string username = "";  //用户名
string password = "";  //密码
using (SqlConnection conn = new SqlConnection(@"Data Source=服务器;Initial Catalog=数据库;User ID=数据用户名;Password=数据库密码;"))
{
    if (conn.State == ConnectionState.Closed)
    {
        conn.Open();
    }
    using (SqlCommand comm = conn.CreateCommand())
    {
        comm.CommandText = " SELECT Password FROM [USER] WHERE USERNAME='" + username + "'";
        using (SqlDataReader dr = comm.ExecuteReader())
        {
            if (!dr.HasRows)
            { 
                //此时不存在该用户名 即用户名出错
                return;
            }
            dr.Read();
            string rpwd = dr[0].ToString();
            if (rpwd != password)
            { 
                //此时密码输入不正确
                return;
            }
            //
            // 验证通过操作
            //
        }
    }
}

------解决方案--------------------

C# code

public  AdminList GetAdminInfo(string name,string pwd)
        {
            Open();
                string sql = string.Format("select userName,passWord from adminList where userName='{0}' and  passWord='{1}'", name, pwd);
                SqlCommand commadn = new SqlCommand(sql,Connection);
                SqlDataReader read = commadn.ExecuteReader();
                //判断是否有数据
                if (read.HasRows)
                {
                    while (read.Read())
                    {
                        user.UserName = read["userName"].ToString();
                        user.UserPwd = read["userPwd"].ToString();
                    }
                    
                }
                else
                {
                    //没有数据则添加空信息
                    user.UserName = "";
                    user.UserPwd = "";
                }
                Connection.Close();
            return user;
        }