What is the goal of Where in c#?

Question

i have this code:

Task NavigateToAsync<TViewModel>() where TViewModel : ViewModelInteres;

i dont understand the goal of where, what is the goal?, can someone explain to me what it is for, Is it mandatory to use the reserved word WHERE?

Answer 1

It is not mandatory at all. You would use where when you want to apply a constraint on the types of your generics.

Some examples:

class MyGeneric<T> where T : class  {...}

T here can only be a class (no structs, like int, byte etc.), this makes it possible to handle better nullable types, for example.

class MyGeneric<T> where T : notnull {...} 

T can be struct or class, but will never be null.

class MyGeneric<T> where T : new() {...} 

T is a class that can be instantiated with an empty constructor. Useful if you want to have generic fabrics.

interface Animal { public int nrPaws {get;} }
class MyGeneric<T> where T : Animal {...} 

T must implement the interface Animal. In this way you can control the methods you can call from MyGeneric.

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where is generally very handy when you want these kind of checks to be done at compile-time rather that throwing error during run-time.

Answer 2

it constrains what types can be used for TViewModel