I'm new to C# and programming in general and wondering about the notation <T>
The question has been asked before, for example here: What does "T" mean in C#?
I just wanted to get some clarification to extend on that.
Can <T> be anything? I understand it is a naming convention by MS and could be named whatever you want, and it is for generic types, int, bools or whatever - but can it extend beyond that?
Can I pass an entire function/method to it just for the sake of it? Or is it strictly for return types if that makes sense?
Generic parameters, specified using the <> notation indicate the type. It is up to the defining class whether or not to limit the types that can be supplied. For example you could have a base class
public class Animal
{}
and a several derived classes
public class Dog : Animal
{}
public class Cat : Animal
{}
You could then make human a class like this
public class Human
{
public void AddPet<T>() where T: Animal
{}
}
In this case the T is constrained to be only things that inherit from Animal
So, you could do this
var me = new Human();
me.AddPet<Dog>();
but not this
var me = new Human();
me.AddPet<Human>();
This is a way of declaring new types/classes that can use another type inside initially not defined. But whenever you instantiate new object of that class, you can to specify that type and it will only be used for that object.
In short each of object of that class might have different type for that value on instantiation.
public class Test<T>
{
public T a;
}
This is a generic class where the type of a isn't known at this stage, but let's see example objects instantiations of that class:
var obj1 = new Test<int>(); // a will be of type int
var obj2 = new Test<List<string>>(); // a will be a list of strings
Type is dependent on object instantiation...
Update: Check docs for more details about generics https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/generic-classes
T represents generic type. It could be any type (user/system defined).
https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic?view=net-6.0
Please mark this answer, if it is useful to you.
Related
I have the following setup and it seems that my object cannot be converted to the generic type. While it is actually the base class. Why doesn't this work? It seems so logical to me.
public class AList<T> where T : A
{
void DoStuff(T foo)
{
}
void CallDoStuff()
{
DoStuff(new A()); // ERROR: Cannot convert A to T
}
}
public class A
{
}
The problem here is that the constraint says that T must be A or a class derived from A.
Now, when you instantiate AList with a concrete type, T is a very specific class. And if you didn't instantiate AList with A itself but with a subclass of it, T is a subclass of A.
You can't convert an instance with a runtime type of the base class to one of its subclasses, as it misses all the information that is being added by the subclass.
Example:
public class Animal
{
public int Foo { get; set; }
}
public class Cat : Animal
{
public int Bar { get; set; }
}
Derived d = new Base();
Would you expect that code to work? Surely not, because a Cat is also a Animal but a Animal is not a Cat.
If you would expect the above code to actually work, ask yourself what is supposed to happen when the following code is executed: d.Bar = 42;
Animal doesn't contain a definition for Bar.
The same is happening in your code - it's just a little bit more obscured with the generics in the game.
T could be also a class that derives from A so you can't put instance of A as a parameter of type T. Something like invoking method, that takes int with and argument that is of type object.
This is the same reason that is an object can not be converted to int implicitly.
The method expects a child class and you are passing the parent, so you need an explicit cast.
Because you are asking T to extend A. So you can replace T with A but not A with T.
if Cat : Animal it doesn't mean you can always convert an Animal to a Cat.
Try and use Activator to give you an instance of A from T because you know that T has to be A due to your constraint, so there isn't a need to use new A() anywhere as you have T, so you can just create an instance of T.
T obj = Activator.CreateInstance<T>();
DoStuff(obj);
Or do a cast of your object as other answers have mentioned, however this will not work in all cases.
T obj = (T)new A();
DoStuff(obj);
I haven't found my use case in the existing questions so here I go.
I have an abstract class that has generics like this :
public abstract class aParameter<T>
{
private T _parameter;
public T Parameter
{
get { return _parameter;}
}
...
}
Then I have my "Type" classes like this :
public class IntParameter : aParameter<Int32>
{
public IntParameter(Int32 value)
{
_parameter = value;
}
}
public class TextParameter : aParameter<String>
{
public TextParameter(String value)
{
_parameter = value;
}
}
Now in my application code, I'd like to create a dictionary with the abstract generic class without specifying the generic type. Like this :
...
private Dictionary<Int32, aParameter> _paramDict = new Dictionary<Int32, aParameter>();
...
