Guide to the Language

 Multiple Constraints
Multiple constraints can be applied to a type parameter by specifying 
them in a comma-separated list. Furthermore, to constrain more than one 
type parameter, additional where clauses can be added. Note that if either 
the class or the struct constraint is used, it must appear first in the list. 
Moreover, if the parameterless constructor constraint is used, it must be 
the last one in the list.
class J
where T : class, I
where U : I, new() {}
 Why Use Constraints
Aside from restricting the use of a generic method or class to only certain 
parameter types, another reason for applying constraints is to increase 
the number of allowed operations and method calls supported by the 
constraining type. An unconstrained type may only use the System.Object 
methods. However, by applying a base class constraint, the accessible 
members of that base class also become available.
class Person
{
public string name;
}
class PersonNameBox where T : Person
{
public string box;
public void StorePersonName(T a)
Chapter 28 GeneriCs

---

169
{
box = a.name;
}
}
The following example uses the parameterless constructor constraint. 
This constraint enables new objects of the type parameter to be 
instantiated.
class MyClass where T : new() {}
Note that if a class has a constraint on its type parameter and a child 
of that class has a type parameter that’s constrained by the base class, that 
constraint must also be applied to the child class’s type parameter.
class MyChild : MyClass
where T : MyClass, new() {}
Chapter 28 GeneriCs

---

171
© Mikael Olsson 2020 
M. Olsson, C# 8 Quick Syntax Reference, 
https://doi.org/10.1007/978-1-4842-5577-3_29