How can I dynamically instance an Object?

Question

I am trying to build a factory class that will feed me singleton-ized instances of different DbContexts.

The main idea is to have a Dictionary<Type,DbContext>that will hold all the instances I need , and a GetDbContext(Type type) method that looks up type in the dictionary and returns it if it already exists. If it doesn't it should create a new Type(), and add it to the corresponding dictionary.

I have no idea how to do contexts.Add(type, new type());

public class DbContextFactory
{
    private readonly Dictionary<Type, DbContext> _contexts;
    private static DbContextFactory _instance;

    private DbContextFactory()
    {
        _contexts= new Dictionary<Type, DbContext>();
    }

    public static DbContextFactory GetFactory()
    {
        return _instance ?? (_instance = new DbContextFactory());
    }

    public DbContext GetDbContext(Type type)
    {
        if (type.BaseType != typeof(DbContext))
            throw new ArgumentException("Type is not a DbContext type");

        if (!_contexts.ContainsKey(type))
            _contexts.Add(type, new type()); //<--THIS is what I have now Idea how to do

        return _contexts[type];
    }
}

Guffa · Accepted Answer · 2013-10-24 03:32:01Z

3

Make it a generic method:

public DbContext GetDbContext<T>() where T : new()
{
    if (typeof(T).BaseType != typeof(DbContext))
        throw new ArgumentException("Type is not a DbContext type");

    if (!_contexts.ContainsKey(type))
        _contexts.Add(typeof(T), new T());

    return _contexts[type];
}

edited Oct 24, 2013 at 3:32

answered Oct 24, 2013 at 2:38

Guffa

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6 Comments

Erik Philips Over a year ago

type is undefined. I think you meant typeof(T).

dorphalsig Over a year ago

@Guffa Yeap, thats the simplest way to do it alright. But whats the where T: New() part? I've never seen that before

dorphalsig Over a year ago

And what would happen if instead of saying where T: new() I say T: DbContext wouldn't I be checking for the correct type in there?

dorphalsig Over a year ago

@Me UTFG: msdn.microsoft.com/en-us/library/twcad0zb.aspx Explains it all. :)

Guffa Over a year ago

@ErikPhilips: Yes, I missed changing that one.

|

Erik Philips · Accepted Answer · 2013-10-24 02:44:10Z

2

You can create C# classes by using an Activator. One method is the .CreateInstance(Type type).

MyClassBase myClass = Activator.CreateInstance(typeof(MyClass)) as MyClass;

However with a DbContext, you will most likely want to pass in a Connection string so use the .CreateInstance(Type type, params Object[] args)

DbContext myContext = Activator.CreateInstance(typeof(MyClass),
  "ConnectionString") as DbContext;

Or as a Generic method:

if (!_contexts.ContainsKey(typeof(T)))
  _contexts.Add(typeof(T),
    (T)Activator.CreateInstance(typeof(T), "ConnectionString");

edited Oct 24, 2013 at 2:44

answered Oct 24, 2013 at 2:38

Erik Philips

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