1. .NET and .NET Core: Languages, Cloud, Mobile and AI
5. Generic Delegate and Lambda
Pan Wuming
2018

2. Topics Coding with Denotations Delegates
Generic to Simplify Delegate Declaration
The Action and Func generic delegates
Covariance and Contravariance in Generic
Lambda Expressions

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

4. 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

6. <Grid>
<Button Content="Show" Name="ShowButton" HorizontalAlignment="Left" Height="123" Margin="52,53,0,0" VerticalAlignment="Top" Width="193" FontSize="20"/>
<Button Content="Delegat to label" Name="ToLabelButton" HorizontalAlignment="Left" Margin="52,271,0,0" VerticalAlignment="Top" Width="247" Height="52" FontSize="20"/>
<Button Content="Delegat to ToMessageBox" Name="ToMessageBox" HorizontalAlignment="Left" Margin="400,271,0,0" VerticalAlignment="Top" Width="315" Height="52" FontSize="20"/>
<Label Content="Label" Name="ShowLabel" HorizontalAlignment="Left" Margin="345,44,0,0" VerticalAlignment="Top" Height="169" Width="384" FontSize="20"/>
</Grid>

7. public delegate void TextShow(string s);
public partial class MainWindow : Window {
TextShow MyShow;
public MainWindow()
InitializeComponent();
ShowButton.Click = ShowButtonClick;
ToLabelButton.Click = ToLabelButtonClick;
ToMessageBox.Click = ToMessageBoxClick; }

8. public void Show(string s){
ShowLabel.Content = s; }
//public event RoutedEventHandler Click;
//public delegate void RoutedEventHandler(object sender, RoutedEventArgs e);

9. public void ShowButtonClick(object sender, RoutedEventArgs e){
MyShow("Use ShowButtonClick Method！"); }
public void ToMessageBoxClick(object sender, RoutedEventArgs e)
MyShow = (s) => MessageBox.Show(s);
public void ToLabelButtonClick(object sender, RoutedEventArgs e)
//?

10. 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

11. … 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

13. <Grid>
<Button Content="Show" Name="ShowButton" HorizontalAlignment="Left" Height="123" Margin="52,53,0,0" VerticalAlignment="Top" Width="193" FontSize="20"/>
<Button Content="Delegat to label" Name="ToLabelButton" HorizontalAlignment="Left" Margin="52,271,0,0" VerticalAlignment="Top" Width="247" Height="52" FontSize="20"/>
<Button Content="Delegat to ToMessageBox" Name="ToMessageBox" HorizontalAlignment="Left" Margin="400,271,0,0" VerticalAlignment="Top" Width="315" Height="52" FontSize="20"/>
<Label Content="Label" Name="ShowLabel" HorizontalAlignment="Left" Margin="345,44,0,0" VerticalAlignment="Top" Height="169" Width="384" FontSize="20"/>
<Button Content="Remove to label" Name="RemoveToLabel" HorizontalAlignment="Left" Margin="52,338,0,0" VerticalAlignment="Top" Width="247" Height="52" FontSize="20"/>
<Button Content="Remove to ToMessageBox" Name="RemoveToMessageBox" HorizontalAlignment="Left" Margin="400,338,0,0" VerticalAlignment="Top" Width="315" Height="52" FontSize="20"/>
</Grid>

14. public delegate void DelTextShow(string s);
public partial class MainWindow : Window {
public DelTextShow MyShow;
DelTextShow L;
DelTextShow M;

15. public MainWindow() {
InitializeComponent();
ShowButton.Click += ShowButtonClick;
ToLabelButton.Click += ToLabelButtonClick;
ToMessageBox.Click += ToMessageBoxClick;
L = ShowToLable;
M = (s) => MessageBox.Show(s);
RemoveToLabel.Click += RemoveToLabelClick;
RemoveToMessageBox.Click += RemoveToMessageBoxClick; }

