question about Static method/class with where T : class,new(), what does it mean

Question

Hi
I am studying an example project and one class is like this:

public class ClassDB<T> where T : class,new(),
{
public static IEnumerable<T> ExecuteGetList(T whereModel)
}

And in another class(classA) , the code is

public static IEnumerable<T> GetList<T>(T whereModel) where T : class,new()
{
return ClassDB<T>.ExecuteGetList(whereModel);

    }

I guess:
The code "class,new()," in classDB means in classA, we don't have to initialize classDB? because "class,new()," in classDB has already new it？ and what does "class,new()" in classA mean?
Please let me know if I have explained clearly.
Thank you very much

Answer 1

Hi MartinWang-5107,
>>where T : class,new(),
Those are generic type constraints.
where T : class
Means that the type T must be a reference type (not a value type).
where T : new()
Means that the type T must have a parameter-less constructor.
In order to understand it better, please refer to the links below:
What does “where T : class, new()” mean?
Constraints on type parameters (C# Programming Guide)
Best Regards,
Daniel Zhang

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