[fix-finder] Enable nullable reference types in `AidlCompiler.cs`

[github-actions]

Problem

src/Xamarin.Android.Tools.Aidl/AidlCompiler.cs does not have nullable reference types enabled. The file has several fields and parameters that can be null but are typed as non-nullable, which hides potential null reference issues at compile time.

Location

• File: src/Xamarin.Android.Tools.Aidl/AidlCompiler.cs

Current Code

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using Irony.Parsing;
using Mono.Cecil;

namespace Xamarin.Android.Tools.Aidl
{
    public class AidlCompiler
    {
        // ...
        Dictionary<string,CompilationUnit> units = new Dictionary<string,CompilationUnit> ();
        string output_path;    // can be null before Run() is called
        string output_ns;      // can be null, checked on line 71

        public event Action<string,string> FileWritten;  // can be null if no subscriber

        // ...
        // Line 75: FileWritten invoked without null check
        FileWritten (file, csharp);

        // Line 82: out parameter set to null without nullable annotation
        public Result Run (string input, out string output, AssemblyDefinition[] references = null, ...)
        {
            // ...
            output = null;  // assigned null but parameter is non-nullable
        }
    }
}

Suggested Fix

1. Add #nullable enable at the top of the file (no preceding blank lines)
2. Make nullable fields explicit:
   • string output_path; → string? output_path;
   • string output_ns; → string? output_ns;
3. Make the event nullable and use null-conditional invoke:
   • public event Action<string,string> FileWritten; → public event Action<string,string>? FileWritten;
   • FileWritten (file, csharp); → FileWritten?.Invoke (file, csharp);
4. Fix method signatures:
   • out string output → out string? output
   • AssemblyDefinition[] references = null → AssemblyDefinition[]? references = null
5. Do NOT use the ! (null-forgiving) operator anywhere — always handle nulls explicitly

Guidelines

• This project targets netstandard2.0, so use throw new ArgumentNullException (nameof(parameter)) for null argument checks (not ArgumentNullException.ThrowIfNull)
• Follow Mono formatting style: space before ( in method calls (e.g., FileWritten?.Invoke (file, csharp))
• Use "" not string.Empty
• Do not add <Nullable>enable</Nullable> to the .csproj — only add the per-file #nullable enable directive
• Use the IsNullOrEmpty() extension method instead of string.IsNullOrEmpty() if applicable

Acceptance Criteria

• #nullable enable added at the top of AidlCompiler.cs
• All nullable fields and parameters properly annotated
• No use of the ! (null-forgiving) operator
• FileWritten event invoked safely with null-conditional operator
• File compiles without nullable warnings
• All existing tests pass

Generated by Nightly Fix Finder · ● 6.7M · ◷

• expires on May 25, 2026, 9:56 PM UTC