VAF search condition on sub types

Hello. I'm a new to M-Files.
Could someone explain me how to make search condition on sub types.

I have Invoice object type and Line Item object is its sub type.
Updating Invoice object I need to find the Line Item objects associated with current Invoice. Help me please to create a proper search buider for such condition.

My event handler:

[EventHandler(MFEventHandlerType.MFEventHandlerBeforeCheckInChangesFinalize)]
public void CalculateInvoicesTotal_Invoice(EventHandlerEnvironment env)
{
   
    ObjVerEx obj = env.ObjVerEx;
    Vault vault = env.Vault;

   
    PropertyValues originalPropertyValues = obj.Properties.Clone();

   
    if (!this.Configuration.Settings.IsEnabled || obj.Type != this.Configuration.Structure.Objects.Invoice)
    {
        return;
    }

    
    MFSearchBuilder builder = new MFSearchBuilder(env.Vault)
        .Class(this.Configuration.Structure.Classes.LineItem)
        .Deleted(false)
        .Property(this.Configuration.Structure.Properties.Owner, MFDataType.MFDatatypeLookup, obj.ID);

    
    List<ObjVerEx> results = builder.FindEx();
	
}

Thanks.