I want to access a property of some class of mine, but get compiler error "CS0572 - Cannot reference a type through an expression".
I have the following setup
public interface IHelper {
void DoHelp();
}
public abstract class ClassWithHelperBase<THelper> where THelper : IHelper {
public THelper Helper { get; }
}
public class ClassWithHelper : ClassWithHelperBase<ClassWithHelper.Helper> {
// use a nested class, since there will be n classes deriving from ClassWithHelper and giving each helper a readable name (in this example ClassWithHelperHelper) is ugly
public class Helper : IHelper {
public static void SomeStaticMethod() { }
public void DoHelp() { }
}
}
public class Test {
private ClassWithHelper myClass;
public void DoTest() {
((ClassWithHelperBase<ClassWithHelper.Helper>) myClass).Helper.DoHelp(); // this works, but is ugly
myClass.Helper.DoHelp(); // what I want, but it's not working
//myClass.Helper.SomeStaticMethod(); // funnily IDE supposes static methods here even though the resulting code is invalid, since I am (obviously) not referencing the class type
}
}
The interface is unnecessary for reproduction, I added it for clarity.
Note: I do not want to call a static method, I just added it, to show the IDE mixes up the member and the class qualifier.
Is there a way to access the property Helper of myClass, without casting myClass or renaming the nested class?
Aka: Why can't the compiler distinguish the member and the nested class?
The problems is due to name collision between Helper class (type) and Helper property. Try this
public interface IHelper
{
void DoHelp();
}
public abstract class ClassWithHelperBase<THelper> where THelper : IHelper
{
public THelper Helper { get; set; }
}
public class ClassWithHelper : ClassWithHelperBase<ClassWithHelper.CHelper>
{
// use a nested class, since there will be n classes deriving from ClassWithHelper and giving each helper a readable name (in this example ClassWithHelperHelper) is ugly
public class CHelper : IHelper
{
public static void SomeStaticMethod() {}
public void DoHelp() { }
}
}
public class Test
{
private ClassWithHelper myClass;
public void DoTest() {
myClass.Helper.DoHelp();
ClassWithHelper.CHelper.SomeStaticMethod();
}
}
Here I renamed Helper class to the CHelper, so compiler can now distinguish class and property and thus the line myClass.Helper.DoHelp(); now works without cast.
If a "do not rename nested class" requirement is absolutely mandatory, then the problem may be also solved by renaming the Helper property in the base class to avoid name collision. However, I can't imagine better name for the property.
Unfortunately, for the static method, you can't reference myClass instance. So, you will need reference the whole type.
Related
I have a NavigationModel class which implements site navigation. Internally there is a private implementation of NavigationNode which I want to be able to declare within the NavigationModel but not outside of it. How would I accomplish this? When I do the following:
public class NavigationModel
{
public List<NavigationNode> NavigationNodes { get; set; }
public NavigationModel()
{
}
private class NavigationNode
{
}
}
The property tells me:
Inconsistent accessibility: property type
'List' is less accessible than
property 'NavigationModel.NavigationNodes'
The error is raised because by declaring NavigationModel as public, you create a public interface that is used to access NavigationModel. Part of this interface are the signatures of the public methods or properties. By that, you'd publish class NavigationNode that is supposed to be private - hence the error.
In order to fix this, you could create a public interface that only contains the parts of NavigationNode that you want to publish. If you do not want to publish anything, the interface is empty. The following sample shows the basic components:
Public interface INavigationNode.
Property of type List<INavigationNode>.
Private class NavigationNode that implements the interface.
public interface INavigationNode
{
// Add parts of NavigationNode that you want to publish
}
public class NavigationModel
{
public List<INavigationNode> NavigationNodes { get; set; }
public NavigationModel()
{
}
private class NavigationNode : INavigationNode
{
}
}
NavigationNode needs to be public for this to work properly. Making it public still keeps the declaration internal to the containing class NavigationModel yet classes outside NavigationModel can reference it.
I've got a class that I want to enforce some generic constraints on when created
Myobject.cs
public class MyObject<T> where T : ObjectBase, new()
{
public MyObject()
{
}
public bool Validate(out ServiceError? message)
{
// validation
}
}
ObjectBase.cs
public abstract class ObjectBase
{
public abstract bool Validate(out ServiceError? message);
}
I'm wondering if there is any way to avoid having to place generic constraints for T every time I have a function such as this:
ObjectRepo.cs
public void Put<T>(MyObject<T> obj)
where T : ObjectBase, new()
{
// code
}
If the constraints for T are specified in the MyObject class, is there a reason I have to re-specify every single time I use MyObject as a parameter?
