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The FuncT TResult ActionT and PredicateT delegate types

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The System namespace contains Func<..., TResult> delegate types with between 0 and 15 generic parameters, returning type TResult. 

private void UseFunc(Func<string> func)
{
    string output = func(); // Func with a single generic type parameter returns that type
    Console.WriteLine(output);
}

private void UseFunc(Func<int, int, string> func)
{
    string output = func(4, 2); // Func with multiple generic type parameters takes all but the first as parameters of that type
    Console.WriteLine(output);
}

The System namespace also contains Action<...> delegate types with different number of generic parameters (from 0 to 16). It is similar to Func<T1, .., Tn>, but it always returns void. 

private void UseAction(Action action)
{
    action(); // The non-generic Action has no parameters
}

private void UseAction(Action<int, string> action)
{
    action(4, "two"); // The generic action is invoked with parameters matching its type arguments
}

Predicate<T> is also a form of Func but it will always return bool. A predicate is a way of specifying a custom criteria. Depending on the value of the input and the logic defined within the predicate, it will return either true or false. Predicate<T> therefore behaves in the same way as Func<T, bool> and both can be initialized and used in the same way. 

Predicate<string> predicate = s => s.StartsWith("a");
Func<string, bool> func = s => s.StartsWith("a");

// Both of these return true
var predicateReturnsTrue = predicate("abc");
var funcReturnsTrue = func("abc");

// Both of these return false
var predicateReturnsFalse = predicate("xyz");
var funcReturnsFalse = func("xyz");

The choice of whether to use Predicate<T> or Func<T, bool> is really a matter of opinion. Predicate<T> is arguably more expressive of the author’s intent, while Func<T, bool> is likely to be familiar to a greater proportion of C# developers.

In addition to that, there are some cases where only one of the options is available, especially when interacting with another API. For example List<T> and Array<T> generally take Predicate<T> for their methods, while most LINQ extensions only accept Func<T, bool>.

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Delegates:
* Delegates
* Declaring a delegate type
* Combine Delegates Multicast Delegates
* The FuncT TResult ActionT and PredicateT delegate types
* Safe invoke multicast delegate
* Delegate Equality
* Assigning a named method to a delegate
* Assigning to a delegate by lambda
* Underlying references of named method delegates
* Passing delegates as parameters
* Encapsulating transformations in functions
* Closure inside a delegate
Table Of Contents
0 Getting Started With C#
1 Literals
2 Operators
3 Conditional Statements
4 Equality Operator
5 Equals and GetHashCode
6 Null-Coalescing Operator
7 Null-Conditional Operators
8 nameof Operator
9 Verbatim String
10 Common String Operations
11 String Format
12 String Concatenate
13 String Manipulation
14 String Interpolation
15 String Escape Sequences
16 StringBuilder
17 Regex
18 DateTime
19 Arrays
20 On Algorithm for circular rotation of an array
21 Enum
22 Tuples
23 Overview of C# collections
24 GUID
25 BigInteger
26 Collection Initializers
27 Looping
28 Iterators
29 IEnumerable
30 Value type vs Reference type
31 Built-in Types
32 Aliases of built-in types
33 Anonymous types
34 Dynamic type
35 Type Conversion
36 Casting
37 Nullable types
38 Constructors and Finalizers
39 Access Modifiers
40 Interfaces
41 Static Classes
42 Singleton Implementation
43 Dependency Injection
44 Partial class and methods
45 Object Initializers
46 Methods
47 Extension Methods
48 Named Arguments
49 Named And Optional Arguments
50 Data Annotation
51 Keywords
52 Recursion
53 Naming Conventions
54 XML Documentation Comments
55 Comments and regions
56 Inheritance
57 Generics
58 Using Statement
59 Using Directive
60 IDisposable interface
61 Reflection
62 IQueryable interface
63 LINQ to Objects
64 LINQ Queries
65 LINQ to XML
66 Parallel LINQ PLINQ
67 XmlDocument
68 XDocument
69 C# 7.0 Features
70 C# 6.0 Features
71 C# 5.0 Features
72 C# 4.0 Features
73 C# 3.0 Features
74 Exception Handling
75 NullReferenceException
76 Handling FormatException
77 Read and Understand Stack trackes
78 Diagnostics
79 Overflow
80 JSON handling
81 Using json.net
82 Lambda Expressions
83 Lambda expressions
84 Generic Lambda Query Builder
85 Properties
86 Initializing Properties
87 INotifyPropertyChanged interface
88 Events
89 Expression Trees
90 Overload Resolution
91 Preprocessor directives
92 Structs
93 Attributes
94 Delegates
95 File and Stream IO
96 Networking
97 Performing HTTP requests
98 Reading and writing .zip files
99 FileSystemWatcher
100 Asynchronous Socket
101 Action Filters
102 Polymorphism
103 Immutability
104 Indexer
105 Checked and Unchecked
106 Stream
107 Timers
108 Stopwatches
109 Threading
110 Async/Await, BackgroundWorker, Task and Thread examples
111 Async Await
112 Synchronization Context in Async/Await
113 BackgroundWorker
114 Task Parallel Library
115 Making a variable thread safe
116 Lock Statement
117 Yield Keyword
118 Task Parallel Library TPL Dataflow Constructs
119 Functional Programming
120 Func delegates
121 Function with multiple return values
122 Binary Serialization
123 ICloneable
124 IComparable
125 Accessing Databases
126 Using SQLite
127 Caching
128 Code Contracts
129 Code Contracts and Assertions
130 Structural Design Patterns
131 Creational Design Patterns
132 Implementing Decorating Design Pattern
133 Implementing Flyweight Design Pattern
134 System.Management.Automation
135 Pointers
136 Pointers and Unsafe Code
137 Simulating C Unions with C# Structs
138 Reactive Extensions Rx
139 AsseblyInfo.cs examples
140 Creating Console Application
141 CLSCompliantAttribute
142 ObservableCollection
143 Hash Functions
144 Generating Random Numbers
145 Cryptography
146 Unsafe Code in .NET
147 C# Script
148 Runtime Compile
149 Interoperability
150 .NET Compiler Platform Roslyn
151 Creating Own MessageBox in Windows Form Application
152 Including Font Resources
153 Garbage Collector
154 Windows Communication Foundation
155 Contributors