Generic extension method to check if any variable backed by a nullable value type is `null`, or whatever their default value is. I use it for dealing with `Guid?` but it would work with any value type such as `int?` to combine checks of it it's null or holds the default value for that type. `NotNullWhen` is for when the project has the Nullable …

IsNullOrDefault.cs

public static partial class ExtensionMethods
{
    public static bool IsNullOrDefault<T>([NotNullWhen(false)] this T? value) where T : struct
    {
        //return value == null || value.Equals(default(T)); Simpler, but my research online recommended using EqualityComparer<T>.Default instead of value.Equals
        return !value.HasValue || EqualityComparer<T>.Default.Equals(value.Value, default);
    }
}