C#

private void TestingButton_Click(object sender, EventArgs e)
{
    Form_EffectiveDate effDate = new Form_EffectiveDate();           
        if (effDate.ShowDialog() == DialogResult.OK)
        {
        EffDateTextBox.Text = Form_EffectiveDate.getEffDate;
        }
}

namespace Manager_Changes
{
    public partial class Form_EffectiveDate : Form
    {
        public delegate void GetEffDate(object sender);
        public GetEffDate getEffDate;

        public Form_EffectiveDate()
        {
            InitializeComponent();
        }

        private void DateOKButton_Click(object sender, EventArgs e)
        {
            getEffDate(effDateTimePicker.Value.ToString());
            DialogResult = System.Windows.Forms.DialogResult.OK;
            this.Close();
        }
    }
}   

SELECT *
    FROM ValidityRules
    WHERE InstrumentId = 1 AND
          BaySerialNumber = 'ABC123' AND
          AssayType = 'MyType' AND
          (ValdityCode = 'Code1' OR
           ValdityCode = 'Code2' OR
           ValdityCode = 'Code3')

// Create the predicate
var predicate = PredicateBuilder.New<ValidityRulesEntity>();

// Add the Instrument, Bay Serial Number, and Assay Type
predicate = predicate.And(x => x.InstrumentId == entity.InstrumentId);
predicate = predicate.And(x => x.BaySerialNumber == entity.BaySerialNumber);
predicate = predicate.And(x => x.AssayType == entity.AssayType);

// Now add the codes from the list. This should be in ADDITION to the above conditions.
foreach (var validityCode in entity.ValidityCodes)
{
    predicate = predicate.Or(x => x.ValidityCodes.Equals(validityCode));
}