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
Related
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.
I have an abstract class:
public abstract class ExampleBase : IExampleBase
{
protected ExampleBase()
{
this.SetupData();
}
protected abstract Dictionary<int, Adress> RelevantData { get; set; }
protected abstract void SetupData();
public void ProcessData()
{
// use RelevantData
}
}
And a derived class:
public class Example : ExampleBase
{
public Example()
{
}
protected override void SetupData()
{
this.RelevantData = new Dictionary<int, Adress>
{ 1, new Adress { ... } },
{ 2, new Adress { ... } }
}
}
In the base class, ReSharper tells me
Virtual member call in constructor
I understand that it's dangerous to call the method because of the execution order.. but how can I resolve this issue?
Context: I want to set up data in each derived class which will then be processed in the base class. I wanted to call the SetupData() method in the base class since it's the same in every derived class.
Derived class:
Set up the data
Base class:
Process the data
You don't. You accept the fact this is dangerous, and (try to) prevent this. This is a design flaw!
You could prevent this for example by moving the call to the highest level class, or make every class responsible for it's own, thus removing the unsafe part of the method call. Then you don't need another class (a base class) to take responsibility for its deriving classes.
If that isn't possible. Make very clear using comments or any other method available that the developer should take this problem into account when updating the code.
Call SetupData in the constructor of Example (and every other derived class) not ExampleBase and make Example a sealed class.
The problem is that SetupData could access something that would be initialized by the Example constructor. But the Example constructor is called only after ExampleBase constructor has finished.
Your base class constructor is called first. If your override method in your subclass depends on anything done in its constructor it won't work. Personally I'd look for a different design, maybe passing the abstract class into the derived class rather than using inheritance.
So a couple lines of code in every derived class
If you need to control the process order then you can do this
public abstract class MyBase
{
public void ProcessData()
{
bool processData = true;
}
public MyBase()
{
bool myBase = true;
}
public MyBase(int pass)
{
bool myBase = true;
}
}
public class Example : MyBase
{
public void GetData() {}
public Example()
: base(1)
{
bool example = true;
GetData();
ProcessData();
}
}
I've got an issue with generics. I've those classes :
abstract class BaseTestClass<T> : where T : class, new()
{
//base test class implementation
public abstract void Run(BaseDataClass<T> data);
}
class BaseDataClass<T> : where T : class, new()
{
//base data class implementation
}
class DataA : BaseDataClass<SettingsA>
{
//some stuff
}
class TestA : BaseTestClass<SettingsA>
{
//Works!
public override void Run(BaseDataClass<SettingsA> data)
{
}
//Doesn't Work!
public override void Run(DataA data)
{
}
}
My question if why can't I use the inherited class in the abstract method ?
[EDIT]
The error at compilation is:
TestA does not implement inherited abstract member Run(BaseDataClass)
You can, but the BaseTestClass<SettingsA> base class simply does NOT have a method to override of the signature Run(DataA), but only one of the signature Run(BaseDataClass<DataA>).
The generic inheritance also means that the generic T type is the same.
You can implement this with an additional generic argument, Type-Safe and without cast:
internal abstract class BaseTestClass<T, Y>
where T : class, new()
where Y : BaseDataClass<T>
{
private T m_data;
//base test class implementation
public abstract void Run(Y data);
}
public class BaseDataClass<T> where T : class, new()
{
}
internal class TestA : BaseTestClass<SettingsA, DataA>
{
public override void Run(DataA data)
{
throw new NotImplementedException();
}
}
class DataA : BaseDataClass<SettingsA>
{
}
class SettingsA
{
}
This is type-safe because the constraint is
where Y : BaseDataClass<T>
If you don't need T directly in you base class, you can only use one generic parameter and remove T
Your second method does not compile because there is no method to be overriden by it, removing the override modificator will make your code compile.
