Question

I’m sorry if this is a duplicate question, I have found some similar ones, but not one that would solve my problem. I have a collection of objects with various parameters and I want to filter them using data from ComboBoxes and TextBoxes.

        var query = from zaj in zajezdy
                    where zaj.Zeme == (String)zemeCombo.SelectedValue
                    && zaj.Oblast == (String)oblastCombo.SelectedValue
                    && zaj.Stredisko == (String)strediskoCombo.SelectedValue
                    && zaj.Doprava.Contains((String)dopravaCombo.SelectedValue)
                    && zaj.Strava.Contains((String)stravaCombo.SelectedValue)
                    && zaj.CenaOd > Int32.Parse(cenaOdText.Text)
                    && zaj.CenaOd < Int32.Parse(cenaDoText.Text)
                    select zaj;

This code works if all the combos have been properly selected. However, if the user leaves some unselected/empty, the query comes back with zero objects in it. How can I detect which parameters are null so that the query ignores them?

Answer 1

I think this is a nice example for Specification usage.

Create object, which will represent zajezd specification:

public interface ISpecification<T>
{
    bool IsSatisfiedBy(T value);
}

public class ZajezdSpecification : ISpecification<Zajezd>
{
    private string _zeme;
    private string _oblast;
    private string _stredisko;
    private string _doprava;
    private string _strava;
    private int _cenaOd;
    private int _cenaDo;

    public ZajezdSpecification(string zeme, string oblast, string stredisko, 
        string doprava, string strava, int cenaOd, int cenaDo)
    {
        _zeme = zeme;
        _oblast = oblast;
        _stredisko = stredisko;
        _doprava = doprava;
        _strava = strava;
        _cenaOd = cenaOd;
        _cenaDo = cenaDo;
    }

    public bool IsSatisfiedBy(Zajezd zajezd)
    {
        if (!String.IsNullOrEmpty(_zeme) && zajezd.Zeme != _zeme)
            return false;

        if (!String.IsNullOrEmpty(_oblast) && zajezd.Oblast != _oblast)
            return false;

        // ... verify anything you want

        return _cenaOd < zajezd.CenaOd && zajezd.CenaOd < _cenaDo;
    }
}

and initialize it with values from UI:

ZajezdSpecification spec = new ZajezdSpecification(
   (string)zemeCombo.SelectedValue,
   (string)oblastCombo.SelectedValue,
   (string)strediskoCombo.SelectedValue,
   ...
   Int32.Parse(cenaDoText.Text)
);

Use this specification to filter your collection:

var query = from zaj in zajezdy
            where spec.IsSatisfiedBy(zaj)
            select zaj;

PS try to use English names in your code.