I want to access a property of a new created object within a generic method, which is constraint by an Interface:
public interface MyInterface
{
int ID { get; set; }
string Name { get; set; }
}
Since the Compiler knows that "T" is of the Type MyInterface it should be possible to access the properties of that inteface:
public T doSomething<T>(String value) where T : MyInterface, new()
{
T entity = new T();
entity.Name = value;
return entity;
}
But it sais: T does not have a definition for 'Name'
If I can use an interface as a constraint here: Why isn't it possible to access its properties?
The code you posted is correct for itself. Maybe you have different versions of your interface (MyInterface in different namespaces)? Check the namespaces / fully qualified names of the interface types. Also the check the assembly versions, if declaring types in another assembly...
public class Foo2 : MyInterface
{
public int ID { get; set; }
public string Name { get; set; }
}
...
var foo = doSomething<Foo2>("test");
Console.WriteLine(foo.Name);
Seems to work as long as your code has the namespace of your interface and concrete class in a using clause. Also, as a matter of convention MyInterface should be IMyInterface.
Related
How can I separately declare a getter and a setter for a property?
For exemple, say I want to create the following hierarchy:
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
string Value { set; }
}
class BasicFoo : IFoo
{
string Value { get; set; }
}
The compiler is compaining because IFoo.Value is hiding IReadOnlyFoo.Value, which is not what I want to do. I want to "merge" the getter and setter declarations.
I've had a look at how the .NET Framwork declares the IReadOnlyList and IList interfaces, but it's done in a different way.
How could I acheive what I want to do ? Can I do that with a property or do I really have to create separate GetValue() and SetValue() methods instead?
When you change your interface definition to
interface IReadOnlyFoo
{
string Value { get; }
}
interface IReadWriteFoo
{
string Value { get; set; }
}
class BasicFoo : IFoo, IReadOnlyFoo
{
public string Value { get; set; }
}
it should work.
When you implement the interface the two members would be merged since you don't have a get method in IFoo.Value.
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
new string Value { set; }
}
class BasicFoo : IFoo
{
public string Value { get; set; }
}
As long as you are using implicit implementations for the interfaces it would behave as you intended. on the other hand, if you wish to have two different behaviors for the members of the interface then you want to use explicit implementations. You can find an example here
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/interfaces/how-to-explicitly-implement-members-of-two-interfaces
All you have to change in your code is to add a getter to the Value property in the IFoo interface.
Semantically speaking, IFoo is a specific kind of IReadOnlyFoo that adds another capability to it's base type (the Setter of the Value property).
This is the exact definition of inheritance in object oriented programming - a child type is a more specific version of it's base type and is adding capabilities to it.
interface IReadOnlyFoo
{
string Value { get; }
}
interface IFoo : IReadOnlyFoo
{
new string Value { get; set; }
}
class BasicFoo : IFoo
{
public string Value { get; set; }
}
This code is perfectly valid and will give you exactly what you're looking for.
This way, if you have a reference of type IReadOnlyFoo to an instance of BasicFoo, the Value property is indeed readonly, but if your reference type is IFoo it's a read / write property.
You can see a live demo on rextester.
This question already has answers here:
Convert List<DerivedClass> to List<BaseClass>
(13 answers)
Closed 5 years ago.
Let's consider that there is an abstract base class and one, or more child classes:
public abstract class BaseInnerClass
{
public int Id { get; set; }
}
public class ConcreteInnerClass : BaseInnerClass
{
public string Name { get; set; }
}
Then, let's assume there is a generic abstract class that has a property of above abstract class type:
public abstract class GeneriAbstractTestClass<T> where T : BaseInnerClass
{
public T InnerClass { get; set; }
}
Then let's make a class that inherits from the class above:
public class ConcreteTestClass : GeneriAbstractTestClass<ConcreteInnerClass>
{
public string ConcreteString { get; set; }
}
So now everything is prepared to ask a question ;) Why it is not possible to do it:
//cannot convert initializer type
GeneriAbstractTestClass<BaseInnerClass> genericClass = new ConcreteTestClass();
while this is allowed:
//ok
BaseInnerClass baseInner = new ConcreteInnerClass();
What's the difference between this two assignments?
This has nothing to do with abstract classes. A simpler example would be
List<BaseInnerClass> base = new List<ConcreteInnerClass>
The fact that type A is derived from type B does not imply that type C<A> is derived from type C<B>. Your example is a little bit more complicated, but it can be explained using the same logic.
