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Change return type in Interface Method of a Class (C#)

Posted on 2009-05-15
Medium Priority
Last Modified: 2012-05-07
I'm in the process of building a set of custom classes to wrap up a custom OLEDB provider, so I'm recreating classes such as MyDbConnection, MyDbCommand etc.  The problem is that when using an interface, such as IDbConnection, it returns an interface type.

For example, I have a MyDbCommand class (using IDbCommand) and a MyDbConnection class (using IDbConnection).  When I provide an method for the MyDbConnection.CreateCommand() it returns a IDbCommand, not a MyDbCommand as I want it to.

When I look at the 'standard' classes such as OleDbConnection, it's CreateCommand() method returns an OleDbCommand object, not a IDbCommand object so I'm guessing this is possible?

The method in the MyDbConnection as the moment looks like:

public IDbCommand CreateCommand()
    return new MyDbCommand();

which compiles, but then requires conversion when used in another project.  Am I missing a keyword/method in order to do this?
Question by:d_brentley
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LVL 26

Expert Comment

by:Anurag Thakur
ID: 24395658
we are using something what you are trying to do but we are using the enterprise library to achieve the functionality

we achieve something like this
LVL 26

Expert Comment

by:Anurag Thakur
ID: 24395691
protected Database Create (string databaseInstance)
      m_dbProvider = ConfigurationManager.ConnectionStrings[databaseInstance].ProviderName;
      switch (m_dbProvider)
            case DbProviderMapping.DefaultOracleProviderName:
            return new OracleDatabase (ConfigurationManager.ConnectionStrings[databaseInstance].ConnectionString);

            case DbProviderMapping.DefaultSqlProviderName:
            return new SqlDatabase (ConfigurationManager.ConnectionStrings[databaseInstance].ConnectionString);

            case DbProviderMapping.DefaultMySqlProviderName:
            return new MySqlDatabase (ConfigurationManager.ConnectionStrings[databaseInstance].ConnectionString);

            return DatabaseFactory.CreateDatabase (databaseInstance);

Accepted Solution

tculler earned 375 total points
ID: 24395928
Interfaces are set-in-stone, and they are that way or a very good reason. If you changed the return type on a global interface, yet then used it at a later time, you'd get that other return type; one that you may not want.

Instead, I would make a seperate method--one to satisfy the interface, and one to avoid the "casting" syntax. It is a good idea to still implement the interface, though.
public IDbCommand CreateCommand()
     // Shut the interface up, return your command
public MyDbCommand CreateMyCommand()
     // Return your command

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