这个方法可以把CommandType为Text的SQL语句提前在数据库中编译为一个临时的StoredProcedure然后再执行，这样对于需要多次执行的DbCommand来说，可以提高一定的执行效率：）注意：请在指定了Command的Connection之后再调用Prepare()方法。
此方法在msdn上的介绍如下：
Creates a prepared (or compiled) version of the command on the data source.
Remarks
If the CommandType property is set to TableDirect, Prepare does nothing. If CommandType is set to StoredProcedure, the call to Prepare should succeed, although it may result in a no-op.
Example
The following example creates an instance of the derived class, OleDbCommand, and opens the connection. The example then prepares a stored procedure on the data source by passing a string that is a SQL Select statement and a string to use to connect to the data source.
[C#]
public void OleDbCommandPrepareEx() {
int id = 20;
string desc = "myFirstRegion" ;
OleDbConnection rConn = new OleDbConnection("Provider=SQLOLEDB;Data Source=localhost;Integrated Security=SSPI;Initial Catalog=Northwind;");
rConn.Open();
OleDbCommand command = new OleDbCommand(null, rConn);
// Create and prepare an SQL statement.
command.CommandText = "insert into Region (RegionID, RegionDescription) values (@id, @desc)" ;
command.Parameters.Add ( "@id", id) ;
command.Parameters.Add ( "@desc", desc) ;
command.Prepare() ; // Calling Prepare after having set the Commandtext and parameters.
command.ExecuteNonQuery();
// Change parameter values and call ExecuteNonQuery.
command.Parameters[0].Value = 21;
command.Parameters[1].Value = "mySecondRegion";
command.ExecuteNonQuery();
}