class. I've abstracted out the relevant features in the sample code below. For the IEquatable<MyClass> implementation, I'd like to be able to compare the _myHelperDelegate attributes of two MyClass instances to determine if both delegates point to the same piece of code. The (_myHelperDelegate == other._myHelperDelegate) clause in the if statement below is clearly the wrong way to go about doing this (it fails to compile, giving the error "Method name expected"). My question is, is there a way to compare two delegates to determine if they reference the same piece of code? If so, how do you do that?
public class MyClass : IEquatable<MyClass>
{
public delegate object HelperDelegate(args);
protected internal HelperDelegate _myHelperDelegate;
public MyClass(HelperDelegate helper)
{
...
_myHelperDelegate = helper;
}
public bool Equals(MyClass other)
{
if (
(_myHelperDelegate == other._myHelperDelegate) &&
(... various other comparison criteria for equality of two class instances... )
)
return true;
return false;
}
}
The following compiles and works as expected.
private void Form1_Load(object sender, EventArgs e)
{
var helper1 = new TestDelegates.Form1.MyClass.HelperDelegate(Testing);
var helper2 = new TestDelegates.Form1.MyClass.HelperDelegate(Testing2);
var myClass1 = new MyClass(helper1);
var myClass2 = new MyClass(helper1);
System.Diagnostics.Debug.Print(myClass1.Equals(myClass2).ToString()); //true
myClass2 = new MyClass(helper2);
System.Diagnostics.Debug.Print(myClass1.Equals(myClass2).ToString()); //false
}
private object Testing()
{
return new object();
}
private object Testing2()
{
return new object();
}
public class MyClass : IEquatable<MyClass>
{
public delegate object HelperDelegate();
protected internal HelperDelegate _myHelperDelegate;
public MyClass(HelperDelegate helper)
{
_myHelperDelegate = helper;
}
public bool Equals(MyClass other)
{
if (_myHelperDelegate == other._myHelperDelegate)
{
return true;
}
return false;
}
}
Per msdn, Delegate.Equals method returns:
true if obj and the current delegate have the same targets, methods, and invocation list; otherwise, false.
Have you tried this?
Old question, but I wrote a simple example program to demonstrate comparing delegates with Delegate.Equals -
public delegate int test1(int t);
public static int asdf(int t)
{
return t + 5;
}
public static int asdf2(int x)
{
return x + 7;
}
public static void CompareDelegates(test1 test1, test1 test2)
{
Console.WriteLine(test1 == test2);
}
public static void Main(string[] args)
{
test1 test1 = asdf;
test1 test2 = asdf2;
test1 test3 = asdf;
CompareDelegates(test1, test1);
CompareDelegates(test1, test2);
CompareDelegates(test2, test3);
CompareDelegates(test1, test3);
}
// Outputs:
//
// True
// False
// False
// True
Related
Given the following set of classes:
public class MyClass
{
public int MyInt { get; set; }
}
public class ObjectProcessor
{
public int ProcessObject(MyClass myClass)
{
return myClass.MyInt ++;
}
}
public class Runner
{
public void Run()
{
var classToPass = new MyClass();
FuncExecutor.ExecuteAction<MyClass>(x => x.ProcessObject(classToPass));
}
}
public static class FuncExecutor
{
public static void ExecuteAction<T>(Expression<Func<ObjectProcessor, int>> expression)
{
// var func = expression.Compile(); ... does having an Expression help?
// How can I get a reference to 'classToPass' at this point?
// The 'classToPass' Type is known to be 'T', in this case 'MyClass'.
}
}
From within the ExecuteAction method, how can I get a reference to the classToPass instance that was passed in to ProcessObject?
EDIT: The comments have highlighted the complexity of trying to parse Expression Trees which could vary widely in their composition.
However, in this particular case there are two facts which cut down this variation considerably:
ProcessObject will only ever take a single parameter.
The parameter type is known in advance.
Code altered to express this.
