Add AV1155: Use the field keyword in auto-properties when additional logic is needed (#316)

Author: dennisdoomen

_rules/1155.md

@@ -0,0 +1,38 @@
+---
+rule_id: 1155
+rule_category: member-design
+title: Use the `field` keyword in auto-properties when additional logic is needed
+severity: 3
+---
+C# 14 introduced the `field` keyword, which provides access to the underlying storage of a property without the need to declare a private field. This eliminates boilerplate while still allowing validation or transformation logic. Furthermore, it protects against other members modifying the underlying value, which breaks encapsulation.
+
+```csharp
+// Before C# 14: required a manual backing field
+private int maxLength;
+public int MaxLength
+{
+    get => maxLength;
+    set
+    {
+        ArgumentOutOfRangeException.ThrowIfLessThan(value, 0);
+        maxLength = value;
+    }
+}
+
+// With C# 14: use the field keyword
+public int MaxLength
+{
+    get;
+    set
+    {
+        ArgumentOutOfRangeException.ThrowIfLessThan(value, 0);
+        field = value;
+    }
+}
+```
+
+Use the `field` keyword when:
+- You need getter or setter logic that goes beyond a simple assignment, but don't want to sacrifice the clarity of an auto-property.
+- Adding a backing field would add noise without adding meaning.
+
+Don't use `field` when the backing field itself carries meaning (e.g. needs its own name or XML documentation).