Don't redefine T on your method if you have it on the class
public class MyObject<T> where T : ObjectBase, new()
{
//public void Put<T>(MyObject<T> obj)//doesn't work
public void Put(MyObject<T> obj)//works
{
// code
}
}
Update
If Put is in another class your best solution it to refactor your code and move all the T related method in the same class (and add the constrain on the class)
Update 2
If update 1 is not possible ,it might worth checking if you can replace your T with ObjectBase or another base class
I want to force my child classes to pass themselves as as the generic parameter to the parent class.
For example :
class BaseClass<T> where T: BaseClass
{
//FullClassName : Tuple [Save,Update,Delete]
Dictionary<string,Tuple<delegate,delegate,delegate>> dict = new Dictionary...;
static BaseClass()
{
RegisterType();
}
private static void RegisterType()
{
Type t = typeof(T);
var props = t.GetProperties().Where(/* Read all properties with the SomeCustomAttribute */);
/* Create the delegates using expression trees and add the final tuple to the dictionary */
}
public virtual void Save()
{
delegate d = dict[t.GetType().FullName];
d.Item1(this);
}
}
class ChildClass : BaseClass<ChildClass>
{
[SomeCustomAttribute]
public int SomeID {get;set;}
[SomeCustomAttribute]
public string SomeName {get; set;}
}
public class Program
{
public static void Main(string[] args)
{
ChildClass c = new ChildClass();
c.Save();
}
}
Obviously the above code won't compile. I'll restate : I want the child class to pass itself as the generic parameter and not any other child of BaseClass.
(The above code is kind of a psuedo code and will still not compile).
You can do this:
public class BaseClass<T> where T: BaseClass<T> { }
public class ChildClass : BaseClass<ChildClass> { }
But this doesn't force you to use ChildClass as the generic parameter. You could do this public class OtherChildClass : BaseClass<ChildClass> { } which would break the "coontract" that you want to enforce.
The direct answer is that if your accessing a static method then typeof(T) will give you the type for reflection.
However, there is probably better solutions than using reflection. Options:
1) Static constructor on the child class.
2) Abstract method declared in the base class.
I do not know the application, but I get concerned about my design if I feel like using a static constructor, I also get concerned if a base class needs to initialize the child class.
I suggest looking at injection as a solution rather than inheritance. It offers superior unit testing and often a better architecture.
More info (after initial post), this is my preferred solution:
public interface IRegesterable
{
void Register();
}
public class Widget : IRegesterable
{
public void Register()
{
// do stuff
}
}
public class Class1
{
public Class1(IRegesterable widget)
{
widget.Register();
}
}
Hope this helps
The ConcurrentDictionary is being used as a Set<Type>. We can check in the Set<Type> if the type has been initialized. If not we run RegisterType on the type.
public abstract class BaseClass
{
//Concurrent Set does not exist.
private static ConcurrentDictionary<Type, bool> _registeredTypes
= new ConcurrentDictionary<Type, bool>();
protected BaseClass()
{
_registeredTypes.GetOrAdd(GetType(), RegisterType);
}
private static bool RegisterType(Type type)
{
//some code that will perform one time processing using reflections
//dummy return value
return true;
}
}
public class ChildClass : BaseClass
{
}
There are several inefficiencies with this pattern though.
object.GetType() is pretty darn slow, and inefficient.
Even with the HashSet behavior, we are checking for initialization on each instanciation. Its as fast as I can get it, but its still pretty superfluous.
I have the following abstract class:
public abstract class BaseClass{
public object contents { get; set; }
public Action<BaseClass> mutator;
public abstract void Initialise();
}
This will be used by several classes, which will override the Initialize method to assign a value to contents, which will in turn be mutated using the mutator delegate at specific points in time.
I have the following static class, with each method intended to be used as a mutator:
public static class Mutators{
public static void VariantA(A inputObj){
// inputObj.contents = something else
}
public static void VariantB(A inputObj) { } // etc. etc.
}
I then have class A, which implements BaseClass. I am trying to assign Mutators.VariantA to the mutator delegate, but i'm not able to.
public class A : BaseClass{
public A(){
mutator = Mutators.VariantA;
}
public override void Initialise(){
/* set the value of contents property here */
}
}
Specifically I get the following error: A method or delegateMutators.VariantA(A)' parameters do not match delegate System.Action<BaseClass>(BaseClass)' parameters (CS0123)
I understand that Mutators.VariantA(A) requires an object of type A, and the Action was declared to accept an input of type BaseClass, however as class A implements BaseClass I thought I would have been able to do this ?
