C#: Remove some FPs for cs/simplifiable-boolean-expression.

[michaelnebel]

It turns out that the query cs/simplifiable-boolean-expression produces some false positives, when the extracted data has incomplete type information.
The following example produces a false positive

public class Container {
    public bool? Field;
}

public void Fn() {
    // Emulating incomplete type information by not declaring container explicitly.
    if (container.Field == true) ;
}

If type information had been complete, for instance

public void Fn(Container container) {
       if (container.Field == true) ;
}

then the query didn't produce an alert prior to the changes in this PR. In this case an alert is NOT produced because the boolean literal true on the right hand side of == is wrapped in a cast.

[Copilot]

Copilot review overview

🟡 Changes recommended

Conditional rewrites still accept non-Boolean or unknown condition types, allowing invalid simplification suggestions.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Require conditional expressions to have Boolean type · New
• Add missing apostrophe in “Can’t” · New

What changed in this PR

Reduces false positives in C# Boolean-expression simplification when type information is incomplete or nullable.

Changes:

• Requires Boolean operand types before suggesting rewrites.
• Adds nullable and incomplete-type regression cases.
• Adds a change note.

File	Description
csharp/​ql/​src/​Language Abuse/​SimplifyBoolExpr.ql	Adds type-aware rewrite predicates.
csharp/​ql/​test/​query-tests/​Language Abuse/​SimplifyBoolExpr/​SimplifyBoolExpr.cs	Tests nullable Boolean comparison.
csharp/​ql/​test/​query-tests/​standalone/​Language Abuse/​SimplifyBoolExpr/​SimplifyBoolExpr.cs	Tests incomplete type information.
csharp/​ql/​src/​change-notes/​2026-09-30-simplify-bool-expr.md	Documents reduced false positives.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Copilot]

Warning

Copilot couldn't run its full agentic review because it didn't start before the timeout. Make sure your repository has a runner available, or add a copilot-code-review.yml file specifying one with the runs-on attribute. See the docs for more details.

Copilot review overview

Review effort: Lite
Findings: 1 · 2

Open (3)

• These statements reference container without any declaration, which can introduce cascading… · New
• The BoolType gating logic is duplicated across binary and conditional rewrites. Consider factoring… · New
• The BoolType gating logic is duplicated across binary and conditional rewrites. Consider factoring… · New

Resolved since last review (2)

• Require conditional expressions to have Boolean type
• Add missing apostrophe in “Can’t”