Club all the values into single string.
Page_Load()
{
string strDate = textbox1.text + " " + textbox2.text + " " + dropdownlist.selectedtext;
DateTime datetime = DateTime.Parse(strDate);
Console.WriteLine(datetime.ToString());
}
I have tried above code, it is working as expected.
Please note: This solution will work if you ensure that correct format of date and time is entered in the textboxes else it will give you an exception.