We Keep Coding

Passing constructor delegate or object for unmanaged resources

In my (simplified) problem I have a method "Reading" that can use many different implementation of some IDisposableThing. I am passing delegates to the constructor right now so I can use the using statement.
Is this approach of passing a delegate of the constructor of my object appropriate?
My problem is that things like List<Func<IDisposable>> etc start looking bit scary (because delegates look like crap in c#) and passing in a object seems more usual and a clearer statement of intent.
Is there a better/different way of managing this situation without delegates?
public void Main()
{
Reading(() => new DisposableThingImplementation());
Reading(() => new AnotherDisposableThingImplementation());
}
public void Reading(Func<IDisposableThing> constructor)
{
using (IDisposableThing streamReader = constructor())
{
//do things
}
}

As I said in the comment, it's difficult to say what's best for your situation, so instead I'll just list your options so you can make an informed decision:
Continue doing what you're doing
Having to use around objects with an unpleasantly complicated-looking type is maybe not ideal visually, but in your situation it may well be perfectly appropriate
Use a custom delegate type
You can define a delegate like:
public delegate IDisposableThing DisposableThingConstructor();
Then anywhere you would write Func<IDisposableThing>, you can just write DisposableThingConstructor instead. For a commonly used delegate type, this may improve code readability, though this too is a matter of taste.
Move the using statements out of Reading
This really depends on whether it's sensible for the lifecycle management of these objects to be a responsibility of the Reading method or not. Given what we have of your code at the moment, we can't really judge this for you. An implementation with the lifecycle management moved out would look like:
public void Main()
{
using(var disposableThing = new DisposableThingImplementation())
Reading(disposableThing);
}
public void Reading(IDisposableThing disposableThing)
{
//do things
}
Use a factory pattern
In this option, you create a class which returns new IDisposableThing implementations. Lots of information can be found on the factory pattern which you may well already know, so I won't repeat it all here. This option may well be overkill for your purposes here, adding a lot of pointless complexity, but depending on how those DisposableThings are constructed, it may have additional benefits which make it worthwhile.
Use a generic argument
This option will only work if all of your IDisposableThing implementations have a parameterless constructor. I'm guessing that's not the case, but in case it is, it's a relatively straightforward approach:
public void Reading<T>() where T : IDisposableThing, new()
{
using(var disposableThing = new T())
{
//do things
}
}
Use an Inversion of Control container
This is another option which would certainly be overkill if used for this purpose alone. I include it mostly for completeness. Inversion of control containers like Ninject will give you easy ways to manage the lifecycles of objects passed into others.
I very much doubt this would be an appropriate solution in your case, especially since the disposable objects are not being used in another class's constructor. If you later run into a situation where you're trying to manage object lifecycle in a larger, complex object graph, this option might be worth revisiting.
Construct the objects outside of the using statement
This is specifically described as "not a best practice" in the MSDN documentation, but it is an option. You can do:
public void Main()
{
Reading(new DisposableThingImplementation());
}
public void Reading(IDisposableThing disposableThing)
{
using (disposableThing)
{
//do things
}
}
At the end of the using statement, the Dispose method will be called, but the object will not be garbage collected because it is still in scope. Trying to use the object after that would be very likely to cause problems because it is not fully initialized. So again, while this is an option, it's unlikely to be a good one.

Is this approach of passing a delegate of the constructor of my object appropriate? My problem is that things like List<Func<IDisposable>> etc start looking bit scary (because delegates look like crap in c#) and passing in a object seems more usual and a clearer statement of intent.
Yes, it's fine. However I understand your concern about passing a list of those things... Perhaps creating a custom delegate with the same signature as Func<IDisposable> and a more explicit name (e.g. SomethingFactory) would be clearer.
Is there a better/different way of managing this situation without delegates?
You could pass a factory or a list of factories to the method. I don't think it's really "better", though; it's mostly the same, since your factory would typically be represented as an interface with a single method, which is essentially the same as a delegate.

Where Is the ability to RegisterAll with Registration Contexts (aka Func<T>)?