To answer very specifically:
public class Runner
{
public void Run()
{
var classToPass = new MyClass();
classToPass.MyInt = 42;
FuncExecutor.ExecuteAction(x => x.ProcessObject(classToPass));
}
}
public class FuncExecutor
{
public static void ExecuteAction(Expression<Func<ObjectProcessor, int>> expression)
{
var lambdaExpression = (LambdaExpression)expression;
var methodCallExpression = (MethodCallExpression)lambdaExpression.Body;
var memberExpression = (MemberExpression)methodCallExpression.Arguments[0];
var constantExpression = (ConstantExpression)memberExpression.Expression;
var fieldInfo = (FieldInfo)memberExpression.Member;
var myClassReference = (MyClass) fieldInfo.GetValue(constantExpression.Value);
Console.WriteLine(myClassReference.MyInt); // prints "42"
}
}
Please note that when you pass the lambda to the ExecuteAction method, you capture a local variable reference (classToPass). The compiler will generate some code to handle that properly. More precisely, it will generate a type with a single member (a field) of type MyClass to hold the reference and use it from this point. That's why you'll get a MemberExpression in the argument expression list.
Since you can't directly manipulate this generated type, you can't just use the member expression Value property. But you can dynamically invoke the member accessor using the MemberInfo and the target reference (an instance of the compiler generated type).
I would not rely on this code.
You can read more about lambda related compiler generated code here, for example: http://thewalkingdev.blogspot.fr/2012/04/c-lambda-expressions-and-closures.html
The easiest way is to pass the instance as parameter and let ExecuteAction take care of calling the process method using that instance. To do this it is necessary to give your code a little bit of structure using a generic object processor interface:
public interface IObjectProcessor<T> {
public int ProcessObject(T instance);
}
public class MyClassProcessor : IObjectProcessor<MyClass> {
public int ProcessObject(MyClass myClass) {
return myClass.MyInt ++;
}
}
public class Runner {
public void Run() {
var classToPass = new MyClass();
var processor = new MyClassProcessor();
FuncExecutor.ExecuteAction<MyClass>(processor, classToPass);
}
}
public class FuncExecutor {
public static void ExecuteAction<T>(IObjectProcessor<T> processor, T obj) {
int result = processor.ProcessObject(obj);
}
}
This design could be a little annoying especially if your processor are "stateless" and if you really need a Func as parameter. In this case you can drop the interface and use static processors:
public class MyClassProcessor
public static int ProcessObject(MyClass myClass) {
return myClass.MyInt ++;
}
}
public class Runner {
public void Run() {
var classToPass = new MyClass();
FuncExecutor.ExecuteAction<MyClass>(MyClassProcessor.ProcessObject, classToPass);
}
}
public class FuncExecutor {
public static void ExecuteAction<T>(Func<T, int> process, T obj) {
int result = process(obj);
}
}
Suppose I have the following code.
static class Store<T> {
public static T A;
public static T B;
public static T C;
}
public static class Store {
public static Value A = new Value(<T>(v) => Store<T>.A = v); //just an example of what I want
public static Value B = new Value(<T>(v) => Store<T>.B = v); //just an example of what I want
public static Value C = new Value(SetC<T>); //just an example of what I want
public static void SetA<T>(T value) { Store<T>.A = value; }
public static void SetB<T>(T value) { Store<T>.B = value; }
public static void SetC<T>(T value) { Store<T>.C = value; }
}
public class Value {
Action<T><T> _valueChanger; //just an example of what I want
public Value(Action<T><T> valueChanger) { //just an example of what I want
_valueChanger = valueChanger;
}
public void SetValue<T> (T value) {
_valueChanger<T>(value); //just an example of what I want
}
}
I want to write Store.A.SetValue(42) so that the value is saved to Store<int>.A. What can I write instead of the lines marked by "just an example of what I want" to make that happen? (I want to explore a solution that doesn't involve dictionaries or something similar)
Rephrasing the question:
I want to modify class Value (define some fields, write a constructor and write the method Value.SetValue(T value) ), then construct three different variables of type Value (A, B, C) in such a way that when I call Store.A.SetValue(42) the value Store<int>.A is changed to 42.
Another variation of the classes:
static class Holder<T> {
T Value { get; set; }
}
static class Store2<T> {
public static Holder<T> A = new Holder<T>();
public static Holder<T> B = new Holder<T>();
public static Holder<T> C = new Holder<T>();
}
public static class Store2 {
public static Value A = new Value2(Store2<>.A); //just an example of what I want
public static Value B = new Value2(Store2<>.B); //passing non-specific generic expression
public static Value C = new Value3({TFree}() => Store2<TFree>.C); //just an example of what I want
}
public class Value2 { //Non-generic class!
