Add documentation for the GCI95 rule

This is for green-code-initiative/creedengo-csharp#92

Author: StanleySweet

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- [#399](https://github.com/green-code-initiative/creedengo-rules-specifications/pull/399) Add C# Rule GCI95 - Use `is` operator instead of `as` operator and null check
 - GCI69 Java : calls to hasMoreElements() and nextElement() methods from java.util.Enumeration interface aren't flagged anymore when called in a for loop
 
 - Correction of various typos in rules documentations

RULES.md

@@ -71,6 +71,7 @@ Some are applicable for different technologies.
 | GCI92    | Use string.Length instead of comparison with empty string         | Comparing a string to an empty string is unnecessary and can be replaced by a call to `string.Length` which is more performant and more readable.                                                                                                                                                                                                                                                                             |                                                                                                                                                                         | 🚫   | 🚫  | 🚫 | 🚫     | 🚫   | ✅  | 🚫   |
 | GCI93    | Return `Task` directly                                            | Consider returning a `Task` directly instead of a single `await`                                                                                                                                                                                                                                                                                                                                                              |                                                                                                                                                                         | ❓    | ❓   | ❓  | ❓      | ❓    | ✅  | ❓    |
 | GCI94    | Use orElseGet instead of orElse                                   | Parameter of orElse() is evaluated, even when having a non-empty Optional. Supplier method of orElseGet passed as an argument is only executed when an Optional value isn’t present. Therefore, using orElseGet() will save computing time.                                                                                                                                                                                   | [Optimized use of Java Optional Else](https://github.com/green-code-initiative/creedengo-challenge/issues/77)                                                           | ✅    | 🚫  | 🚫 | 🚫     | 🚫   | 🚫 | 🚫   |
+| GCI95    | Use `x is string` instead of `x as string != null` | Using the `as` operator followed by a null check is more verbose and can be simplified using the `is` operator with pattern matching. When using the `is` pattern, the type check and cast happen in one concise statement, improving readability and reducing boilerplate. Therefore, using `is` with pattern matching will simplify code and improve maintainability. | [Roslynator Rule](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS1172/) [Microsoft Documentation](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/type-testing-and-cast) | 🚫 | 🚫 | 🚫 | 🚫 | 🚫 | ✅ | 🚫   |
 | GCI203   | Detect unoptimized file formats                                   | When it is possible, to use svg format image over other image format                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                         | 🚧   | 🚀  | 🚀 | ✅      | 🚀   | 🚀 | 🚫   |
 | GCI404   | Avoid list comprehension in iterations                            | Use generator comprehension instead of list comprehension in for loop declaration                                                                                                                                                                                                                                                                                                                                             |                                                                                                                                                                         | 🚫   | 🚫  | 🚫 | ✅      | 🚫   | 🚫 | 🚫   |
 | GCI522   | Sobriety: Brightness Override                                     | To avoid draining the battery, iOS and Android devices adapt the brightness of the screen depending on the environment light.                                                                                                                                                                                                                                                                                                 |                                                                                                                                                                         | 🚫   | 🚫  | ✅  | 🚫     | 🚫   | 🚫 | 🚫   |

src/main/rules/GCI95/GCI95.json

@@ -0,0 +1,16 @@
+{
+  "title": "Use 'is' operator instead of 'as' operator and null check",
+  "type": "CODE_SMELL",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant/Issue",
+    "constantCost": "1min"
+  },
+  "tags": [
+    "roslynator",
+    "readability",
+    "simplification",
+    "performance"
+  ],
+  "defaultSeverity": "Minor"
+}

src/main/rules/GCI95/csharp/GCI95.asciidoc

@@ -0,0 +1,53 @@
+Using the `as` operator followed by a null check is more verbose and can be simplified using the `is` operator with pattern matching.
+
+When using the `is` pattern, the type check and cast happen in one concise statement, improving readability and reducing boilerplate.
+
+Therefore, using `is` with pattern matching will simplify code and improve maintainability.
+
+== Non compliant Code Example
+
+[source,csharp]
+----
+var myObj = obj as MyType;
+if (myObj != null)
+{
+    myObj.DoSomething();
+}
+----
+
+== Compliant Code Example
+
+[source,csharp]
+----
+if (obj is MyType myObj)
+{
+    myObj.DoSomething();
+}
+----
+
+[source,csharp]
+----
+var anotherObj = maybeObj as AnotherType;
+if (anotherObj != null)
+{
+    Console.WriteLine(anotherObj.Value);
+}
+----
+
+== Compliant Code Example
+
+[source,csharp]
+----
+if (maybeObj is AnotherType anotherObj)
+{
+    Console.WriteLine(anotherObj.Value);
+}
+----
+
+==  Additional Notes
+
+- Pattern matching avoids redundant null checks.
+- It ensures the cast object is both non-null and of the expected type.
+- It promotes safer and more modern C# practices introduced in C# 7.0 and later.
+- Also in [certain cases](https://gist.github.com/StanleySweet/cf6a62ddb1db27c4229804f8a706f323) the assembly is better with is less instructions usually means more performance and less energy consumption.
+