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Assertions to check logic should always be true

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Assertions are used not to perform testing of input parameters, but to verify that program flow is corect – i.e., that you can make certain assumptions about your code at a certain point in time. In other words: a test done with Debug.Assert should always assume that the value tested is true.

Debug.Assert only executes in DEBUG builds; it is filtered out of RELEASE builds. It must be considered a debugging tool in addition to unit testing and not as a replacement of code contracts or input validation methods.

For instance, this is a good assertion: 

var systemData = RetrieveSystemConfiguration();
Debug.Assert(systemData != null);

Here assert is a good choice because we can assume that RetrieveSystemConfiguration() will return a valid value and will never return null.

Here is another good example: 

UserData user = RetrieveUserData();
Debug.Assert(user != null);
Debug.Assert(user.Age > 0);
int year = DateTime.Today.Year - user.Age;

First, we may assume that RetrieveUserData() will return a valid value. Then, before using the Age property, we verify the assumption (which should always be true) that the age of the user is strictly positive.

This is a bad example of assert: 

string input = Console.ReadLine();
int age = Convert.ToInt32(input);
Debug.Assert(age > 16);
Console.WriteLine("Great, you are over 16");

Assert is not for input validation because it is incorrect to assume that this assertion will always be true. You must use input validation methods for that. In the case above, you should also verify that the input value is a number in the first place.

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Code Contracts and Assertions:
* Assertions to check logic should always be true
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