> I suppose I should've been more specific. I'd like SqlParameter to
> automagically deduce that MyClass is a SqlDbType.VarChar (as it does for
> System.String). It works fine if I manually set the DbType, or cast to a
> string before adding the value - but I'd like it to be a no-brainer.
I think if you add an implicit conversion to SqlString, it should
work, but I haven't tested this. The MetaType retrieval routine then will
produce the proper type, instead of the wrong type. Internally it appears to
work with Sql* types, instead of native .NET types. I'm not sure though,
please take a peek with reflector in SqlClient.SqlCommand.BuildParamList.
> For background, the scenario is that I have a custom class for person
> identifiers used in the domain classes as a sort of strongly typed
> string. There's an implicit conversion operator on this class to convert
> to System.String (and an explicit one to convert from string). This
> works fine when, for instance, assigning to TextBox.Text.
>
> Since AddWithValue takes an Object type, there's no compiler error
> adding instances of MyClass, it simply fails at runtime because it can't
> figure out the DbType. Obviously, SqlParameter can deduce the DbType of
> Framework primitives - but is there no hook for adding your own? Am I
> stuck with, "whenever you add this as a database parameter make sure you
> explicitly set the DbType or cast to a string first"?
Strictly, this is a Udt case for the parameter, I think. So it doesn't
know if you want to threat it as a Udt, or that you want to threat the object
as a value of a different type. The thing is that it can't know to which type
to cast unless it casts and tests.
As to be expected, the class is sealed so you can't override something
to add your own behavior.
FB
>
> --MB
>
> > -----Original Message-----
> > From: Discussion of advanced .NET topics. [mailto:ADVANCED-
> > [EMAIL PROTECTED] On Behalf Of Frans Bouma
> > Sent: Thursday, October 04, 2007 2:26 PM
> > To: [email protected]
> > Subject: Re: [ADVANCED-DOTNET] SqlParameter Type Mapping
> >
> > > Given a custom type, how can you define what value gets passed when
> > it is
> > > used as a database parameter? Consider:
> > >
> > > public class MyClass {
> > > public string Value;
> > > public MyClass(string value) { this.Value = value; }
> > > }
> > >
> > > using (SqlCommand cmd = new SqlCommand()) {
> > > // I want myClass to be a string when passed to the database!
> > > cmd.Parameters.AddWithValue("@param", new MyClass("test"));
> > > }
> > >
> > > Right now, I get a "No mapping exists from object type MyClass to a
> > known
> > > managed provider native type" - which certainly implies that a
> > mapping
> > > *could* exist. I've tried implicit conversion operators to
> > System.String
> > > and SqlTypes.SqlString and IConvertible with no luck.
> > >
> > > Ideally, it should seamlessly convert to Odbc, OleDb, etc. parameter
> > > values as well (note that it's the value I want coerced - not
> > converting
> > > to an actual SqlParameter).
> > >
> > > I would think an attribute, interface or conversion operator would
> be
> > the
> > > trick - but spelunking with Reflector and Google isn't turning
> > anything
> > > up. Anyone have any ideas?
> >
> > Set parameter.SqlDbType to SqlDbType.VarChar afterwards.
> >
> > FB
> >
> >
> >
> > ===================================
> > This list is hosted by DevelopMentor(r) http://www.develop.com
> >
> > View archives and manage your subscription(s) at
> > http://discuss.develop.com
>
> ===================================
> This list is hosted by DevelopMentor. http://www.develop.com
>
> View archives and manage your subscription(s) at http://discuss.develop.com
===================================
This list is hosted by DevelopMentor® http://www.develop.com
View archives and manage your subscription(s) at http://discuss.develop.com