In a previous version, I used interfaces instead of abstract class for aParameter but I wanted to migrate it to abstract so that I could simplify my "Type" classes and not repeat identical code for each "Type".
Any thoughts here?
EDIT
I forgot to mention that it will be part of a library that is meant to be distributed among the company so I really need to secure the allowed types. I can't allow for just objects to be fed into my code.
You probably still going to need the non-generic interface (or nongeneric abstract base class as Brandon says), unless you drop to working with objects.
The reason is that aParameter<String> is not the same type as aParameter<int32> so you'll not be able to get them into the one dictionary without some help.
My preferred method around this is to declare a non-generic interface and use that as the basis for the dictionary, then implement it in each of your typed generics, and also in an abstract base class. That way you can inherit from the base class where possible, or just implement the interface if one of your types needs to inherit from something else - you get maximum flexibility.
Make a non-generic abstract base class and have your generic abstract class inherit from it.
The purpose of generic types is (among other things) that you can only write code once that can be used for multiple types. However, as C# is strongly types, the compiler needs to know what types it is dealing with in the generic class. This is why you have to specify the type of the generic (i.e. the type in angled brackets)
If you don't specify the type of the generic then, in your example, the compiler wouldn't know the type of the Parameter property.
Depending on exactly what you're doing with it, there are a number of possible approaches:
Use a Dictionary<int, object>.
When reading the values, you can use a bunch of if/elses to check the specific type of the object and convert to the appropriate type. e.g.
if(obj.GetType() == typeof(TextParameter))
{
TextParameter p = obj as TextParameter
// Do stuff
}
else if obj.GetType() == typeof(IntParameter))
{
IntParameter p = obj as IntParameter
// Do stuff
}
Have multiple dictionaries. Dictionary<int, TextParameter>, Dictionary<int, IntParameter>, etc.
If there are methods/properties in aParameter that are not type dependent then move them to a lower level non-generic abstract class. This could give you at least some of your functionality without having to resort to type conversions.
I'm working on a small class library at work, and it naturally involves using generics for this task. But there is this thing that I don't really understand with generics:
Why would I need to use generic type parameters, and then constrain the the type parameter to a specific base class or interface.
Here's an example to what I mean:
public class MyGenericClass<T> where T : SomeBaseClass
{
private T data;
}
And here's the implementation without generics
public class MyClass
{
private SomeBaseClass data;
}
Are these two definitions the same (if yes, then i don't see the advatage of using generics here)?
If not, what do we benefit from using generics here?
As with almost all uses of generics, the benefit comes to the consumer. Constraining the type gives you the same advantages that you get by strongly typing your parameter (or you can do other things like ensure that there's a public parameterless constructor or ensure that it's either a value or reference type) while still retaining the niceties of generics for the consumer of your class or function.
Using generics also, for example, allows you to obtain the actual type that was specified, if that's of any particular value.
This example is a little contrived, but look at this:
public class BaseClass
{
public void FunctionYouNeed();
}
public class Derived : BaseClass
{
public void OtherFunction();
}
public class MyGenericClass<T> where T: BaseClass
{
public MyGenericClass(T wrappedValue)
{
WrappedValue = wrappedValue;
}
public T WrappedValue { get; set; }
public void Foo()
{
WrappedValue.FunctionYouNeed();
}
}
...
var MyGenericClass bar = new MyGenericClass<Derived>(new Derived());
bar.Foo();
bar.WrappedValue.OtherFunction();
The difference is that the former defines the new class as a specific type; the latter simply defines a plain class with a field of that type.
It's all about type safety. Using generics you can return a concrete type (T) instead of some base type which defines the API you need in your generic class. Therefore, the caller of your method won't have to cast the result to the concrete type (which is an error-prone operation).
The main difference is in usage. In the first case, the usage can have:
MyGenericClass<SomeDerivedClass> Variable
Variable.data.SomeDerivedProperty = X
And so that when you use that class, you can still access anything from SomeDerivedClass without casting back to it.
The second example will not allow this.
MyClass.data = SomeDerivedClassInstance
MyClass.data.SomeDerivedProperty = X //Compile Error
((SomeDerivedClass)MyClass.data).SomeDerivedProperty = X //Ewwwww
You will have to cast back up to the SomeDerivedClass (which is unsafe) to use something specific to the derived class.
