I have a generic class that has one type parameter (T). I needed to store a collection of these generic objects that are of different types, so I created an interface that the generic class implements as suggested here. There is a property in the generic class of type T that I need to access when iterating through the generic list that contains the collection of Interface objects. So far the only way I have been able to get the value is to call a method using reflection.
interface ISomeClass {
//?
}
class SomeClass<T> : ISomeClass {
T ValueINeed { get; set;}
}
class ClassThatHasListOfGenericObjects{
List<ISomeClass> _l = new List<ISomeClass>();
public AddToList<T>(T someClass) : where T : ISomeClass {
_l.Add(someClass);
}
public SomeMethod(){
foreach(ISomeClass i in _l){
i.ValueINeed; //I don't know how to access the property in the generic class
}
}
}
As I see it you have two options. The easy option is to expose the value (as an object) on the interface (and possibly its type as well). Here's how that would look:
interface ISomeClass
{
object ValueINeed { get; set; }
// Only needed if you care about static type rather than using ValueINeed.GetType()
Type TypeOfValue { get; }
}
class SomeClass<T> : ISomeClass
{
public T ValueINeed { get; set; }
public Type TypeOfValue { get { return typeof(T); } }
object ISomeClass.ValueINeed { get { return ValueINeed; } set { ValueINeed = (T)value; } }
}
This has the disadvantage that there's a bit of casting going on and you might need to invoke reflection to do certain things with the value. It has the advantage that it's easy to understand and implement.
The other alternative would be to encode an "existential type" which truly represents a SomeClass<T> for some unknown T (like a SomeClass<?> in Java). This is much more complicated and hard to follow, but avoids any casts:
interface ISomeClassUser<X>
{
X Use<T>(SomeClass<T> s);
}
interface ISomeClassUser
{
void Use<T>(SomeClass<T> s);
}
interface ISomeClass
{
X Apply<X>(ISomeClassUser<X> user);
void Apply(ISomeClassUser user);
}
class SomeClass<T> : ISomeClass
{
public T ValueINeed { get; set; }
public X Apply<X>(ISomeClassUser<X> user) { return user.Use(this); }
public void Apply(ISomeClassUser user) { user.Use(this); }
}
// Assumes you want to get a string out, use a different generic type as needed
class XmlUser : ISomeClassUser<string>
{
public string Use<T>(SomeClass<T> s)
{
string str = "";
// do your conditional formatting here, branching on T as needed
// ...
return str;
}
}
class ClassThatHasListOfGenericObjects
{
List<ISomeClass> _l = new List<ISomeClass>();
XmlUser user = new XmlUser();
public string SomeMethod()
{
string s = "";
foreach (ISomeClass i in _l)
{
s += i.Apply(user);
}
return s;
}
}
Add ValueINeed to the interface and you'll be able to call it in SomeMethod().
I think you might just need a little refactoring. Looks like you're almost there
interface ISomeClass<T> {
T ValueINeed { get; set; }
}
class SomeClass<T> : ISomeClass {
T ValueINeed { get; set;}
}
class ClassThatHasListOfGenericObjects{
List<ISomeClass> _l = new List<ISomeClass>();
public AddToList<T>(T someClass) : where T : ISomeClass {
_l.Add(someClass);
}
public SomeMethod(){
foreach(ISomeClass i in _l){
i.ValueINeed; //this will work now, since it's in the interface
}
}
}
The elements' types you are using is of ISomeClass, so if want to access a member property you need to either cast i to SomeClass or add the property deceleration to the interface
interface ISomeClass {
T ValueNeeded
{
get;
set;
}
}
Note that you still need to implement the property in SomeClass.
Related
First, some background on my code.
I have a interface for items that can be saved/used with a data store/repository type service:
public interface IItem
{
Guid Id { get; set; }
}
public interface IDataStore<T> where T: IItem { ... }
An example of such an item:
public class ItemNormalExample : IItem
{
public Guid Id { get; set; }
public string RandomProperty { get; set; }
}
And then I also have special/specific type of item, which other items inherit from. This is where my problem lies.
public interface ISpecialItem : IItem
{
string ImportantString { get; set; }
}
public class ItemSpecialBase : ISpecialItem
{
public Guid Id { get; set; }
public string ImportantString { get; set; }
}
public class ItemSpecialExample : ItemSpecialBase
{
public string RandomProperty { get; set; }
}
I am using Autofac to resolve instances of this data store of each concrete implementation of IItems. I.e.:
builder.RegisterType<DataStore<ItemNormalExample>>().As<IDataStore<ItemNormalExample>>();
This works as intended, however, for my objects inheriting from the ItemSpecialBase base class, I would like these to be instantiated as a DataStore of the base class type instead of the actual child class.
