Constrain on custom generic type in .NET

Introduction

Generics is most important topic included in C# 2.0. Generics are use full in number of way when we do coding in project. One of the most important use is we want to use collections with type safety and without boxing/unboxing. But here I am going to discuss about the type of constrain we apply one the custom type we create using generics.

Reference Type

Constrain ensure that type argument is Reference Type. i.e Class, Interface, Delegates, Array etc. Syntax

       Class A<T> where T : class 

Example

Note : Always come first when multiple constrain applied.

Value Type

Constrain ensure that type argument is Value Type. i.e int, float, struct, enum etc. But not nullable types. Syntax

       Class A<T> where T : struct

Note :

Constructor Type

Syntax

       Class A<T> where T : new() 

Derived Type

Constrain ensure that type argument must derived or implemented form specified type. Syntax

       Class A<T> where T : MyClass
       Class A<T> where T : InterfaceME

Example

class A<T> where T : Stream

class A<T> where T : IDisposible

Points to remeber

You can specify multiple interface for type argument in this constrain but only one. Because no type can derived from more than once class.

Note:

Specified class must not be struct, a sealed class or not belonging to special type

System.Object
System.Enum
System.ValueType
System.Delegate

Summary

Its good to focus on Constrain type when you are implementing own custom types. This Constrain also applicable to generic methods with type argument.