运用ExcecuteScalar方法时出错，帮忙改正啊！
代码如下：
  SqlConnection conn = new SqlConnection("server=localhost;database=shoupiao;uid=sa;pwd=");
  string strsql = "select sum(price) from chpyish where shijian='time1'";
  conn.Open();
  SqlCommand cmd = new SqlCommand(strsql, conn);
  double d = (double)cmd.ExecuteScalar();
  this.Label4.Text = Convert.ToString(d);
  conn.Close();
  异常详细信息: double d = (double)cmd.ExecuteScalar();这行指定的转换无效。

------解决方案--------------------
首先得判断ExecuteScalar()返回的值是否为null
double d = 0;
object result = cmd.ExecuteScalar();
if(result != null) 
{
d = Convert.ToDouble(result);
}