Something like this (This doesn't work):
builder.RegisterType<DataStore<ItemSpecialBase>>().As<IDataStore<ItemSpecialExample>>();
My intend is for every object inheriting from ItemSpecialBase to use the same DataStore. My DataStore class supports this and works fine.
How can I accomplish this using Autofac?
Looks like you have run into a Variant Generic Interface problem!
Specifically, when you do...
builder
.RegisterType<DataStore<ItemSpecialBase>>()
.As<IDataStore<ItemSpecialExample>>();
...you're attempting to cast a Storage<Base> into an IStorage<Child>, which is known as Contravariance.
The Contravariance problem can be simplified as so:
class Animal { }
class Cat : Animal { }
class Dog : Animal { }
interface IStorage<T> {
void Put(T thing);
T? Get();
}
class Storage<T> : IStorage<T> {
public void Put(T t) { }
public T? Get() { return default(T); }
}
void Main() {
var builder = new ContainerBuilder();
// What you're attempting to do:
builder.RegisterType<Storage<Animal>>().As<IStorage<Cat>>();
builder.Build();
// Is equivalent to this:
IStorage<Cat> cats = new Storage<Animal>();
// The compiler will stop you from doing either.
}
The problem is that generics are validated at compile-time. And the compiler notices that, while you can Put a Cat in an Animal Storage, when you try to Get something out of storage it cannot guarantee you'll get a Cat - the storage may contain Dogs. (note the in and out, which are relevant to the covariant/contravariant discussion).
One thing you can do is to acknowledge to the compiler that you will only attempt to put a Child class in, and will not try to get a Child out:
// Notice the `in` constraint
interface IStorage<in T> {
// We can pass `T` in as parameter
void Put(T thing);
// We cannot return `T` anymore, because we promised to
// only use T in the `in` direction, and not out.
object? Get();
}
// This stays the same. `in` can only be enforced against interfaces.
class Storage<T> : IStorage<T> {
public void Put(T t) { }
public object? Get() { return default(T); }
}
void Main() {
var builder = new ContainerBuilder();
// This is now allowed:
builder.RegisterType<Storage<Animal>>().As<IStorage<Cat>>();
builder.Build();
// And this too:
IStorage<Cat> cats = new Storage<Animal>();
}
I have a problem with designing my interfaces
I have these interfaces :
interface IField {
}
interface IScreenField : IField {
}
interface ITable {
CustomCollection<IField> CustomCollection { get; set; }
}
interface IScreen
{
AnotherCustomCollection<IScreenField> AnotherCustomCollection { get; set; }
}
IScreen interface should inherit from ITable but it shows an error that I have to implement a collection of IField but I already implement a collection of IScreenField
What is the soltuion for this ?
I uploaded a sample project to explain the issue more
You can check the error message in Screen class that says :
Screen does not implement interface member ITable.Fields. Screen.Fields cannot implement ITable.Fields becuase it does not have the matching return type of CusomCollection<IField>
Here is the sample :
Sample project
This description of this example helps you to solve the problem: If IExample2 inherits another Interface, when implementing IExample2 u need to implement
all the method(properties etc...) that has been declared in interface + the method of inhered interfaces from IExample2. Remember that when you implement an interface you have to implement all of members of that interface (you have to implement even the members of all interfaces that are in chain) and all the returns types has to be the same in interface and in class.
interface IExample
{
void Method1();
}
interface IExample2 : IExample
{
void Method2();
}
class Screen : IExample2
{
public void Method2()
{
}
public void Method1()
{
}
}
Chain Example
interface IExample
{
void Method1();
}
interface IExample2 : IExample
{
void Method2();
}
interface IExample3 : IExample2
{
void Method3();
}
This is the answer I posted on your previous post. Reposting it here as-is since your previous question was put on hold before I could
submit the answer.
It’s very difficult to identify the problem without looking at the full code but based on what you have said, I believe, you have implemented IScreenField explicitly and the compiler is not able to find any implementation of IField.
Checkout following for more info:
https://www.codeproject.com/Articles/1000374/Explicit-Interface-VS-Implicit-Interface-in-Csharp
Update: After looking at the code
First of all you need to understand difference between Implicit and Explicit implementations of an Interface:
Implicit: you access the interface properties and properties as if they were part of the class.
