Updated AV1570 (dennisdoomen#114)

* Removed AV1570

because it advocates outdated language usage.

* Review feedback

Src/Cheatsheet/Cheatsheet.md

@@ -91,7 +91,7 @@ NOTE: Requires Markdown Extra. See http://michelf.ca/projects/php-markdown/extra
 * Don’t allow methods and constructors with more than three parameters (AV1561)  
 * Don’t use `ref` or `out` parameters (AV1562)  
 * Avoid methods that take a `bool` flag (AV1564)  
-* Always check the result of an `as` operation (AV1570)  
+* Prefer `is` patterns over `as` operations (AV1570)  
 * Don’t comment-out code (AV1575)  
 
 <br/>

Src/Guidelines/1500_MaintainabilityGuidelines.md

@@ -408,8 +408,21 @@ Often, a method taking such a flag is doing more than one thing and needs to be
 ### <a name="av1568"></a> Don't use parameters as temporary variables (AV1568) ![](images/3.png)
 Never use a parameter as a convenient variable for storing temporary state. Even though the type of your temporary variable may be the same, the name usually does not reflect the purpose of the temporary variable.
 
-### <a name="av1570"></a> Always check the result of an `as` operation (AV1570) ![](images/1.png)
-If you use `as` to obtain a certain interface reference from an object, always ensure that this operation does not return `null`. Failure to do so may cause a `NullReferenceException` at a much later stage if the object did not implement that interface.
+### <a name="av1570"></a> Prefer `is` patterns over `as` operations (AV1570) ![](images/1.png)
+
+If you use 'as' to safely upcast an interface reference to a certain type, always verify that the operation does not return `null`. Failure to do so may cause a `NullReferenceException` at a later stage if the object did not implement that interface.
+Pattern matching syntax prevents this and improves readability. For example, instead of:
+
+	var remoteUser = user as RemoteUser;
+	if (remoteUser != null)
+	{
+	}
+
+write:
+
+	if (user is RemoteUser remoteUser)
+	{
+	}
 
 ### <a name="av1575"></a> Don't comment out code (AV1575) ![](images/1.png)
 Never check in code that is commented out. Instead, use a work item tracking system to keep track of some work to be done. Nobody knows what to do when they encounter a block of commented-out code. Was it temporarily disabled for testing purposes? Was it copied as an example? Should I delete it?