Holder{TFree}<TFree> _holder; //just an example of what I want
public Value(Holder{TFree}<TFree> holder) { //just an example of what I want
_holder = holder;
}
public void SetValue<T> (T value) {
_holder{T}.Value = value; //just an example of what I want
}
}
public class Value3 { //Non-generic class! (Another variation)
Func{TFree}<Holder<TFree>> _holderFactory; //just an example of what I want
public Value(Func{TFree}<Holder<TFree>> holderFactory) { //just an example of what I want
_holderFactory = holderFactory;
}
public void SetValue<T> (T value) {
Holder<T> holder = _holderFactory{T}(); //just an example of what I want
holder.Value = value;
}
}
Solution:
An easy reflection-free and collection-free solution was found using the answers to another question ( Emulating delegates with free generic type parameters in C# and Emulating delegates with free generic type parameters in C#). The solution is Delegates to generic operations where the generic type is unknown. How to create something like that?.
Use an array to store the values and access them through a property using an index
public static class Store<T>
{
public static readonly T[] Values = new T[3];
public static T A { get { return Values[0]; } set { Values[0] = value; } }
public static T B { get { return Values[1]; } set { Values[1] = value; } }
public static T C { get { return Values[2]; } set { Values[2] = value; } }
}
public static class Store
{
public static readonly Value A = new Value(0);
public static readonly Value B = new Value(1);
public static readonly Value C = new Value(2);
}
public class Value
{
private int _index;
public Value(int index)
{
_index = index;
}
public void SetValue<T>(T value)
{
Store<T>.Values[_index] = value;
}
public T GetValue<T>()
{
return Store<T>.Values[_index];
}
}
Since the constructor of Value is not aware of any generic type parameter, you cannot have any reference to a specific Store<T>.
UPDATE
Be aware of the fact that a copy of Store<T> will be created for every distinct type argument that you supplied for T. See this example
Store.A.SetValue(42);
Store.A.SetValue("Douglas Adams");
Store.A.SetValue(new DirectoryInfo(#"C:\"));
Store.A.SetValue(new List<int>());
var x1 = Store.A.GetValue<int>(); // --> 42
var x2 = Store.A.GetValue<string>(); // --> "Douglas Adams"
var x3 = Store.A.GetValue<DirectoryInfo>(); // --> DirectoryInfo{ C:\ }
var x4 = Store.A.GetValue<List<int>>(); // --> List<int>{ Count = 0 }
By using the debugger, you will see that four different values are stored in A at the same time! Of cause these are four differents A's that exist in four diffferent Store<T>.
The problem turned out to be solvable. Mike-z gave me a nearly right solution for the delegate-to-generic-method problem ( Emulating delegates with free generic type parameters in C#) which I modified to be a full solution: ( Emulating delegates with free generic type parameters in C#).
The solution this question becomes easy too. Interfaces can contain generic methods and we can use the interface-valued variables to store links to generic methods without specifying concrete type arguments. The following code utilizes the Store<T> class without modifications and uses the ISetter interface and ASetter/BSetter/CSetter "closures" to hold references to different generic members. The Value class stores the references in a ISetter-typed variable and uses the generic member which the _setter links to once the type argument T becomes available.
public interface ISetter {
void SetValue<T>(T value);
}
public static class Store {
public static Value A = new Value(new ASetter());
public static Value B = new Value(new BSetter());
public static Value C = new Value(new CSetter());
class ASetter : ISetter {
public void SetValue<T>(T value) { Store<T>.A = value; }
}
class BSetter : ISetter {
public void SetValue<T>(T value) { Store<T>.B = value; }
}
class CSetter : ISetter {
public void SetValue<T>(T value) { Store<T>.C = value; }
}
}
public class Value {
ISetter _setter;
public Value(ISetter setter) {
_setter = setter;
}
public void SetValue<T> (T value) {
_setter.SetValue<T>(value);
}
}
I have the following scenario:
public class SomeClass {
// Have some other data members as well
public int i ;
}
public class TestClass {
public bool SomeFunction() {
SomeClass a = new SomeClass();
SomeClass b = new SomeClass();
if (a == b) // this is where I am getting compile error
return true;
return false;
}
public static bool operator==(SomeClass a, SomeClass b) {
if (a.i == b.i)
return true;
// compare some other members as well
return false;
}
}
Is this possible to achieve in C#?
Thanks for the help!
No, it's not possible to override an operator from a class that is not involved in the operation.