Explicit: you can only access properties and properties when treating the class as the implemented interface.
The problem with the Screen class is that it implements IScreen interface, which in-turn implements ITable interface. Both these interfaces have a property named Fields.
The problem surfaced due to this and you are required to
explicitly implement the Fields property in Screen class.
NOTE: It is irrelevant that you have different return types. Since you have implemented Fields property in both interfaces, it is assumed that you are expecting different values when the property is accessed from each of the interfaces.
public class Screen : IScreen
{
public string Name { get; set; }
AnotherCustomCollection<IBaseField> IScreen.Fields
{
get
{
return default(AnotherCustomCollection<IBaseField>);
}
}
CustomCollection<IField> ITable.Fields
{
get
{
return default(CustomCollection<IField>);
}
}
public string Title { get; set; }
public string Description { get; set; }
}
Now how to access them? To access Fields property of each of these Interfaces you need to access Screen object as those interfaces.
Ex:
var screen = new Screen();
var fields = screen.Fields; // Error
var fields = (screen as IScreen).Fields; // returns property value of IScreen Fields
var fields = (screen as ITable).Fields; // returns property value of ITable Fields
Here is the complete code: https://dotnetfiddle.net/5KS0Xd
Hope this was helpful. All the best and happy coding.
You could do something like this:
public class Screen : IScreen
{
public string Name { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public AnotherCustomCollection<IBaseField> Fields { get; set; }
CustomCollection<IField> ITable.Fields
{
get
{
throw new System.NotSupportedException();
}
}
}
And apparently the compiler likes that, and if you do something like this:
public static void Main(string[] args)
{
var collection = new List<IScreen>()
{
new Screen
{
Fields = new AnotherCustomCollection<ScreenInterface.IBaseField>
{
new TextField()
{
Name = "Hello"
}
}
}
};
var y = collection.First();
//Prints "Hello"
Console.WriteLine(string.Join(" ", y.Fields.Select(x => x.Name)));
Console.ReadLine();
}
But, if you are working with the upper interface (ITable)
public static void Main(string[] args)
{
var collection = new List<ITable>() //here
{
new Screen
{
Fields = new AnotherCustomCollection<ScreenInterface.IBaseField>
{
new TextField()
{
Name = "Hello"
}
}
}
};
var y = collection.First();
//Throws NotSupportedException
Console.WriteLine(string.Join(" ", y.Fields.Select(x => x.Name)));
Console.ReadLine();
}
My guess is that there isn't the concept of generic inheritance, and that may be proved if you switch the conditional generic parameter of AnotherCustomCollection from IBaseField to IField, and instead of throwing the exception, return the public Fields property on Screen.ITable.Fields. Compiler will automatically recognize the concrete property and everything will work.
So, for this to work, either define an implicit operator or a custom getter:
public class Screen : IScreen
{
public string Name { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public AnotherCustomCollection<IBaseField> Fields { get; set; }
CustomCollection<IField> ITable.Fields
{
get
{
var customCollection = new CustomCollection<IField>();
customCollection.AddRange(Fields);
return customCollection;
}
}
}
Is it possible to add different type of generic objects to a list?. As below.
public class ValuePair<T>
{
public string Name { get; set;}
public T Value { get; set;
}
and let say I have all these objects...
ValuePair<string> data1 = new ValuePair<string>();
ValuePair<double> data2 = new ValuePair<double>();
ValuePair<int> data3 = new ValuePair<int>();
I would like to hold these objects in a generic list.such as
List<ValuePair> list = new List<ValuePair>();
list.Add(data1);
list.Add(data2);
list.Add(data3);
Is it possible?
In general, you'd have to either use a List<object> or create a non-generic base class, e.g.
public abstract class ValuePair
{
public string Name { get; set;}
public abstract object RawValue { get; }
}
public class ValuePair<T> : ValuePair
{
public T Value { get; set; }
public object RawValue { get { return Value; } }
}
Then you can have a List<ValuePair>.
