public void btnQuery_Click(object sender,EventArgs e)
{
      ...
       string s1 = this.DropDownList1.SelectedItem.Text.Trim();
       string s2 = this.DropDownList2.SelectedItem.Text.Trim();
       ...
}

------解决方案--------------------
WinForm的ComboBox啊，本来还以为是DropDownList呢

C# code

private void button1_Click(object sender, EventArgs e)
{
  string s1 = this.comboBox1.SelectedValue; // this.comboBox1.SelectedText;
  string s2 = this.comboBox2.SelectedValue; //this.comboBox2.SelectedText;
}

------解决方案--------------------
    string s1 = this.comboBox1.SelectedValue.ToString();
   string s2 = this.comboBox2.SelectedText.Trim();