How to set substitution type in OracleParameter

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C#

Oracle

Hi All!
I know Builder C++ and Delphi have Oracle components(TOracleDataSet for example) which have Substitution type. How can I get it in C#?
I tried to do this:

private string query = "select * from tb_zone t where t.employee=:p1 :p2 ";
       public void test()
       {
           using (OracleConnection connection = new OracleConnection(connectionString))
           {
               connection.Open();
               OracleCommand cmd = new OracleCommand(query, connection);
               cmd.Parameters.AddWithValue("p1", 8434);
               //Substitution               
               cmd.Parameters.AddWithValue("p2", " and t.rkc is not null "); 
               MessageBox.Show( cmd.ExecuteOracleScalar().ToString());
           }
       }

But it doesn't work!
I think the query becomes like below :
select * from tb_zone t where t.employee=8434 " and t.rkc is not null "
The second parameter wrapped by quotes.
Sorry for my English :)