Coming from dynamically typed languages i'm having a tough time getting to grips with working with types in this way :(
Is there any way I can point to a function with an input of the abstract type in this way ? Do I need to look at some other design pattern ?
Thanks
I understand that Mutators.VariantA(A) requires an object of type A, and the Action was declared to accept an input of type BaseClass, however as class A implements BaseClass I thought I would have been able to do this ?
Absolutely not.
An Action<BaseClass> has to be able to accept any BaseClass object. So for example, if your code were valid, I would be able to write:
Action<BaseClass> mutator = Mutators.VariantA;
mutator.Invoke(new B());
(Where B is another class derived from BaseClass.)
The fact that B derives from BaseClass makes it valid for the invocation - but it's not going to help your VariantA method work nicely.
It's not really clear why you have a mutator here - I strongly suspect you should abstract BaseClass from its mutations. I still don't follow what you're trying to achieve, but this design pattern isn't going to help you get there in a type-safe way.
You could write:
public abstract class BaseClass<T> where T : BaseClass<T> {
public object Contents { get; set; }
public Action<T> Mutator { get; set; }
public abstract void Initialise();
}
... then:
public class A : BaseClass<A> {
public A() {
Mutator = Mutators.VariantA;
}
}
... as then you'd be writing something which can mutate "A" values. But in my experience this sort of generic nesting gets really messy, really quickly.
I've used your current example and changed the Method Signature of one of the classes to the following and it works
public abstract class BaseClass
{
public object contents { get; set; }
public Action<BaseClass> mutator;
public abstract void Initialise();
}
public static class Mutators
{
public static void VariantA(BaseClass baseClass)
{
// inputObj.contents = something else
}
public static void VariantB(A inputObj) { } // etc. etc.
}
public class A : BaseClass
{
public A()
{
mutator = Mutators.VariantA;
}
public override void Initialise()
{
/* set the value of contents property here */
}
}
Here is an example. I have two classes, one inherited, and both have a function with the same name, but different arguments:
public class MyClass
{
//public class members
public MyClass()
{
//constructor code
}
public void Copy(MyClass classToCopy)
{
//copy code
}
}
public class InheritedClass : MyClass
{
//public class members
public InheritedClass():base()
{
//constructor code
}
public void Copy(InheritedClass inheritedClassToCopy)
{
//copy code
}
}
My question is how do I make the base class' copy method (MyClass.Copy) non-inheritable or non-visible in InheritedClass? I don't want to be able to do this:
MyClass a;
InheritedClass b;
b.Copy(a);
Does this make sense, or should I keep this functionality in there? Can what I'm asking even be done?
Does this make sense, or should I keep this functionality in there? Can what I'm asking even be done?
Trying to hide a public method like this when used by a base class is problematic. You're purposely trying to violate the Liskov substitution principle.
You can't do what you are wanting to do here; C# does not allow negative variance in inherited members. (almost no languages truly do, actually)
It may be that you don't want an inherited class here at all, though; what you may really want is an interface. Or... your two classes here may not have the correct relationship; perhaps they should both instead be common siblings of a third class, which is their parent.
You can use explicit interface implementation to hide this method from the inheritor. But you will need to add an interface of course and you will need to cast your type to the interface to call your method:
public interface MyInterface
{
void Copy(MyClass classToCopy)
}
public class MyClass : MyInterface
{
void MyInterface.Copy(MyClass classToCopy)
{
//copy code
}
}
This is not possible. An inherited class inherits all public and protected members, methods and properties. Using the sealed modifier with make it non-overridable, but still accessible to your inherited class.
What everyone else said, but if I am inferring your goal correctly, it is to make sure that InheritedClass users never use the MyClass method. In that case, exclude it from MyClass and make two classes that inherit it.
Make MyBaseClass abstract if it should not be instantiated (most likely).
(Edited -- you probably would want to include copy code for anything that's part of the base class in the base class)
public abstract class MyBaseClass
{
public MyClass()
{
//constructor code
}
protected void Copy(MyBaseClass classToCopy)
{
//copy code
}
// other methods that all inherited classes can use
}
public class MyClass: MyBaseClass
{
public MyClass():base()
{
//constructor code
}
public void Copy(MyClass myClassToCopy)
{
base.Copy(myClassToCopy);
//specific copy code for this extensions in this class
}
}
public class InheritedClass : MyBaseClass
{
public InheritedClass():base()
{
//constructor code
}
public void Copy(InheritedClass inheritedClassToCopy)
{
base.Copy(myClassToCopy);
//specific copy code for this extensions in this class
}
}