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Can you please explain to me what where T : class, new() means in the following line of code?
void Add<T>(T item) where T : class, new(); 
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it means that the type used as T when the generic method is used must be a class - i.e. it cannot be a struct or built in number like int or double // Valid: var myStringList = DoThis<string>(); // Invalid - compile error var myIntList = DoThis<int>(); Copy link CC BY-SA 2.5.
The new operator creates a new instance of a type. You can also use the new keyword as a member declaration modifier or a generic type constraint.
T is called type parameter, which can be used as a type of fields, properties, method parameters, return types, and delegates in the DataStore class. For example, Data is generic property because we have used a type parameter T as its type instead of the specific data type. Note.
That is a constraint on the generic parameter T. It must be a class (reference type) and must have a public parameter-less default constructor.
That means T can't be an int, float, double, DateTime or any other struct (value type).
It could be a string, or any other custom reference type, as long as it has a default or parameter-less constructor.
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Those are generic type constraints. In your case there are two of them:
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. Having this constraint will allow you to do something like T field = new T(); in your code which you wouldn't be able to do otherwise.
You then combine the two using a comma to get:
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