If you want to have a method which will run only when the parameter is of type DataA, but still execute the interface method implementation, you can do this way:
class TestA : BaseTestClass<SettingsA>
{
//Works!
public override void Run(BaseDataClass<SettingsA> data)
{
}
public void Run(DataA data)
{
//dp some stuff
Run((BaseDataClass<SettingsA>)data);
}
}
But note that this is not bullet proof, you could miss boxed calls, the better way is do this way:
class TestA : BaseTestClass<SettingsA>
{
//Works!
public override void Run(BaseDataClass<SettingsA> data)
{
var myDataA = data as DataA;
if (myDataA != null)
{
//your parameter is a DataA;
}
}
}
It does not work because you cannot override one method with two methods. Take the override off of one of the methods in TestA, and it will all work. There's no point in overriding a single method twice, anyhow, within a single class.
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 */
}
}
I have a database table which contains an ID column and a Name column. I am tasked with designing a program that accepts one of the IDs as an argument to Main().
Bold is edit 2
I need to use that ID which must exist in the database, to correspond to some code to run. Each row in the table corresponds to slightly different code, but a lot of them share a lot of code. I need a design that will minimize code duplication.
So far what I've developed is an abstract base class that has an abstract Int32 field ID to enforce derived classes having their corresponding ID in the database. That way I can reflect over the derived classes to find the one whose ID matches the Main() argument and instantiate that class. Then I just call the virtual methods from Main() which runs the most derived code that has been defined.
public abstract class Base {
public abstract Int32 Id { get; }
public void Foo() {
// Do something
}
}
public class Derived {
public override Int32 Id { get { return 42; } }
public void Foo() {
// Do something more specific
}
}
Does anyone have any better ideas how to achieve what I want? I like the idea of keeping the ID right in the class definition, but I'm open to changing that if it makes sense.
Thanks!
EDIT:
One thing I don't like about this is that I have to reflect over each derived type and instantiate that type to check the ID. Does anyone have a better idea on how to do that?
Instead of using a property to define the ID of the class, use a custom attribute. That way, you don't have to instantiate the object to check what its ID is.
When your program runs, it can scan the assembly for all classes with that attribute tag, and find the one with the matching ID, instantiate that class, and then run it's Foo method. If you perform this kind of lookup multiple times per application run, you could instatiate all the classes with your custom attribute and then put them into a Dictionary to provide quick lookups by ID.
Your code might look something like this:
[AttributeUsage(AttributeTargets.Class)]
public class CommandAttribute {
public CommandAttribute(int id) {
ID = id;
}
public int ID { get; private set; }
}
public abstract class Command {
public abstract void Execute();
}
[Command(2)]
public class MyCommand : Command {
public override void Execute() {
//Do something useful
}
}
The other advantage of using a custom attribute is that you have to explicitly tag everything that is a candidate for being instantiated and executed by ID, rather than assuming than anything derived from your base class is a candidate. If you are sharing code between the classes, you might want to make a common base class for them that derives from your base class, but should not be instantiated or executed on its own.
One thing I don't understand is, what is the point of the "Name" field if the class you want to run is identified by the ID? If you can decide what the name of each ID is, then you could use the name field as the fully qualified type name of the class you want to execute, which then avoid having to scan through all the types in your assembly (or application domain, depending upon the scope of your search). That setup is a bit more prone to typos, however.
It sounds like you need to implement a factory pattern.
I would define an interface:
public interface IWidget
{
void Foo();
}
Then the base class:
public abstract class WidgetBase : IWidget
{
public void Foo()
{
this.Bar()
}
protected virtual void Bar()
{
// Base implementation
}
}
The factory:
public static WidgetFactory
{
public static IWidget Create(int id)
{
// Get class name from id, probably use the name in your database.
// Get Type from class name
// Get constructor for Type
// Create instance using constructor and return it.
}
}
A derived class:
public class DerivedWidget : WidgetBase
{
protected override void Bar()
{
// call base implementation
base.Bar();
// derived implementation
}
}
In your main:
public void Main(int id)
{
var widget = WidgetBase.Create(id);
widget.Foo();
}
I like #Xint0's idea of using a Factory for this kind of task, but I thought I'd still contribute another answer.
A better way to implement your original design would be to pass the ID to the base constructor as follows:
public abstract class Base {
public Int32 Id { get; private set; }
protected Base(Int32 id) {
this.Id = id;
}
public void Foo() {
// Do something
}
}
public class Derived : Base {
public Derived : base(42) {}
public void Foo() {
// Do something more specific
}
}