在DB4O中按类型查询

Question

How do you pass a class type into a function in C#?

As I am getting into db4o and C# I wrote the following function after reading the tutorials:

    public static void PrintAllPilots("CLASS HERE", string pathToDb)
    {
        IObjectContainer db = Db4oFactory.OpenFile(pathToDb);
        IObjectSet result = db.QueryByExample(typeof("CLASS HERE"));
        db.Close();
        ListResult(result);
    }

Answer 1

There are two ways. The first is to explicitly use the Type type.

public static void PrintAllPilots(Type type, string pathToDb)
{
  ...
  IObjectSet result = db.QueryByExample(type);
}

PrintAllPilots(typeof(SomeType),somePath);

The second is to use generics

public static void PrintAllPilots<T>(string pathToDb)
{
  ...
  IObjectSet result = db.QueryByExample(typeof(T));
}

PrintAllPilots<SomeType>(somePath);

Answer 2

The answers given by by Jon, Jared, and yshuditelu use query-by-example which is largely unused DB4o querying mechanism, and could potentially be deprecated in the future.

The preferred methods of querying on DB4O for .NET is native queries and LINQ.

// Query for all Pilots using DB4O native query:
var result = db.Query<Pilot>();

Or alternatively using Linq-to-DB4O:

// Query for all Pilots using LINQ
var result = from Pilot p in db
             select p;

Both of these work provided you know the type (eg Pilot) at compile time. If you don't know the type at compile time, you can instead use a DB4O SODA query:

var query = db.Query();
query.Constrain(someObj.GetType());
var results = query.Execute();

edit Why use LINQ instead of SODA, Query-by-Example (QBE), or Native Query (NQ)? Because LINQ makes it very natural to do query expressions. For example, here's how you'd query for pilots named Michael:

var michaelPilots = from Pilot p in db
                    where p.Name == "Michael"
                    select p;

And LINQ is composable, meaning you can do things like this:

var first20MichaelPilots = michaelPilots.Take(20);

And you'll still get an efficient query executed in DB4O when you iterate over the results. Doing the same in SODA or QBE or NQ is ugly at best.

Answer 3

I think this is what you want:

public static void PrintAllPilots(Type classType, string pathToDb)
{
    IObjectContainer db = Db4oFactory.OpenFile(pathToDb);
    IObjectSet result = db.QueryByExample(classType);
    db.Close();
    ListResult(result);
}

Answer 4

You can either do it manually by using Type :

public static void PrintAllPilots(Type type, string pathToDb)

Or you could use generics to infer the type:

public static void PrintAllPilots<T>(string pathToDb)
{
   //...
   var result = db.QueryByExample(typeof(T));
}