I don't think that there is a huge amount of difference except that the generic version is constraining your Class, whereas the second is just a constraint on a member of the class. If you added more members and methods to your first Class, you would have the same constraint in place.
A colleague of mine posted a question on an internal forum which got me thinking about whether this was possible through C#. Basically, he's got an interface as follows:
public interface IProvider<T>
{
T GetT();
}
Is it possible to use something that implements that interface as a type parameter to another generic class and have access to the type T without re-specifying it? For example:
public class Foo<P> where P : IProvider<T>
{
P p;
T GetInnerT() { return p.GetT(); }
}
This does not compile, because the type T is not defined and hence can't be used as a parameter for IProvider. Is something like this even possible? Just curious!
No, he'd need Foo to be generic in both P and T:
public class Foo<P, T> where P : IProvider<T>
otherwise there's no T for the IProvider<T> constraint to be aware of - the fact that it's part of the declaration of IProvider<T> is coincidental. The T in the above declaration is entirely separate (from the compiler's point of view) from the T in IProvider<T>. For example, this would be an equivalent declaration:
public class Foo<TProvider, TProvided> where TProvider : IProvider<TProvided>
Another thing to bear in mind is that an implementation of IProvider<T> could implement it multiple times, for different type arguments:
public class BigProvider : IProvider<string>, IProvider<int>, IProvider<char>
Now what would Foo<BigProvider> mean? It would be ambiguous... whereas with my declaration above, you'd do:
var x = new Foo<BigProvider, int>();
to mean the int-providing aspect of BigProvider.
No, it's not possible, because your definition doesn't allow for the provision of type T when declaring Foo. Consider:
var x = new Foo<string>();
What is T in this case? There is no way to know at compile-time nor at runtime.
As you yourself said: the type T is not defined, and for the class to be valid you have to provide a means of definition. For example:
public class Foo<P, T> where P : IProvider<T>
Consider, I have the following 3 classes / interfaces:
class MyClass<T> { }
interface IMyInterface { }
class Derived : IMyInterface { }
And I want to be able to cast a MyClass<Derived> into a MyClass<IMyInterface> or visa-versa:
MyClass<Derived> a = new MyClass<Derived>();
MyClass<IMyInterface> b = (MyClass<IMyInterface>)a;
But I get compiler errors if I try:
Cannot convert type 'MyClass<Derived>' to 'MyClass<IMyInterface>'
I'm sure there is a very good reason why I cant do this, but I can't think of one.
As for why I want to do this - The scenario I'm imagining is one whereby you ideally want to work with an instance of MyClass<Derived> in order to avoid lots of nasty casts, however you need to pass your instance to an interface that accepts MyClass<IMyInterface>.
So my question is twofold:
Why can I not cast between these two types?
Is there any way of keeping the niceness of working with an instance of MyClass<Derived> while still being able to cast this into a MyClass<IMyInterface>?
This does not work because C# only supports covariance on the type parameters of interfaces and delegates. If your type parameter exists only in output positions (i.e. you only return instances of it from your class and don't accept it as an argument) you could create an interface like this:
interface IClass<out T> { }
class MyClass<T> : IClass<T> { }
Which would allow you to do this:
IClass<Derived> a = new MyClass<Derived>();
IClass<IMyInterface> b = a;
Honestly that is about as close as you are going to get and this requires the C# 4 compiler to work.
The reason you cannot do this in general is because most classes are not simple empty examples. They have methods:
class MyClass<T>
{
static T _storage;
public void DoSomethingWith(T obj)
{
_storage = obj;
}
}
interface IMyInterface { }
class Derived : IMyInterface { }
MyClass<Derived> a = new MyClass<Derived>();
Now, a has a method DoSomethingWith that accepts a Derived and stores it in a static variable of type Derived.
MyClass<IMyInterface> b = (MyClass<IMyInterface>)a;
If that was allowed, b would now appear to have a method DoSomethingWith that accepts anything that implements IMyInterface, and would then internally attempt to store it in a static variable of type Derived, because it's still really the same object referred to by a.
So now you'd have a variable of type Derived storing... who knows what.