InvalidCastException driving me nuts

J

JPerrin

I'm attempting to create a generic data access layer that can load any
assembly that has objects implementing the "IDb..." interfaces.
Everything is working perfectly when I use a SqlConnection, however when
I attempt to use a MySqlConnection and cast it to an IDbConnection I'm
getting an InvalidCastException, even though the MySqlConnection class
implements the IDbConnection interface. Here's a snippet from my code:

------

if( this._assembly == null )
{
this._assembly = Assembly.Load( this.AssemblyName );
}

Object obj = Activator.CreateInstance(
this._assembly.GetType( this._connectionType ) );

Console.WriteLine( obj.GetType().ToString() );
Type [] types = obj.GetType().GetInterfaces();
foreach( Type type in types )
{
Console.Write( type.Name + ", " );
}

IDbConnection conn = (IDbConnection)obj;
conn.ConnectionString = this._connectionString;

return conn;

------

The exception is thrown at the "IDbConnection conn =
(IDbConnection)obj;" line. The output from my Console.WriteLine calls
is as follows:

ByteFX.Data.MySqlClient.MySqlConnection
IComponent, IDisposable, IDbConnection, ICloneable,

As you can see, the Activator.CreateInstance method is returning an
object of the correct type, and the little foreach loop proves that the
object *does* indeed implement IDbConnection. When I run the same code
with a SqlConnection object it works perfectly.

If anybody has any ideas as to what may be happening here, I'd love to
hear them!

- -
JP
 
J

Jon Skeet [C# MVP]

JPerrin said:
I'm attempting to create a generic data access layer that can load any
assembly that has objects implementing the "IDb..." interfaces.
Everything is working perfectly when I use a SqlConnection, however when
I attempt to use a MySqlConnection and cast it to an IDbConnection I'm
getting an InvalidCastException, even though the MySqlConnection class
implements the IDbConnection interface. Here's a snippet from my code:
Click to expand...

I suggest you find out exactly *which* IDbConnection it implements -
and where that type was loaded from. For example, if the assembly
containing MySqlConnection contains the definition for IDbConnection as
well, you'll have problems.

See http://www.pobox.com/~skeet/csharp/plugin.html for more information
about how this kind of thing can go wrong.
 
J

JPerrin

Jon said:
I suggest you find out exactly *which* IDbConnection it implements -
and where that type was loaded from. For example, if the assembly
containing MySqlConnection contains the definition for IDbConnection as
well, you'll have problems.

See http://www.pobox.com/~skeet/csharp/plugin.html for more information
about how this kind of thing can go wrong.
Click to expand...

Thanks Skeeter, that article really helped to point me in the right
direction. Here's what was happening:

- I was dynamically loading a ByteFX.MySqlClient assembly that was
pre-compiled with the .NET 1.1 framework.

- My project was compiled with VS.NET 2002, which uses the 1.0 version
of the framework.

When I attempted to cast the loaded MySqlConnection object as an
IDbConnection object, the runtime was unable to perform the cast because
of the difference in .NET framework versions. As soon as I created the
ByteFX assembly with VS.NET 2002, and thus compiled it against 1.0,
everything worked just fine.

I am now once again a happy camper.

- -
JP
 
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