"Null-Correctness Assisting Attributes: NotNull"

// NotNull indicates that a ref or out parameter, that is marked as nullable, will never be set as null after the method returns.

public void AppendHello([NotNull] ref string? a) { 
	if (a == null) a = "Hello";
	else a = a + "Hello";
}


Code snippet taken from "Two Decades of C#: A Reference - C# 8".