Fix false positive for Equals/CompareTo with nullable reference types

[Copilot]

Problem

CodeQL queries ClassDoesNotImplementEquals.ql and IncorrectEqualsSignature.ql incorrectly flag classes implementing Equals(object?) as not implementing Equals(object):

#nullable enable

class MyClass : IEquatable<MyClass>
{
    public bool Equals(MyClass? other) => /* ... */;
    
    // False positive: "Class 'MyClass' does not implement Equals(object)"
    public override bool Equals(object? obj) => /* ... */;
    
    public override int GetHashCode() => /* ... */;
}

Changes

Removed redundant instanceof ObjectType parameter type checks from framework method identifiers in System.qll:

• SystemIComparableInterface.getCompareToMethod()
• SystemObjectClass.getEqualsMethod()
• SystemObjectClass.getStaticEqualsMethod()
• SystemObjectClass.getReferenceEqualsMethod()

These methods identify specific framework methods where declaring type + method name + parameter count already provide unique identification. The parameter type checks were unnecessary and caused false positives with nullable reference types.

Tests

• Added NullableTest.cs with nullable Equals implementations
• Extended library tests in Equals.cs for nullable patterns

Original prompt

---

This section details on the original issue you should resolve

<issue_title>False positive - C# IEquatable: Equals(object) not implemented alert when Equals(object?) is implemented</issue_title>
<issue_description>Description of the false positive

CodeQL will complain about a missing override for Equals(object obj) not existing, if the Equals method is implemented within a nullable object as parameter. Using Equals(object? obj) is valid.

Two different alerts appeared on a PR.

Code sample

Sadly, working in closed source, but this sample should do. I do not know if this triggers the false positive in practice, as I can't push it to test, but it should give you a good example. I also added a small a bit of code to show that Equals(object? obj) is valid and working when using Equals().

using System;
using System.Linq;

#nullable enable
public class Program
{
	public class MyClass : IEquatable<MyClass>
	{
		private object property;
		public MyClass()
		{
			property = 1;
		}

		public bool Equals(MyClass? other)
		{
			Console.WriteLine("MyClass? called");
			if (ReferenceEquals(null, other))
			{
				return true;
			}

			return Equals(property, other.property);
		}

		public override bool Equals(object? obj)
		{
			Console.WriteLine("object? called");
			if (ReferenceEquals(null, obj))
			{
				return false;
			}

			if (ReferenceEquals(this, obj))
			{
				return true;
			}

			if (obj.GetType() != GetType())
			{
				return false;
			}

			return Equals((MyClass)obj);
		}

		public override int GetHashCode()
		{
			return HashCode.Combine(property);
		}
	}

	public static void Main()
	{
		var a = new MyClass();
		var b = new MyClass();
		Console.WriteLine(a.Equals(b)); // Equals(MyClass? other)
		Console.WriteLine(Equals(a, b)); // Equals(object? obj)
		Console.WriteLine(IEquatable<MyClass>.Equals(a, b)); // Equals(object? obj)
	}
}

Check 1 (failure check)

The CodeQl error message goes as follows for the override bool Equals(object? obj) line:
Class 'MyClass' does not implement Equals(object), but it implements IEquatable.Equals
The IEquatable.Equals points to bool Equals(MyClass? other).

Error 2 (alert):
The warning goes for the same function override bool Equals(object? obj)
The declaring(MyClass) type of this 'Equals(Object)' method does not override 'Equals(object)'.

Source for alerts
https://github.com/github/codeql/blob/main/csharp/ql/src/API%20Abuse/ClassDoesNotImplementEquals.ql

https://github.com/github/codeql/blob/28b6aa8616a393ebb45186e3ba4df004a0f3ef4e/csharp/ql/src/API%20Abuse/IncorrectEqualsSignature.ql</issue_description>

Comments on the Issue (you are @copilot in this section)

@hvitved I believe the fix here may be to remove all the `result.getParameter(0).getType() instanceof ObjectType` occurrences from https://github.com/github/codeql/blob/28b6aa8616a393ebb45186e3ba4df004a0f3ef4e/csharp/ql/lib/semmle/code/csharp/frameworks/System.qll.

• Fixes False positive - C# IEquatable: Equals(object) not implemented alert when Equals(object?) is implemented #21128

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.