I can register a single registration item with instanceCreator context (aka Func<T>), but there doesn't seem to be the same allowance with a RegisterAll.
TL;DR - Find the accepted answer and look at update 2 (or skip down to Update 3 on this question)
This is what I want to do:
container.RegisterAll<IFileWatcher>(
new List<Func<IFileWatcher>>
{
() => new FileWatcher(
#".\Triggers\TriggerWatch\SomeTrigger.txt",
container.GetInstance<IFileSystem>()),
() => new FileWatcher(
#".\Triggers\TriggerWatch\SomeOtherTrigger.txt",
container.GetInstance<IFileSystem>())
});
I tried adding an extension based on a previous Stack Overflow answer for multiple registrations, but it seems that last one in wins:
public static class SimpleInjectorExtensions
{
public static void RegisterAll<TService>(this Container container,
IEnumerable<Func<TService>> instanceCreators)
where TService : class
{
foreach (var instanceCreator in instanceCreators)
{
container.RegisterSingle(typeof(TService),instanceCreator);
}
container.RegisterAll<TService>(typeof (TService));
}
}
I'm also curious why there is a need for RegisterAll to exist in the first place. This is the first dependency injection container out of 5 that I've used that makes the distinction. The others just allow you to register multiple types against a service and then load them all up by calling Resolve<IEnumerable<TService>> (autofac) or GetAllInstances<TService> (both SimpleInjector and Ninject).
Update
For more clarity, I'm trying to build a list of items that I can pass to a composite that handles each of the individual items. It suffers from the same problem as the above since it falls into a group of tasks that all get registered to be run based on schedules, triggers, and events (Rx). To remove the register all for a moment and rip out some of the other stuff:
container.Register<ITask>(() => new FileWatchTask(
container.GetInstance<IFileSystem>(),
container.GetInstance<IMessageSubscriptionManagerService>(),
configuration,
container.GetAllInstances<IFileWatcher>()));
You can see that I am grabbing all instances of the previously registered file watchers.
What I need to know is a simple workaround for this issue and when it will be implemented (or if not, why it won't be). I will also accept that this is not possible given the current limitations of Simple Injector's design. What I will not accept is that I need to change and adapt my architecture to meet the limitations of a tool.
Update 2
Let's talk about OCP (Open Closed Principle aka the O in SOLID) and the impression I'm getting in how SimpleInjector breaks this particular principle in some cases.
Open Closed Principle is just that, open for extension, but closed for modification. What this means is that you can alter the behavior of an entity without altering its source code.
Now let's shift to an example that is relevant here:
var tasks = container.GetAllInstances<ITask>();
foreach (var task in tasks.OrEmptyListIfNull())
{
//registers the task with the scheduler, Rx Event Messaging, or another trigger of some sort
task.Initialize();
}
Notice how clean that is. To be able to do this though, I need to be able to register all instances of an interface:
container.RegisterAll<ITask>(
new List<Func<ITask>>{
() => new FileWatchTask(container.GetInstance<IFileSystem>(),container.GetInstance<IMessageSubscriptionManagerService>(),configuration,container.GetAllInstances<IFileWatcher>()),
() => new DefaultFtpTask(container.GetInstance<IFtpClient>(),container.GetInstance<IFileSystem>()),
() => new DefaultImportFilesTask(container.GetInstance<IFileSystem>())
}
);
Right? So the lesson here is that this is good and meeting OCP. I can change the behavior of the task runner simply by adding or removing items that are registered. Open for extension, closed for modification.
Now let's focus on trying to do it the way suggested in the answer below (prior to the second update, which finally answers this question), which the author is giving the impression to be a better design.
Let's start with what the answer from the maintainer mentions is good design for registration. The viewpoint that I'm getting is that I have to make a sacrifice to my code to somehow make the ITask more flexible to work with SimpleInjector:
container.Register<ITask<SomeGeneric1>(() => new FileWatchTask(container.GetInstance<IFileSystem>(),container.GetInstance<IMessageSubscriptionManagerService>(),configuration,container.GetAllInstances<IFileWatcher>()));
container.Register<ITask<SomeGeneric2>(() => new DefaultFtpTask(container.GetInstance<IFtpClient>(),container.GetInstance<IFileSystem>()));
container.Register<ITask<SomeGeneric3>(() => new DefaultImportFilesTask(container.GetInstance<IFileSystem>()));
Now let's see how that makes our design change:
var task1 = container.GetInstances<ITask<SomeGeneric1>();
task1.Initialize();
var task2 = container.GetInstances<ITask<SomeGeneric2>();
task2.Initialize();
var task3 = container.GetInstances<ITask<SomeGeneric3>();
task3.Initialize();
Ouch. You can see how every time I add or remove an item from the container registration, I now need to also update another section of code. Two places of modification for one change, I'm breaking multiple design issues.
You might say why am I asking the container for this? Well this is in the startup area, but let's explore if I wasn't.
So I will use constructor injection to illustrate why this is bad. First let's see my example as construction injection.
public class SomeClass {
public SomeClass(IEnumerable<ITask> tasks){}
}
Nice and clean.
Now, let's switch back to my understanding of the accepted answer's view (again prior to update 2):
public class SomeClass {
public SomeClass(ITask<Generic1> task1,
ITask<Generic2> task2,
ITask<Generic3> task3
) {}
}
Ouch. Everytime I have to edit multiple areas of code, and let's not even get started at how poor this design is.
What's the lesson here? I'm not the smartest guy in the world. I maintain (or try to maintain :)) multiple frameworks and I don't try to pretend I know more than or better than others. My sense of design might be skewed or I might be limiting others in some unknown way that I have not even thought of yet. I'm sure the author means well when he gives advice on design, but in some cases it may come across annoying (and a little condescending), especially for those of us that know what we are doing.
Update 3
So the question was answered in Update 2 from the maintainer. I was trying to use RegisterAll because it hadn't occurred to me that I could just use Register<IEnumerable<T>> (and unfortunately the documentation didn't point this out). It seems totally obvious now, but when people are making the jump from other IoC frameworks, they are carrying some baggage with them and may miss this awesome simplification in design! I missed it, with 4 other DI containers under my belt. Hopefully he adds it to the documentation or calls it out a little better.

