1. 6 Delegate and Lambda Expressions
.NET and .NET Core
6 Delegate and Lambda Expressions
Pan Wuming
2017

2. Topics Extension Methods Delegates
Generic to Simplify Delegate Declaration
Lambda Expressions
References：
C# Programming Guide（MSDN）
Covariance and Contravariance in Generics（MSDN）

3. Coding with Denotations
Denoting Collection of Names
Denoting Behavior Group
Referring to Behaviors
Enumerations
Interfaces
Delegates

4. Extension Methods
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
Extension methods are defined as static methods but are called by using instance method syntax.

6. Delegates
A delegate is a type that represents references to methods with a particular parameter list and return type.

7. Delegates
Delegate types are derived from the Delegate class in the .NET Framework.
Delegate types are sealed.
The instantiated delegate can be passed as a parameter, or assigned to a property.
The instantiated delegate can wrap an instance method or a static method

8. Multicasting A delegate can call more than one method when invoked
Delegates with more than one method in their invocation list derive from MulticastDelegate
MulticastDelegate is a subclass of System.Delegate.
To add an extra method to the delegate's list of methods
To remove a method from the invocation list

10. … d1 = obj.Method1; d1 += obj.Method2; 1 2 obj.Method1 _target obj
_methodPtr MethodClass.Method1
_prev null 1
_target obj
_methodPtr MethodClass.Method2
_prev
_target obj
_methodPtr MethodClass.Method1
_prev null
obj.Method2 2

11. Call The Members Defined In System.Delegate

12. Comparing delegates

13. Is the delegate referred by handler Derived from MulticastDelegate？
A Question
Is the delegate referred by handler Derived from MulticastDelegate？

14. Generic to Simplify Delegate Declaration
Generic delegates
The Action and Func generic delegates
Covariance and Contravariance in Generics

15. Generic Delegates A delegate can define its own type parameters.
In and out Generic Modifier

17. The Action Generic Delegates in .NET Framework Class Library
Description
Action
Encapsulates a method that has no parameters and does not return a value.
Action< T>
Encapsulates a method that has a single parameter and does not return a value.
Action< T1, T2>
Encapsulates a method that has two parameters and does not return a value.
...
Action< T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15>
Encapsulates a method that has 15 parameters and does not return a value.
Action< T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16>
Encapsulates a method that has 16 parameters and does not return a value.

18. The Func Generic Delegates in .NET Framework Class Library
Description
Func< TResult>
Encapsulates a method that has no parameters and returns a value of the type specified by the TResult parameter.
Func< T, TResult>
Encapsulates a method that has one parameter and returns a value of the type specified by the TResult parameter.
Func< T1, T2, TResult>
Encapsulates a method that has two parameters and returns a value of the type specified by the TResult parameter.
...
Func< T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, TResult>
Encapsulates a method that has 16 parameters and returns a value of the type specified by the TResult parameter.

19. Covariance and Contravariance
Covariance: Enables you to use a more specific type than originally specified.
Contravariance: Enables you to use a more generic (less derived) type than originally specified.
Invariance: Means that you can use only the type originally specified
The in keyword specifies that the type parameter is contravariant.
The out keyword specifies that the type parameter is covariant.

20. Contravariance

21. Covariant Return Type and Contravariant Parameter Type
Null coalescing
x ?? y
Evaluates to y if x is null, to x otherwise

22. Lambda Expressions Anonymous Methods The lambda operator =>
Specifying input parameters (if any) on the left side of =>
Putting the expression or statement block on the other side
Lambdas can refer to outer variables
button1.Click += delegate(System.Object o, System.EventArgs e)
{ System.Windows.Forms.MessageBox.Show("Click!"); };

24. Summarizing the Applications of Delegates
Callback Function
Event members in a Type
Replace simple Interface implementation

25. Actual passing and assigning direction is contravariant !
See you next class!
Actual passing and assigning direction is contravariant !