You can make a class that implements IEualityComparer<SomeClass>, which can be used instead of the standard comparison in some cases, for example in a dictionary:
var x = new Dictionary<SomeClass, string>(new SomeClassEqualityComparer());
If you just want to use the comparison in your own class, you could make it a regular static method instead of overriding an operator:
public static bool SomeClassEqual(SomeClass a, SomeClass b) {
if (a.i == b.i) {
return true;
}
// compare some other members as well
return false;
}
Usage:
if (SomeClassEqual(a, b))
To begin with, you can't use return true; on a void method.
Second, overriding operators should be applied to the host class. In your case, inside SomeClass rather than inside TestClass.
Third, when you implement == overriding operator, you should also implement != .
Here is your code, revised and working:
public class SomeClass
{
// Have some other data members as well
public int i;
public static bool operator ==(SomeClass a, SomeClass b)
{
if (a.i == b.i)
return true;
// compare some other members as well
return false;
}
public static bool operator !=(SomeClass a, SomeClass b)
{
return !(a == b);
}
}
public class TestClass
{
public bool SomeFunction()
{
SomeClass a = new SomeClass();
SomeClass b = new SomeClass();
if (a == b) // this is where I am getting compile error
return true;
return false;
}
}
In order to explain my problem here is an example
namespace CheckAbstarct
{
class Program
{
static void Main(string[] args)
{
myAbstarctClass mac1 = ObjectFactory.ObjectCreator("aaa");
myAbstarctClass mac2 = ObjectFactory.ObjectCreator("bbb");
mac1.changeMyString();
mac2.changeMyString();
string myString = (string)mac2.returnMyObject();
DateTime myObject = (DateTime) mac1.returnMyObject();
object obj1 = mac1.returnMyObject();
object obj2 = mac2.returnMyObject();
myMethod(obj1); //---> This is not compiling
myMethod(obj2); //---> This is not compiling
myMethod(myString); //---> works fine
myMethod(myObject); //---> works fine
Console.ReadKey();
}
public static void myMethod(DateTime dt)
{
}
public static void myMethod(string st)
{
}
}
abstract class myAbstarctClass
{
protected string mMyString;
public myAbstarctClass()
{
mMyString = "myAbstarctClass ";
}
public abstract void changeMyString();
public abstract object returnMyObject();
}
class MyNewAbstractClass1 : myAbstarctClass
{
DateTime mObject;
public MyNewAbstractClass1(string myString)
{
mMyString = myString;
mObject = new DateTime().Date;
}
public override void changeMyString()
{
mMyString += " MyNewAbstractClass1";
Console.WriteLine(mMyString);
}
public override object returnMyObject()
{
return mObject;
}
}
class MyNewAbstractClass2 : myAbstarctClass
{
string mString;
public MyNewAbstractClass2(string myString)
{
mMyString = myString;
mString = mMyString;
}
public override void changeMyString()
{
mMyString += " MyNewAbstractClass2";
Console.WriteLine(mMyString);
}
public override object returnMyObject()
{
return mString;
}
}
static class ObjectFactory
{
public static myAbstarctClass ObjectCreator(string myString)
{
switch (myString)
{
case "aaa":
return new MyNewAbstractClass1(myString);
case "bbb":
return new MyNewAbstractClass2(myString);
default:
return null;
}
}
}
}
My problem is that in Main() I don't know what type the returnMyObject() method returns so I can't send it to MyMethod. Is there a way to cast the objects ??
Because in your design of returnMyObject() you went back to the most common object references, you will have to find out in runtime:
if (obj1 is string)
myMethod((string)obj1); //--->cast it
else if (obj1 is DateTime)
myMethod((DateTime) obj1);
You could check the object's type at runtime:
public static void myMethod(Object o)
{
if (o is DateTime)
myMethod((DateTime)o);
else if (o is string)
myMethod((string)o);
}
Although in your case, you might just as well pass a myAbstarctClass instance to myMethod, and then call returnMyObject() there.
You can either use dynamic feature from C# 4.0 or change design to utilize some kind of double dispatch technique
dynamic obj1 = mac1.returnMyObject();
dynamic obj2 = mac2.returnMyObject();
Use Polymorphism mechanisms so you don't need to know the type of object.
Make myMethod an abstract method of myAbstarctClass and provide implementations in both MyNewAbstractClass1 and MyNewAbstractClass2.
Modify myAbstractClass1.returnMyObject() to return myAbstarctClass (not object).
The test code in Main can then be written:
...
myAbstarctClass obj1 = mac1.returnMyObject();
myAbstarctClass obj2 = mac2.returnMyObject();
obj1.myMethod(); // calls MyNewAbstractClass1.myMethod()
// no if statement required!
obj2.myMethod(); // calls MyNewAbstractClass2.myMethod()
// no if statement required!