Note that you can define another concrete type:
public class EvilConcreteInnerClass : BaseInnerClass
{
}
If what you wanted was possible, then the following would work:
GeneriAbstractTestClass<BaseInnerClass> genericClass = new ConcreteTestClass();
genericClass.InnerClass = new EvilConcreteInnerClass(); // OK, because the compiler sees `T` as `BaseInnerClass`
genericClass variable points to an object whose T generic parameter is ConcreteInnerClass, so assigning EvilConcreteInnerClass to the property would result in a run-time exception.
Actually. You can do this. But you need to specify interface with covariant out T generic, because it is type safe to make those casts.
Example
namespace ConsoleTest
{
class Program
{
static void Main(string[] args)
{
var a = new Generic<Concrete>();
IGeneric<Base> c = new Generic<Base>();
c = a;
}
}
public interface IGeneric<out T> where T: Base
{
T Inner { get; }
}
public class Generic<T> : IGeneric<T>
where T : Base
{
public T Inner { get; set; }
}
public class Concrete : Base
{
}
public class Base
{
}
}
Delegates also not restricted if they specify covariant out generic templates.
It means those casts you want is OK as long as you use readonly generic properties. So, like #Kapol said and provided you example why it is not type safe to allow setters on properties or pass T into function.
Summary
Use ReadOnly interfaces if you want to use those kinds of casts.
In my abstract base class AbstractType, I have an abstract auto-implemented property Value of unknown type. All my derived classes implement this property with their own types, such string or double. Normally, I know you would just make it AbstractType<T> and have the property be T Value { ... }. However, I don't have the ability to use generics in this case. In AbstractType, I'm trying to implement a method that returns a new derived class from AbstractType, so if I use generics, the caller has to know the type. If I make Value type object, then the caller has to wrap the object to the correct type - very inconvenient type/instance checking.
Here's what my class structure looks like (the method is simplified for the sake of demonstration):
abstract class AbstractType
{
public abstract ??? Value { get; set; }
AbstractType FromValue(int i)
{
if (i == 0)
return new NumberType();
else
return new StringType();
}
}
class NumberType : AbstractType
{
public override double Value { get; set; }
}
class StringType : AbstractType
{
public override string Value { get; set; }
}
Is there any way to do this without using generics?
In My class I have
class MyClass : IMyInterface
{
//I want MyClass only to be able to accept object of type List<SomethingElse>
public List<ISomething> ListOfSomethings {get; set;}
}
interface IMyInterface{
List<ISomething> ListOfSomethings {get; set;}
}
class SomethingElse : ISomething{
}
class SomethingMore : Isomething{
}
Basically I want to know if it is possible to constrain the type that the list uses in MyClass, so if someone tried coding it to the wrong type (i.e. a List of SomethingMore) it would throw an exception.
EDIT: If this is not possible to do, is there an alternative solution that would work?
You can constrain the T (type) of the list items (and any other T) by using the where restriction:
for more details see Constraints on Type Parameters
Interfaces:
interface ISomething { }
Allows to only use Ts that implement the interface ISomething.
interface IMyInterface<T> where T : ISomething
{
List<T> ListOfSomethings { get; set; }
}
Classes:
class SomethingElse : ISomething { }
class SomethingMore : ISomething { }
class MyClass1 : IMyInterface<SomethingElse>
{
public List<SomethingElse> ListOfSomethings { get; set; }
}
class MyClass2 : IMyInterface<SomethingMore>
{
public List<SomethingMore> ListOfSomethings { get; set; }
}
You can restrict the T whereever it suits you. Here for example on the class itself.
This allows only SomethingElse
class MyClass3<T> : IMyInterface<T> where T : SomethingElse
{
public List<T> ListOfSomethings { get; set; }
}
an example with a Dictionary:
var dic = new Dictionary<string, IMyInterface<ISomething>>();
dic.Add("MyClass1", (IMyInterface<ISomething>)new MyClass1());
dic.Add("MyClass2", (IMyInterface<ISomething>)new MyClass2());
if you won't cast it everytime then the only solution that I can currently think of is to create your custom dictionary and encapsulate the casting:
class MyDictionary : Dictionary<string, IMyInterface<ISomething>>
{
public void Add(string key, MyClass1 value)
{
base.Add(key, (IMyInterface<ISomething>)value);
}
public void Add(string key, MyClass2 value)
{
base.Add(key, (IMyInterface<ISomething>)value);
}
}
var dic2 = new MyDictionary();
dic2.Add("MyClass1", new MyClass1());
dic2.Add("MyClass2", new MyClass2());
//I want MyClass only to be able to accept object of type List<SomethingElse>
Then you can't define it as List<ISomething>, but rather use a different interface or even a concrete type. If you define it as List<ISomething>, than it automatically accepts anything, that implements ISomething interface. There's no way around that.
C# is a typesafe by language design, so there is no way the consumer of your list can inject into it a type which is not SomethingElse or SomethingMore.
