[c#] how to set the default value to the drop down list control?

I have a drop down list control on my web page. I have bind the datatable to the dropdownlist control as follows -

lstDepartment.DataTextField = "DepartmentName";
    lstDepartment.DataValueField = "DepartmentID";
    lstDepartment.DataSource = dtDept;
    lstDepartment.DataBind();

in the page load event i want to set the default value to the drop down list control from my other table field.

how to do this?

Assuming that the DropDownList control in the other table also contains DepartmentName and DepartmentID:

lstDepartment.ClearSelection();

foreach (var item in lstDepartment.Items) 
{
  if (item.Value == otherDropDownList.SelectedValue)
  {
    item.Selected = true;
  }
}

lstDepartment.DataTextField = "DepartmentName";
lstDepartment.DataValueField = "DepartmentID";
lstDepartment.DataSource = dtDept;
lstDepartment.DataBind();
'Set the initial value:
lstDepartment.SelectedValue = depID;
lstDepartment.Attributes.Remove("InitialValue");
lstDepartment.Attributes.Add("InitialValue", depID);

And in your cancel method:

lstDepartment.SelectedValue = lstDepartment.Attributes("InitialValue");

And in your update method:

lstDepartment.Attributes("InitialValue") = lstDepartment.SelectedValue;

After your DataBind():

lstDepartment.SelectedIndex = 0;  //first item

or

lstDepartment.SelectedValue = "Yourvalue"

or 
//add error checking, just an example, FindByValue may return null
lstDepartment.Items.FindByValue("Yourvalue").Selected = true;

or
//add error checking, just an example, FindByText may return null
lstDepartment.Items.FindByText("Yourvalue").Selected = true;

if you know the index of the item of default value,just

lstDepartment.SelectedIndex = 1;//the second item

or if you know the value you want to set, just

lstDepartment.SelectedValue = "the value you want to set";