Console.ReadKey();
Edit: This can be further simplified, since the returnMyObject() methods are no longer necessary - they just return the object you already have. The test code is now simply:
mac1.myMethod();
mac2.myMethod();
// etc...
Console.ReadKey();
No, you have to either create switch with all possibilities, or something like Dictionary<Type, Delegate>
or you can just make myMethod(object obj)
it's called Multiple dispatch (http://en.wikipedia.org/wiki/Multiple_dispatch) and there are some libraries that can do it
Since you seem to be using your class as a container for a type (eg. DateTime, string), perhaps Generics would a be better choice than Inheritance:
namespace CheckAbstract
{
class Program
{
static void Main(string[] args)
{
myTemplateClass<DateTime> mac1 = new myTemplateClass<DateTime>(new DateTime().Date);
myTemplateClass<string> mac2 = new myTemplateClass<string>("cat dog");
mac1.changeMyString();
mac2.changeMyString();
string myString = (string)mac2.returnMyObject();
DateTime myObject = (DateTime) mac1.returnMyObject();
myMethod<string>(myString);
myMethod<DateTime>(myObject);
Console.ReadKey();
}
public static void myMethod<T>(T obj)
{
}
}
class myTemplateClass<T>
{
T mObject;
string mMyString;
public myTemplateClass(T init)
{
mMyString = init.ToString();
mObject = init;
}
public void changeMyString()
{
mMyString += " " + mObject.ToString();
Console.WriteLine(mMyString);
}
public T returnMyObject()
{
return mObject;
}
}
}
Is there any way to override a class method with a lambda function?
For example with a class definition of
class MyClass {
public virtual void MyMethod(int x) {
throw new NotImplementedException();
}
}
Is there anyway to do:
MyClass myObj = new MyClass();
myObj.MyMethod = (x) => { Console.WriteLine(x); };
Chris is right that methods cannot be used like variables. However, you could do something like this:
class MyClass {
public Action<int> MyAction = x => { throw new NotImplementedException() };
}
To allow the action to be overridden:
MyClass myObj = new MyClass();
myObj.MyAction = (x) => { Console.WriteLine(x); };
No. However if you declare the method as a lambda in the first place, you can set it, though I would try to do that at initialization time.
class MyClass {
public MyClass(Action<int> myMethod)
{
this.MyMethod = myMethod ?? x => { };
}
public readonly Action<int> MyMethod;
}
This however cannot implement an interface that has a MyMethod declared, unless the interface specifies a lambda property.
F# has object expressions, which allow you to compose an object out of lambdas. I hope at some point this is part of c#.
No. Methods cannot be used like variables.
If you were using JavaScript, then yes, you could do that.
You can write this code:
MyClass myObj = new MyClass();
myObj.TheAction = x => Console.WriteLine(x);
myObj.DoAction(3);
If you define MyClass in this way:
class MyClass
{
public Action<int> TheAction {get;set;}
public void DoAction(int x)
{
if (TheAction != null)
{
TheAction(x);
}
}
}
But that shouldn't be too surprising.
Not directly, but with a little code it's doable.
public class MyBase
{
public virtual int Convert(string s)
{
return System.Convert.ToInt32(s);
}
}
public class Derived : MyBase
{
public Func<string, int> ConvertFunc { get; set; }
public override int Convert(string s)
{
if (ConvertFunc != null)
return ConvertFunc(s);
return base.Convert(s);
}
}
then you could have code
Derived d = new Derived();
int resultBase = d.Convert("1234");
d.ConvertFunc = (o) => { return -1 * Convert.ToInt32(o); };
int resultCustom = d.Convert("1234");
Depending on what you want to do, there are many ways to solve this problem.
A good starting point is to make a delegate (e.g. Action) property that is gettable and settable. You can then have a method which delegates to that action property, or simply call it directly in client code. This opens up a lot of other options, such as making the action property private settable (perhaps providing a constructor to set it), etc.
E.g.
class Program
{
static void Main(string[] args)
{
Foo myfoo = new Foo();
myfoo.MethodCall();
myfoo.DelegateAction = () => Console.WriteLine("Do something.");
myfoo.MethodCall();
myfoo.DelegateAction();
}
}
public class Foo
{
public void MethodCall()
{
if (this.DelegateAction != null)
{
this.DelegateAction();
}
}
public Action DelegateAction { get; set; }
}