Xamarin PCL and sqlite.net

Question

I would have the need, in a pcl Xamarin, to pass to a function a type, which is then used to perform a query. The code that I tried to use is this:

public object sqlLeggi(string sql, Type myType){
 SQLiteCommand cmd = DB.CreateCommand(sql);
 var results = cmd.ExecuteQuery< myType > ();
[..]
}

but it does not work, give me this error:

The type or namespace 'myType' could not be found.

Does anyone know if it is possible to do such a thing?

Thank you very much

Note that this has nothing to do with portable class libraries, xamarin, monotouch, executequery or sqlite. This is just a confusion between a Type object and a generic type parameter. — Lasse V. Karlsen

Lasse V. Karlsen Lasse V. Karlsen · Accepted Answer · 2014-03-24T13:45:59

The myType in the two statements play very different roles:

public object sqlLeggi(string sql, Type myType){

Here, myType is a Type object, referencing an instance of the Type class.

var results = cmd.ExecuteQuery< myType > ();

Here, myType is a Type identifier, which is a syntactic construct referring to a specific type, one that would actually be named myType in this case.

Now, there is usually two ways to handle this specific problem:

Look at the object type in cmd and see if there is an overload or alternative method to ExecuteQuery that takes a Type object parameter instead
Make your method generic so that you don't have a Type object to begin with.

The first case would presumably be written in this way:

var results = cmd.ExecuteQuery(myType);

The second like this:

public myType sqlLeggi<myType>(string sql{
    SQLiteCommand cmd = DB.CreateCommand(sql);
    var results = cmd.ExecuteQuery< myType > ();
    [..]
}

Note that:

I made the method return myType instead of object
myType is now specified as a generic parameter to the method: sqlLeggi<myType>

Naming convention in this case would dictate that your generic type parameter be named T or something beginning with T so here is my advice:

public T sqlLeggi<T>(string sql{
    SQLiteCommand cmd = DB.CreateCommand(sql);
    var results = cmd.ExecuteQuery<T>();
    [..]
}

Xamarin PCL and sqlite.net

1 Answers