# Is With This
# Detects 'is' operator used with 'this' in C#
id: is-with-this
name: is Should Not Be Used With This
severity: warning
category: maintainability
defect_class: correctness
inline_tier: warning
language: csharp

message: "'is' should not be used with 'this' — use GetType() comparison instead"

description: |
  Using 'is' with 'this' checks the declared type, not the runtime type.
  Use GetType() == typeof(ExpectedType) for exact type comparison,
  or virtual methods for polymorphic behavior.

  ✅ FIX: Use GetType() or virtual dispatch

  ```csharp
  if (this.GetType() == typeof(DerivedClass)) {  // Runtime type check
      // ...
  }
  ```

query: |
  (binary_expression
    left: (this_expression) @THIS
    right: (_) @TYPE)

metavars:
  - THIS
  - TYPE

tags:
  - maintainability
  - csharp
  - api-design
  - pitfall

examples:
  bad: |
    if (this is MyClass) {  // BAD - checks declared type
        // ...
    }

  good: |
    if (this.GetType() == typeof(MyClass)) {  // GOOD - runtime check
        // ...
    }

has_fix: false