SQL代码返回单个值，应该使用SqlCommand对象的ExceuteScalar方法

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Data.SqlClient;namespace CommandScalar{    class Program    {        static void Main(string[] args)        {         SqlConnection conn = new SqlConnection(@"Data Source=PC2012032219VBB;Initial Catalog=Northwind;Integrated Security=True");         //create command(with both text and connection)         string sql = @"select count(*) from employees";         SqlCommand cmd = new SqlCommand(sql, conn);         try         {             conn.Open();             Console.WriteLine("Number of Employees is {0}", cmd.ExecuteScalar());         }         catch (SqlException ex)         {             Console.WriteLine(ex.ToString());         }         finally         {             conn.Close();             Console.WriteLine("Connection Closed.");         }         Console.ReadKey();        }    }}

cmd.ExecuteScalar()返回的是object类型,如果把值赋给变量，注意数据类型转换.