Using one (or more ) conditions in filter expression

it's not exactly solved (first problem) this is just different approach, I though there was some static mechanism for this. 

Second problem is "solved" too:
If I have example 5 gridview columns and for that 5 textboxes which are my control parameters for filtering, for every combination of using those textboxes I must build custom FilterExpression and FilterParameters collection which has exact number of parameters which are textbox.text!="".

Than I must call everytime this routine (to build specific expression) on postback at page load event, because if I for example use update command at gridview, my filtering expression get's somehow overridden by my (default) Select Command in sql datasource and I don't preserve filtered gridview on postback, that is how I see it.

Because FilterParameters collection is preserved at postback I must clear it, to start a new clean process

In filter() routine FilterParameters collection is also built

protected void Page_Load(object sender, EventArgs e)
    {
          if (isPostBack)
           {
           SQLLocal.FilterParameters.Clear();
           SQLLocal.FilterExpression = filter();
            SQLLocal.DataBind();
            }
       }

       
           
     

It seems That if you define two filter parameters that you need to use both.

Now I programmaticly builded filter, so everytime (based on value of textbox fields) I define exact number of parameters which I will really use. This works fine but only on first postback I get correct results for all combinations of fields(textboxes). When I click second time, my filter doesn't show results as I would expect . Why is that?

protected void Page_Load(object sender, EventArgs e)
    {


        SQLLocal.FilterExpression = filter();
        SQLLocal.DataBind();
   
    }