C# 将列的总和反映到WinForm上的标签

我想将列的总和反映到WinForm上的标签上。我不知道这个代码有什么问题：

private void btnT_Click(object sender, EventArgs e)
{
 SqlConnection cn = new SqlConnection("data source = TURKY-PC ; initial 
 catalog = coffeeshopDB ; integrated security = true ; ");            
 SqlCommand cmd;                      
 SqlDataReader dr;        
 cmd = new SqlCommand("select SUM (cost) from billTB", cn);             
 cn.Open();       
 dr = cmd.ExecuteReader();    
 while (dr.Read())            
   {                
     btnT.Text = dr["cost"].ToString();     
   }         
  dr.Close();      
  cn.Close();       
}

---

出现的错误异常是：

System.IndexOutOfRangeException:“cost”

如果要按名称引用列，则必须命名

总和的结果。在这种情况下，您也不需要循环，因为您知道只有一行

private void btnT_Click(object sender, EventArgs e)
{
 SqlConnection cn = new SqlConnection("data source = TURKY-PC ; initial 
 catalog = coffeeshopDB ; integrated security = true ; ");            
 SqlCommand cmd;                      
 SqlDataReader dr;        
 cmd = new SqlCommand("select SUM (cost) as TotalCost from billTB", cn);             
 cn.Open();       
 dr = cmd.ExecuteReader();    
 dr.Read();            
 btnT.Text = dr["TotalCost"].ToString();     
 dr.Close();      
 cn.Close();       
}