C#

new SqlConnection(@"Server = .\sqlexpress; database = YourDbName ; integrated security = sspi;");

private bool stillInitializing = false;    //<========== NEW CODE ===================
public MyForm()
{
    this.stillInitializing = true;    //<========== NEW CODE ===================

    InitializeComponent();

    this.stillInitializing = false;    //<========== NEW CODE ===================
}

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (this.stillInitializing)
    {
        return;
    }

    // your code goes here
}

private void Form1_Load(object sender, EventArgs e)
        {
            this.stillInitializing = true;                                  //NEW CODE

            unitMeasureCollection.GetMulti(null);
            vendorCollection.GetMulti(null);
            shipMethodCollection.GetMulti(null);
            purchaseOrderDetailCollection.GetMulti(null);

            /*PredicateExpression filter = new PredicateExpression(UnitMeasureFields.UnitMeasureCode == "BOX");
            unitMeasureCollection1.GetMulti(filter);
            */

            comboBoxUnitMeasure.DataSource = unitMeasureCollection;
            comboBoxUnitMeasure.ValueMember = "UnitMeasureCode";

            listBoxVendor.DataSource = vendorCollection;
            listBoxVendor.DisplayMember = "Name";
            listBoxVendor.ValueMember = "VendorID";

            listBoxShipMethod.DataSource = shipMethodCollection;
            listBoxShipMethod.DisplayMember = "Name";
            listBoxShipMethod.ValueMember = "ShipMethodID";

            dataGridView1.DataSource = purchaseOrderDetailCollection;

            PopulateCategoriesAndSubCategories();

            //To clear textboxes affected by lisbox event that fired automatically
            ClearTextBoxes();
            DisableButtons();

            this.stillInitializing = false;                                       //NEW CODE
        }

The only values for a custom command that you can define in your application or use in OnCustomCommand are those between 128 and 256. Integers below 128 correspond to system-reserved values.