ADO.NET参数详解

    这个问题以前总是遇到，但是每次都是找到解决方法就没有具体找原因。昨天再次遇到参数问题，才想着一定要搞清楚了。
    ADO.NET中不同数据提供者所用参数格式如下：
    Provider             Named/Positional            Parameter Marker
    SqlClient            Named                       @parmname
    OracleClient         Named                       parmname (or parmname)
    OleDb                Positional                  ?
    Odbc                 Positional                   ?

    在DbCommand.CommandType=CommandType.Text时，DbParameter就要符合上述要求。如 SqlParameter则要使用参数名来决定参数值，而oledb则是根据参数顺序来决定参数值。但是当DbCommand.CommandType= CommandType.StoreProcedure时，则可以采用采用占位符或名字来确定参数值。
    这种不统一使得要写出跨越各种数据提供者的程序变得复杂了。同时要实现统一的数据层当然还要考虑不同数据库的sql语句区别（所以要尽量采用标准的sql 语句）。本来ADO.net提供了很好的类结构(DbConnection, DbCommand等以Db开头的类)来实现这种很重要的功能，可是这些原因使得我们还是要自己写不少代码。
    下面是一段示例代码：

     // create proc TestParam
     // @courseid varchar(50)
     // as
     // select * from course where courseid=@courseid

     using  System;
     using  System.Collections.Generic;
     using  System.Data.Common;
     using  System.Data.OleDb;
     using  System.Data.SqlClient;
     using  System.Text;

     namespace  ADOParameterTest
    {
         class  Program
        {
             enum  ConnectionType :  byte  { Sql, Ole };

         static   string  sqlConnectionString  =   " Data Source=CTEC-LLY;Initial catalog=examdb;Integrated Security=True " ;
         static   string  oleConnectionString  =   " Provider=sqloledb;Data Source=CTEC-LLY;Initial Catalog=examdb;User Id=sa;Password=sa " ;

         static  DbConnection GetConnection(ConnectionType ct){
             if  (ct  ==  ConnectionType.Sql)
          
            {
                 return   new  SqlConnection(sqlConnectionString);
            }
             else
            {
                 return   new  OleDbConnection(oleConnectionString);
            }
        }

         static   void  TestOleCommandText()
        {
            Console.WriteLine( " TestOleCommandText " );
             using (DbConnection con  =  GetConnection(ConnectionType.Ole))
            {
                con.Open();
                DbCommand cmd  =  con.CreateCommand();
                 // cmd.CommandText = "declare @courseid as varchar(50); set @courseid=’0001’;select * from course where courseid=@courseid";
                cmd.CommandText  =   " select * from course where courseid=? " ;
                 // odbc&oledb只能是?做占位符，这时跟参数名无关跟参数顺序是相关的
                DbParameter p  =  cmd.CreateParameter();
                p.ParameterName  =   " courseid " ;
                p.Value  =   " 0001 " ;
                cmd.Parameters.Add(p);
                 // 下面被注释的参数设置方式也是对的，可以注释上面四行而采用下面的语句
                 // 记住参数名是没有意义的，顺序决定参数
                 // cmd.Parameters.Add(new OleDbParameter("@courseid", "0001"));
                ShowResult(cmd.ExecuteReader());
            }
        }

         static   void  TestOleStoredProcedure()
      
         {
            Console.WriteLine( " TestOleStoredProcedure " );
             using  (DbConnection con  =  GetConnection(ConnectionType.Ole))
            {
                con.Open();
                DbCommand cmd  =  con.CreateCommand();
                cmd.CommandType  =  System.Data.CommandType.StoredProcedure;
                cmd.CommandText  =   " TestParam " ;
                Console.WriteLine( " 用@param做占位符 " );
                 // odbc&oledb只能是?做占位符，这时跟参数名无关跟参数顺序是相关的
                DbParameter p  =  cmd.CreateParameter();
                p.ParameterName  =   " @courseid " ;
                p.Value  =   " 0001 " ;
                cmd.Parameters.Add(p);
                 // 下面被注释的参数设置方式也是对的，可以注释上面四行而采用下面的语句
                 // 记住参数名是没有意义的，顺序决定参数
                 // cmd.Parameters.Add(new OleDbParameter("@courseid", "0001"));
                ShowResult(cmd.ExecuteReader());

                Console.WriteLine( " 用?做占位符 " );
                p.ParameterName  =   "" ;
                ShowResult(cmd.ExecuteReader());

            }
        }

        static   void  TestSqlCommandText()
        {
            Console.WriteLine( " TestSqlCommandText " );
             using  (DbConnection con  =  GetConnection(ConnectionType.Sql))
            {
          
            con.Open();
                DbCommand cmd  =  con.CreateCommand();
                 // sql只能是@param做占位符，跟oledb相反，跟参数名有关跟参数顺序无关
                 // oracle只能用:param做占位符，跟参数名有关跟参数顺序无关
                cmd.CommandText  =   " select * from course where courseid=@courseid " ;
                DbParameter p  =  cmd.CreateParameter();
                p.ParameterName  =   " @courseid " ;
                p.Value  =   " 0001 " ;
                cmd.Parameters.Add(p);
                 // 下面被注释的参数设置方式也是对的，可以注释上面四行而采用下面的语句
                 // 记住只有参数名有意义，顺序无关
                 // cmd.Parameters.Add(new OleDbParameter("@courseid", "0001"));
                ShowResult(cmd.ExecuteReader());
            }
        }

         static   void  TestSqlStoredProcedure()
        {
            Console.WriteLine( " TestSqlStoredProcedure " );
             using  (DbConnection con  =  GetConnection(ConnectionType.Sql))
            {
                con.Open();
                DbCommand cmd  =  con.CreateCommand();
                 // sql只能是@param做占位符，跟oledb相反，跟参数名有关跟参数顺序无关
                 // oracle只能用:param做占位符，跟参数名有关跟参数顺序无关
                cmd.CommandType  =  System.Data.CommandType.StoredProcedure;
                cmd.CommandText  =   " TestParam " ;

                Console.WriteLine( " 用@param做占位符 " );
              
                DbParameter p  =  cmd.CreateParameter();
                p.ParameterName  =   " @courseid " ;
                p.Value  =   " 0001 " ;
                cmd.Parameters.Add(p);
                 // 下面被注释的参数设置方式也是对的，可以注释上面四行而采用下面的语句
                 // 记住只有参数名有意义，顺序无关
                 // cmd.Parameters.Add(new OleDbParameter("@courseid", "0001"));
                ShowResult(cmd.ExecuteReader());

                Console.WriteLine( " 用?做占位符 " );
                p.ParameterName  =   "" ;
                ShowResult(cmd.ExecuteReader());
            }
        }

         static   void  ShowResult(DbDataReader reader)
        {
             int  count  =  reader.FieldCount;
             while (reader.Read())
            {
                Console.WriteLine( " ------------------------------- " );
                 for ( int  i = 0 ; i < count; i ++ )
                {
                    Console.WriteLine( string .Format( " Filed[{0}]={1} " , i, reader.GetValue(i).ToString()));
                }
            }
            reader.Close();
            Console.WriteLine();
        }

         static   void  Main( string [] args)
        {
             try
            {
                TestOleCommandText();
                TestSqlCommandText();
                TestOleStoredProcedure();
                TestSqlStoredProcedure();
            }
             catch  (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.ReadLine();
        }
    }
}