一：通过System.Data.OracleClient(需要安装Oracle客户端并配置tnsnames.ora)
1. 添加命名空间System.Data.OracleClient引用
2. using System.Data.OracleClient;
3. 
string connString = "User ID=IFSAPP;Password=IFSAPP;Data Source=RACE;";
OracleConnection conn = new OracleConnection(connString);
try
{
    conn.Open();
    MessageBox.Show(conn.State.ToString());
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

二：通过System.Data.OracleClient(需要安装Oracle客户端不需配置tnsnames.ora)
1. 添加命名空间System.Data.OracleClient引用
2. using System.Data.OracleClient;
3.
string connString = "User ID=IFSAPP;Password=IFSAPP;Data Source=(DESCRIPTION = (ADDRESS_LIST= (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))) (CONNECT_DATA = http://www.mamicode.com/(SERVICE_NAME = RACE)))";
OracleConnection conn = new OracleConnection(connString);
try
{
    conn.Open();
    MessageBox.Show(conn.State.ToString());
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

三：通过System.Data.OleDb和Oracle公司的驱动
1. 添加命名空间System.Data.OracleClient引用
2. using System.Data.OleDb;
3.
string connString = "Provider=OraOLEDB.Oracle.1;User ID=IFSAPP;Password=IFSAPP;Data Source=(DESCRIPTION = (ADDRESS_LIST= (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))) (CONNECT_DATA = http://www.mamicode.com/(SERVICE_NAME = RACE)))";
OleDbConnection conn = new OleDbConnection(connString);
try
{
    conn.Open();
    MessageBox.Show(conn.State.ToString());
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

四：通过System.Data.OleDb和微软公司的Oracle驱动
1. 添加命名空间System.Data.OracleClient引用
2. using System.Data.OleDb;
3.
string connString = "Provider=MSDAORA.1;User ID=IFSAPP;Password=IFSAPP;Data Source=(DESCRIPTION = (ADDRESS_LIST= (ADDRESS = (PROTOCOL = TCP)(HOST = 127.0.0.1)(PORT = 1521))) (CONNECT_DATA = http://www.mamicode.com/(SERVICE_NAME = RACE)))";
OleDbConnection cnn = new OleDbConnection(connString);
try
{
    conn.Open();
    MessageBox.Show(conn.State.ToString());
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

备注：
a.XP操作系统已经安装了微软公司的Oracle驱动C:\Program Files\Common Files\System\Ole DB\msdaora.dll
b.该驱动需要Oracle客户端的三个文件(oraocixe10.dll、oci.dll、ociw32.dll)放在System32下即可

五：使用ODP连接
1. 下载安装ODP.NET(http://www.oracle.com/technetwork/developer-tools/visual-studio/downloads/index.html)
2. 安装完全成后会产生一序列文件。
3. 找到这个安装目录，打开文件夹%ORACLE_HOME%\Network\Admin在这个下面建立一个tnsnames.ora的文件，其内容可以参考其下的Sample目录下面的配置
Oracle.RACE =
(DESCRIPTION=
   (ADDRESS_LIST=
     (ADDRESS=
       (PROTOCOL=TCP)
       (HOST=127.0.0.1)
       (PORT=1521)
     )
   )
   (CONNECT_DATA=http://www.mamicode.com/
     (SID=RACE)
     (SERVER=DEDICATED)
   )
)
Oracle.RACE为连接字符串名称，可以随便取。等号后面的字符串可以在Enterprise Manager Console工具中连接数据库后的TNS描述符中拷过来
4. 引用Oracle.DataAccess命名空间
5. using Oracle.DataAccess.Client;
6. 示例代码：
string connString = "DATA SOURCE=Oracle.RACE;PERSIST SECURITY INFO=True;USER ID=IFSAPP;password=IFSAPP";
OracleConnection conn = new OracleConnection(connString);
try
{
    conn.Open();
    OracleCommand cmd = new OracleCommand(cmdText,conn);
    OracleDataReader reader = cmd.ExecuteReader();
    this.DataGridView1.DataSource = reader;
    this.DataGridView1.DataBind();
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

六：使用第三方驱动
第三方驱动有 Devart，下载驱动 http://www.devart.com/dotconnect/oracle/，但是是商业版，需要购买许可或破解
连接格式 User ID=myUsername;Password=myPassword;Host=ora;Pooling=true;Min Pool Size=0;Max Pool Size=100;Connection Lifetime=0;
1. 引用Devart.Data.Oracle命名空间
2. using Devart.Data.Oracle;
3.
OracleConnection conn = new OracleConnection();
conn.ConnectionString = "";
conn.Unicode = true;
conn.UserId = "IFSAPP";
conn.Password = "IFSAPP";
conn.Port = 1521;
conn.Server = "127.0.0.1";
conn.Sid = "RACE";
try
{
    conn.Open();
    //execute queries, etc
}
catch (Exception ex)
{
    ShowErrorMessage(ex.Message.ToString());
}
finally
{
    conn.Close();
}

C# 连接 Oracle 的几种方式