C# Generic — 01
First, check how to define a generic class.
We defined the generic class as above. We can call it Repository[Class name] of T.
So we can define some classes in a school management system.
So now we have two Entity classes.
Let’s create a generic repository class.
After creating instances.
Generic class inheritance
If we inherited a generic class to a non-genric class, we need to define the type in there. (change the superclass _dataset access modifier to protected )
But also we can create a generic sub-class.
Constraints on generic type parameters
If we add the below method to the repository class, it will give an error
Because type T doesn’t know whether the concrete type will have an Id property or not, so we need to add a constraint that all the types that pass to this will have Id prop.
So we can encapsulate the Id and use it as a superclass
Change the Repository class
We only define IEntity type in type constraint. So that means T can be value type or reference type.
But if we added EntityBase , that means we need to be T as EntityBase type. That means T must be reference type.
If we only add IEntity , we can’t return null from a below method.
Because T can be null-nullable value type.
So we can update as below.
As a important point if we want to return null as explicitly we need to define that T is a reference type. So we add that T is a reference type add class keyword.
We can add ? mark as this can be nullble reference type.
If we want to return a instance of type T we need to add new() to the end.
Impotant thing is we need to make sure passing type need to have default constructor.