Now, there is one exception to this: covariant/contravariant types in C# 4. For example, you can write:
var streamSequenceList = new List<IEnumerable<Stream>>();
IEnumerable<MemoryStream> memoryStreams = null; // For simplicity
IEnumerable<NetworkStream> networkStreams = null; // For simplicity
IEnumerable<Stream> streams = null; // For simplicity
streamSequenceList.Add(memoryStreams);
streamSequenceList.Add(networkStreams);
streamSequenceList.Add(streams);
This isn't applicable in your case because:
You're using a generic class, not an interface
You couldn't change it into a generic covariant interface because you've got T going "in" and "out" of the API
You're using value types as type arguments, and those don't work with generic variable (so an IEnumerable<int> isn't an IEnumerable<object>)
Not unless you have a non-generic base-type ValuePair with ValuePair<T> : ValuePair (it would work for an interface too), or use List<object>. Actually, though, this works reasonably:
public abstract class ValuePair
{
public string Name { get; set; }
public object Value
{
get { return GetValue(); }
set { SetValue(value); }
}
protected abstract object GetValue();
protected abstract void SetValue(object value);
}
public class ValuePair<T> : ValuePair
{
protected override object GetValue() { return Value; }
protected override void SetValue(object value) { Value = (T)value; }
public new T Value { get; set; }
}
No, it is not possible. You could create, in your case, a base class ValuePair from which ValuePair<T> derives. Depends on your purposes.
it's not possible as far as I know.
the line:
List<ValuePair> list = new List<ValuePair>();
you wrote in your sample is not providing a concrete type for T and this is the issue, once you pass it, you can only add object of that specific type.
Is it possible to add different type of generic objects to a list?. As below.
public class ValuePair<T>
{
public string Name { get; set;}
public T Value { get; set;
}
and let say I have all these objects...
ValuePair<string> data1 = new ValuePair<string>();
ValuePair<double> data2 = new ValuePair<double>();
ValuePair<int> data3 = new ValuePair<int>();
I would like to hold these objects in a generic list.such as
List<ValuePair> list = new List<ValuePair>();
list.Add(data1);
list.Add(data2);
list.Add(data3);
Is it possible?
In general, you'd have to either use a List<object> or create a non-generic base class, e.g.
public abstract class ValuePair
{
public string Name { get; set;}
public abstract object RawValue { get; }
}
public class ValuePair<T> : ValuePair
{
public T Value { get; set; }
public object RawValue { get { return Value; } }
}
Then you can have a List<ValuePair>.
Now, there is one exception to this: covariant/contravariant types in C# 4. For example, you can write:
var streamSequenceList = new List<IEnumerable<Stream>>();
IEnumerable<MemoryStream> memoryStreams = null; // For simplicity
IEnumerable<NetworkStream> networkStreams = null; // For simplicity
IEnumerable<Stream> streams = null; // For simplicity
streamSequenceList.Add(memoryStreams);
streamSequenceList.Add(networkStreams);
streamSequenceList.Add(streams);
This isn't applicable in your case because:
You're using a generic class, not an interface
You couldn't change it into a generic covariant interface because you've got T going "in" and "out" of the API
You're using value types as type arguments, and those don't work with generic variable (so an IEnumerable<int> isn't an IEnumerable<object>)
Not unless you have a non-generic base-type ValuePair with ValuePair<T> : ValuePair (it would work for an interface too), or use List<object>. Actually, though, this works reasonably:
public abstract class ValuePair
{
public string Name { get; set; }
public object Value
{
get { return GetValue(); }
set { SetValue(value); }
}
protected abstract object GetValue();
protected abstract void SetValue(object value);
}
public class ValuePair<T> : ValuePair
{
protected override object GetValue() { return Value; }
protected override void SetValue(object value) { Value = (T)value; }
public new T Value { get; set; }
}
No, it is not possible. You could create, in your case, a base class ValuePair from which ValuePair<T> derives. Depends on your purposes.
it's not possible as far as I know.
the line:
List<ValuePair> list = new List<ValuePair>();
you wrote in your sample is not providing a concrete type for T and this is the issue, once you pass it, you can only add object of that specific type.
I have a 3rd party badly designed library that I must use.
It has all sorts of types it works with, we'll call them SomeType1, SomeType2 etc.
None of those types share a common base class but all have a property named Value with a different return type.
All I want to do is to be able to Mixin this class so I'll be able to call someType1Instance.Value and someType2Instance.Value without caring what the concreate type it is and without caring what the return type is (I can use object).
