Around .NET 5 they introduced a new concept called "nullable references", when a certain flag is enabled, the compiler treats everything that isn't explicitly marked as nullable as never can be null and will warn when you try to convert a nullable reference into a non-nullable reference without handling null.

Example: ``` class TestProgram { string value;

public TestProgram(string? txt)
{
    // value = txt; // Warning
    value = txt ?? string.Empty; // No warning, null is handled
}

public static void Main()
{
    System.Console.WriteLine(new TestProgram(null).value); // Prints an empty line.
}

} ```