class Population
{
    public string CountryName { get; set; }
    public int TotalPopulation { get; set; }
    public List<Population> GetPopulation()
    {
        List<Population> obj = new List<Population>();
        obj.Add(new Population{CountryName="Select All",TotalPopulation=0 });
        obj.Add(new Population{CountryName="country 1",TotalPopulation= 100});
        obj.Add(new Population{CountryName="country 2",TotalPopulation= 95 });
        obj.Add(new Population{CountryName="country 3",TotalPopulation= 90 });
        obj.Add(new Population{CountryName="country 4",TotalPopulation= 80 });
        return obj;
    }

}

comboBox1.DataSource = new Population().GetPopulation();
comboBox1.DisplayMember = "CountryName";
comboBox1.ValueMember = "TotalPopulation";

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (Convert.ToInt32(comboBox1.SelectedValue) == 0)
    {
        int total = 0;
        for (int i = 0; i < comboBox1.Items.Count; i++)
        {
            comboBox1.Select();
            total += Convert.ToInt32(comboBox1.SelectedValue);
        }
        //now assign the value in label
        label1.Text = total.ToString(); 
    }
}

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (Convert.ToInt32(comboBox1.SelectedValue) == 0)
    {
        int total = 0;
        List<Population> temp = (List<Population>)comboBox1.DataSource;
        foreach (var item in temp)
        {
            total += item.TotalPopulation;
        }
        label1.Text = total.ToString();
    }
}