Invalid cast error occurs in calculating elements in the listbox

Question

I am trying to calculate the median value from Listbox data in C#. However, occurs invalid cast exception occurs in foreach statement as below (in bold) :

foreach (string item in ListBox1.Items)

can anyone help me how to fix this error? will be highly appreciated.

Thank you.

System.InvalidCastException: Unable to cast object of type 'System.Int32' to type 'System.String'

private void calcButton_Click(object sender, EventArgs e)
{
    medianList.Clear();
    ListBox2.Items.Clear();

    if (sourceList.Count == 0)
    {
        double tInt;
        foreach (string item in ListBox1.Items)
        {
            if (double.TryParse(item, out tInt) == true)
                sourceList.Add(tInt);
        }
    }

    if (ListBox1.Items.Count > 0)
    {
        if (RadioButton1.Checked)
            MiddleMedian();
        else
            AllMedian();
    }
    else
        return;

    DisplayResults();
    sourceList.Clear();
}