16. public void ShowToLable(string s){
ShowLabel.Content = s; }
//public event RoutedEventHandler Click;
//public delegate void RoutedEventHandler(object sender, RoutedEventArgs e);

17. public void ShowButtonClick(object sender, RoutedEventArgs e){
try
MyShow("Use ShowButtonClick Method！"); }
catch
ShowLabel.Content = "Label";
} }
public void ToMessageBoxClick(object sender, RoutedEventArgs e)
MyShow += M;
public void ToLabelButtonClick(object sender, RoutedEventArgs e)
MyShow += L;

18. public void RemoveToLabelClick(object sender, RoutedEventArgs e){
MyShow -= L; }
public void RemoveToMessageBoxClick(object sender, RoutedEventArgs e)
MyShow -= M;

19. Generic to Simplify Delegate Declaration
Problem: It does not necessary to assign different name to delegates
Generics as "code template“ to define delegates in advance in class library
Generics
The runtime knows what type of data structure you are using and can store it in memory more efficiently.
List<int> ListGeneric = new List<int> { 5, 9, 1, 4 };

20. public delegate void TextShow(string s);
public delegate void DelTextShow(string s);
public partial class AClass {
public DelTextShow MyShow;
public TextShow MyShowNew;
public AClass()
MyShow = (s) => MessageBox.Show(s);
MyShowNew = (s) => MessageBox.Show(s); }

21. The Action and Func generic delegates
A delegate can define its own type parameters.
In and out Generic Modifier
public delegate void Action<in T1, in T2>(T1 arg1,T2 arg2);
public delegate TResult Func<in T1, out TResult>(T1 arg1);

22. public delegate void DelTextShow(string s);
public partial class MainWindow : Window {
public DelTextShow MyShow;
DelTextShow L;
DelTextShow M;
public partial class MainWindow2 : Window {
Action<string> MyShow;
Action<string> L;
Action<string> M;

23. 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.

24. 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.

25. 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.

26. Contravariance class GenericDelegate { public GenericDelegate()
Action<BaseClass> b = (target) =>
target.Write(); };
Action<DerivedClass> d = b;
d(new DerivedClass());
Action<DerivedClass> e = (target) =>
target.DerivedClassWrite();
e(new DerivedClass());
//Action<BaseClass> f = e; }
class BaseClass {
public void Write()
Console.WriteLine("BaseClassWrite"); }
class DerivedClass : BaseClass
public void DerivedClassWrite()
Console.WriteLine("DerivedClassWrite");
语法错误。

27. Actual passing and assigning direction is contravariant !
课堂演练：
将前述代码中 Action 改为 Func，修改代码，以使能够运行。
Actual passing and assigning direction is contravariant !

28. 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

29. public void MethodA(int input){
int j = 0;
Func<bool> del = () => { j = 10; return j > input; };
Func<bool> del1 = () => j > input;
Func<int,bool> del2 = (x) => { return x == j; };
MethodB(() => j > input); }
public void MethodB(Func<bool> delIn) { }

30. Calling lambda expression will change the values of variables!
在C# interactive 中执行如下代码
> int j = 0;
Func<bool> del = () => { j = 10; return j > 6; };
del();
Console.WriteLine(j.ToString()); 10
Calling lambda expression will change the values of variables!

31. > class ClassB
. { .
public Func<bool> MethodA(int k) {
int j = 0;
Func<bool> del = () => {return j > k; };
return del; }
public void MethodB(Func<bool> delB)
Console.WriteLine(delB().ToString());
. }
. var c = new ClassB();
. c.MethodB(c.MethodA(5));
False

32. > class ClassB
. { .
public Func<bool> MethodA(int k) {
int j = 0;
Func<bool> del = () => {return j > k; };
j = 10;
return del; }
public void MethodB(Func<bool> delB)
Console.WriteLine(delB().ToString());
. }
. var c = new ClassB();
. c.MethodB(c.MethodA(5));
True

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