So my code is currently:
public interface ISomeType<V>
{
V Value {get; set;}
}
public interface ISomeTypeWrapper
{
object Value { get; set; }
}
public class SomeTypeWrapper<T> : ISomeTypeWrapper
where T : ISomeType<???>
{
T someType;
public SomeTypeWrapper(T wrappedSomeType)
{
someType = wrappedSomeType
}
public object Value
{
get { return someType.Value; }
set { someType.Value = value != null ? value : default(T); }
}
}
public class SomeType1
{
public int Value { get; set; }
}
public class SomeType2
{
public string Value { get; set; }
}
The problem is that I don't know what T might be until runtime due to the fact that I get a dictionary of objects.
I can iterate the dictionary and use reflection to create a SomeWrapperType on runtime but I would like to avoid it.
How can I mixin the concreate type of SomeType to ISomeType?
How can I know what V type parameter is? (wish I had typedefs and decltype like in c++)
How can I, with the minimum of use of reflection possible Mixin those classes with the interface/base class?
You could try the Duck Typing Extensions for Windsor. It means you will need to register each of your types.
container
.Register(Component.For(typeof(SomeType1)).Duck<ISomeType>())
.Register(Component.For(typeof(SomeType2)).Duck<ISomeType>());
You could probably use linq and the register AllTypes syntax to reduce code if the names are similar.
Alternatively in the short term create a factory which can return you the objects you need, implement a concrete object for each type. No you are using the interface you can remove the factory at a later date and replace it with something else with minimal impact:
public class SomeTypeWrapperFactory
{
public ISomeType<int> CreateWrapper(SomeType1 someType1)
{
return new SomeType1Wrapper(someType1);
}
public ISomeType<string> CreateWrapper(SomeType2 someType2)
{
return new SomeType2Wrapper(someType2);
}
}
public class SomeType1Wrapper : ISomeType<int> { ... }
public class SomeType2Wrapper : ISomeType<int> { ... }
Regardless of how you implement the wrapper, be the individually or using a god like class you have the ability to change how the wrapping is done and keep the rest of your code clean.
Why SomeTypeWrapper but not SomeObjectWrapper?
public class SomeObjectWrapper : ISomeType
{
Object _someObject;
PropertyInfo _valuePropertyInfo;
public SomeObjectWrapper(Object wrappedSomeObject)
{
_someObject = wrappedSomeObject;
_valuePropertyInfo = _someObject.GetType().GetProperty("Value", System.Reflection.BindingFlags.Public);
}
public object Value
{
get { return _valuePropertyInfo.GetValue(_someObject, null); }
set { _valuePropertyInfo.SetValue(_someObject, value, null); }
}
}
Edited With .NET 3.5 using LinFu
You may use LinFu instead of Castle. However, you would be using reflection anyway, both with Castle's and with Linfu's DynamicProxy, only hidden in the guts of the libraries instead of being exposed in your code. So if your requirement to avoid the use of reflection is out of performance concerns, you wouldn't really avoid it with this solution.
In that case I would personally choose Orsol's solution.
However: here's an example with LinFu's ducktyping.
public interface ISomeType {
object Value{get; set;}
}
public class SomeType1
{
public int Value { get; set; }
}
public class SomeType2
{
public string Value { get; set; }
}
public class SomeTypeWrapperFactory
{
public static ISomeType CreateSomeTypeWrapper(object aSomeType)
{
return aSomeType.CreateDuck<ISomeType>();
}
}
class Program
{
public static void Main(string[] args)
{
var someTypes = new object[] {
new SomeType1() {Value=1},
new SomeType2() {Value="test"}
};
foreach(var o in someTypes)
{
Console.WriteLine(SomeTypeWrapperFactory.CreateSomeTypeWrapper(o).Value);
}
Console.ReadLine();
}
}
Since you don't know the type of the SomeType's until runtime, I would not use mixins, but the visitor pattern (I know this doesn't answer the question on how to use mixins for this, but I just thought I'd throw in my 2 cents).
With .NET 4 using dynamic
See Bradley Grainger's post here on using c#4's dynamic keyword to implement the visitor pattern.
In your case, reading all the "Value" properties from your dictionary of SomeType's could work like this:
public class SomeType1
{
public int Value { get; set; }
}
public class SomeType2
{
public string Value { get; set; }
}
public class SomeTypeVisitor
{
public void VisitAll(object[] someTypes)
{
foreach(var o in someTypes) {
// this should be in a try-catch block
Console.WriteLine(((dynamic) o).Value);
}
}
}
class Program
{
public static void Main(string[] args)
{
var someTypes = new object[] {
new SomeType1() {Value=1},
new SomeType2() {Value="test"}
};
var vis = new SomeTypeVisitor();
vis.VisitAll(someTypes);
}
}