If you need to constrain to some of types derived from common interface, like SomethingElse, I would go
Hide a property that exposes a List itself
private List<ISomething> ListOfSomethings {get; set;}
Add a public member function, say FromList
public void FromList(List<SomethingElse> somethings)
{
ListOfSomethings = somethings;
}
This function becomes the only way to assign a list to a class, and considering that it accepts only lists of certain type, you are created desired limitation.
You can do this using explicit interface implementation:
class MyClass : IMyInterface
{
List<ISomething> IMyInterface.ListOfSomethings
{
get { return this.ListOfSomethings.Cast<ISomething>().ToList(); }
set { this.ListOfSomethings = value.Cast<SomethingMore>().ToList(); }
}
List<SomethingMore> ListOfSomethings { get; set; }
}
Please note that it is not recommended to make such a restriction, because this violates the Liskov substitution principle: The user of your class might be using the IMyInterface interface and has no idea that its type is restricted.
Another problem is exposing a List<T> like this in an interface: The caller can either call list methods like Add or Remove or set the entire List instance. This is probably not what you want. If you want to expose a readonly collection, use a getter with an array or enumerable type.
I have some classes with common properties, however, I cannot make them derive from a base type (LINQ-to-SQL limitations).
I would like to treat them as if they had a base type, but not by using Reflection (performance is critical).
For example:
public class User
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle
{
public int Id { get; set; }
public string Label { get; set; }
}
In this case I would be happy if I had the Id property available, regardless of the type I'm holding.
Is there any way in C# to to something similar to this:
public static int GetId<T>(T entity) where T // has an int property 'Id'
{
return entity.Id;
}
I guess I could have used dynamic, however, I'm looking for a way to restrict the code in compile time from using this method for an object that has no Id property.
You can use interfaces:
public interface IHasId
{
int Id { get; }
}
public class User : IHasId { ... }
public class Vehicle : IHasId { ... }
public static int GetId<T>(T entity) where T : IHasId
{
return entity.Id;
}
However, if you are not able to modify the classes to add the interface, you won't be able to do this. No compile-time checks will verify that a property exists on T. You'd have to use reflection - which is slow and obviously not ideal.
There is no way to guarantee a type has a given member without constraining to a common base type or interface. One way to work around this limitation is to use a lambda to access the value
public static int Use<T>(T value, Func<T, int> getIdFunc) {
int id = getIdFunc(value);
...
}
Use(new User(), u => u.Id);
Use(new Vehicle(), v => v.Id);
You can create an interface with the common properties and make your classes implement it:
public interface IEntity
{
int Id { get; set; }
}
public class User : IEntity
{
public int Id { get; set; }
public string FirstName { get; set; }
}
public class Vehicle : IEntity
{
public int Id { get; set; }
public string Label { get; set; }
}
public static int GetId<T>(T entity) where T : IEntity
{
return entity.Id;
}
You could simplify GetId like this:
public static int GetId(IEntity entity)
{
return entity.Id;
}
The other answers mentioning the interface approach are certainly good, but I want to tailor the response to your situation involving Linq-to-SQL.
But first, to address the question title as asked
Can C# constraints be used without a base type?
Generally, the answer is no. Specifically, you can use struct, class, or new() as constraints, and those are not technically base types, and they do give some guidance on how the type can be used. That doesn't quite rise to the level of what you wish to do, which is to limit a method to types that have a certain property. For that, you will need to constrain to a specific interface or base class.
For your specific use case, you mention Linq-to-SQL. If you are working from models that are generated for you, then you should have options to modify those classes without modifying the generated model class files directly.
You probably have something like
// code generated by tool
// Customer.cs
public partial class Customer // : EntityBaseClasses, interfaces, etc
{
public int ID
{
get { /* implementation */ }
set { /* implementation */ }
}
}
And other similar files for things such as Accounts or Orders or things of that nature. If you are writing code that wishes to take advantage of the commonly available ID property, you can take utilize the partial in the partial class to define a second class file to introduce a common interface type to these models.
public interface IIdentifiableEntity
{
int ID { get; }
}
And the beauty here is that using it is easy, because the implementation already exists in your generated models. You just have to declare it, and you can declare it in another file.
public partial class Customer : IIdentifiableEntity { }
public partial class Account : IIdentifiableEntity { }
// etc.
This approach has proven valuable for me when using a repository pattern, and wishing to define a general GetById method without having to repeat the same boilerplate in repository after repository. I can constrain the method/class to the interface, and get GetById for "free."
Either you need to make both classes implement an interface with the properties you need, and use that in the generic constraint, or you write separate methods for each type. That's the only way you'll get compile-time safety.