Add NullIf for reference types

[Copilot]

NullIf existed only for value types (where T : struct). Reference types had no equivalent.

Changes

• FunctionalExtensions.cs — adds a second static class ReferenceTypeFunctionalExtensions in the same file containing NullIf<T>(Func<T, bool>) for where T : class, implemented using an extension block. A comment explains why it must live in a separate class: C# (CS0111) prohibits two generic methods with identical parameter types differing only in class vs struct constraints in the same class. The implementation uses Pipe to safely short-circuit on null receivers (value?.Pipe(v => !predicate(v) ? v : null)), so the predicate is never invoked when the receiver is null.
• FunctionalExtensionsTests.cs — adds NullIf_ReferenceType_Test, including a null-receiver case that verifies the predicate is not invoked.

Usage

"hello".NullIf(v => v == "hello")  // null
"world".NullIf(v => v == "hello")  // "world"
((string)null!).NullIf(v => ...)   // null (predicate never called)

[Tyrrrz]

Use extension blocks @copilot

[Copilot]

Done in b3adbfe — FunctionalExtensions2 now uses an extension<T>(T value) where T : class { ... } block.

[Copilot]

Pull request overview

Adds a reference-type overload of NullIf to PowerKit’s functional extensions so NullIf can be used consistently for both struct and class values.

Changes:

• Added NullIf<T>(Func<T, bool>) for where T : class via a new extensions class in FunctionalExtensions.cs
• Added a unit test covering basic reference-type usage

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
PowerKit/Extensions/FunctionalExtensions.cs	Introduces a reference-type NullIf extension (placed in a second static class due to constraint-based overload limitations).
PowerKit.Tests/FunctionalExtensionsTests.cs	Adds a new test validating NullIf behavior on strings.

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[Copilot]

For reference types, this extension method can be invoked on a null receiver; in that case it will call predicate(value) with value == null, which can easily throw and makes null not behave as a no-op. Consider short-circuiting when value is null (return null without invoking the predicate) so the method is safe to use with nullable reference values.

Suggested change

	/// Returns <see langword="null" /> if the value matches the specified predicate; otherwise, returns the value.
	/// </summary>
	public T? NullIf(Func<T, bool> predicate) => !predicate(value) ? value : null;
	/// Returns <see langword="null" /> if the value is <see langword="null" /> or matches the specified predicate;
	/// otherwise, returns the value.
	/// </summary>
	public T? NullIf(Func<T, bool> predicate) =>
	value is null ? null : !predicate(value) ? value : null;

[Tyrrrz]

@copilot implement this suggestions, but with the Pipe method

[Copilot]

Done in 1d874b6 — NullIf now uses value?.Pipe(v => !predicate(v) ? v : null), so a null receiver short-circuits without invoking the predicate. The test also covers this case.