Design & Maintainability 

Class Design

• A class or interface should have a single purpose (AV1000)
• An interface should be small and focused (AV1003)
• Use an interface to decouple classes from each other (AV1005)
• Don't suppress compiler warnings using the new keyword (AV1010)
• It should be possible to treat a derived object as if it were a base class object (AV1011)
• Don't refer to derived classes from the base class (AV1013)
• Avoid exposing the other objects an object depends on (AV1014)
• Avoid bidirectional dependencies (AV1020)
• Classes should have state and behavior (AV1025)
• Classes should protect the consistency of their internal state (AV1026)

Member Design

• Allow properties to be set in any order (AV1100)
• Don't use mutually exclusive properties (AV1110)
• A property, method or local function should do only one thing (AV1115)
• Don't expose stateful objects through static members (AV1125)
• Return an IEnumerable<T> or ICollection<T> instead of a concrete collection class (AV1130)
• Properties, arguments and return values representing strings or collections should never be null (AV1135)
• Define parameters as specific as possible (AV1137)

Miscellaneous Design

• Throw exceptions rather than returning some kind of status value (AV1200)
• Provide a rich and meaningful exception message text (AV1202)
• Don't swallow errors by catching generic exceptions (AV1210)
• Properly handle exceptions in asynchronous code (AV1215)
• Always check an event handler delegate for null (AV1220)
• Use a protected virtual method to raise each event (AV1225)
• Don't pass null as the sender argument when raising an event (AV1235)
• Use generic constraints if applicable (AV1240)
• Evaluate the result of a LINQ expression before returning it (AV1250)
• Do not use this and base prefixes unless it is required (AV1251)

Maintainability

• Methods should not exceed 7 statements (AV1500)
• Make all members private and types internal sealed by default (AV1501)
• Avoid conditions with double negatives (AV1502)
• Don't use "magic" numbers (AV1515)
• Only use var when the type is very obvious (AV1520)
• Declare and initialize variables as late as possible (AV1521)
• Assign each variable in a separate statement (AV1522)
• Favor object and collection initializers over separate statements (AV1523)
• Don't make explicit comparisons to true or false (AV1525)
• Don't change a loop variable inside a for loop (AV1530)
• Avoid nested loops (AV1532)

• Always add a block after the keywords if, else, do, while, for, foreach and case (AV1535)
• Always add a default block after the last case in a switch statement (AV1536)
• Finish every if-else-if statement with an else clause (AV1537)
• Be reluctant with multiple return statements (AV1540)
• Don't use an if-else construct instead of a simple (conditional) assignment (AV1545)
• Encapsulate complex expressions in a property, method or local function (AV1547)
• Call the more overloaded method from other overloads (AV1551)
• Only use optional arguments to replace overloads (AV1553)
• Avoid using named arguments (AV1555)
• Don't declare signatures with more than 3 parameters (AV1561)
• Don't use ref or out parameters (AV1562)
• Avoid signatures that take a bool flag (AV1564)
• Prefer is patterns over as operations (AV1570)
• Don't comment out code (AV1575)

Framework Guidelines

• Use C# type aliases instead of the types from the System namespace (AV2201)
• Prefer language syntax over explicit calls to underlying implementations (AV2202)
• Build with the highest warning level (AV2210)
• Use lambda expressions instead of anonymous methods (AV2221)
• Only use the dynamic keyword when talking to a dynamic object (AV2230)
• Favor async/await over Task continuations (AV2235)

Naming & Layout 

Naming

• Use US English (AV1701)
• Don't prefix fields (AV1705)
• Don't use abbreviations (AV1706)
• Name members, parameters and variables according to their meaning and not their type (AV1707)
• Name types using nouns, noun phrases or adjective phrases (AV1708)
• Name generic type parameters with descriptive names (AV1709)
• Don't repeat the name of a class or enumeration in its members (AV1710)
• Avoid short names or names that can be mistaken for other names (AV1712)
• Properly name properties (AV1715)
• Name methods and local functions using verbs or verb-object pairs (AV1720)
• Use a verb or verb phrase to name an event (AV1735)
• Postfix asynchronous methods with Async or TaskAsync (AV1755)

• Use an underscore for irrelevant parameters (AV1739)

Documentation

• Write comments and documentation in US English (AV2301)
• Document all public, protected and internal types and members (AV2305)
• Write XML documentation with other developers in mind (AV2306)
• Avoid inline comments (AV2310)
• Only write comments to explain complex algorithms or decisions (AV2316)

Layout

• Maximum line length is 130 characters
• Indent 4 spaces, don't use tabs
• Keep one space between keywords like if and the expression, but don't add spaces after ( and before )
• Add a space around operators, like +, -, ==, etc.
• Always add curly braces after the keywords if, else, do, while, for, foreach and case (AV1535)
• Always put opening and closing curly braces on a new line
• Don't indent object/collection initializers and initialize each property on a new line
• Don't indent lambda statement blocks
• Keep expression-bodied-members on one line; break long lines after the arrow sign
• Put the entire LINQ statement on one line, or start each keyword at the same indentation

• Add parentheses around every binary expression, but don't add parentheses around unary expressions

• Be reluctant with #region (AV2407)
• Use expression-bodied members appropriately (AV2410)