From your first example (using the List<Func<IFileWatcher>>) I understand that you want to register a collection of transient filewatchers. In other words, every time you iterate the list, a new file watcher instance should be created. This is of course very different than registering a list with two (singleton) filewatchers (the same instances that are always returned). There's however some ambiguity in your question, since in the extension method you seem to register them as singleton. For the rest of my answer, I'll assume you want transient behavior.
The common use case for which RegisterAll is created, is to register a list of implementations for a common interface. For instance an application that has multiple IEventHandler<CustomerMoved> implementations that all need to be triggered when a CustomerMoved event got raised. In that case you supply the RegisterAll method with list of System.Type instances, and the container is completely in control of wiring those implementations for you. Since the container is in control of the creation, the collection is called 'container-controlled'.
The RegisterAll however, merely forward the creation back to the container, which means that by default the list results in the creation of transient instances (since unregistered concrete types are resolved as transient). This seems awkward, but it allows you to register a list with elements of different lifestyles, since you can register each item explicitly with the lifestyle of choice. It also allows you to supply the RegisterAll with abstractions (for instance typeof(IService)) and that will work as well, since the request is forwarded back to the container.
Your use case however is different. You want to register a list of elements of the exact same type, but each with a different configuration value. And to make things more difficult, you seem to want to register them as transients instead of singletons. By not-passing the RegisterAll a list of types, but an IEnumerable<TService> the container does not create and auto-wire those types , we call this a 'container-uncontrolled' collection.
Long story short: how do we register this? There are multiple ways to do this, but I personally like this approach:
string[] triggers = new[]
{
#".\Triggers\TriggerWatch\SomeTrigger.txt",
#".\Triggers\TriggerWatch\SomeOtherTrigger.txt"
};
container.RegisterAll<IFileWatcher>(
from trigger in triggers
select new FileWatcher(trigger,
container.GetInstance<IFileSystem>())
);
Here we register a LINQ query (which is just an IEnumerable<T>) using the RegisterAll method. Every time someone resolves an IEnumerable<IFileWatcher> it returns that same query, but since the select of that query contains a new FileWatcher, on iteration new instances are always returned. This effect can be seen using the following test:
var watchers = container.GetAllInstances<IFileWatcher>();
var first1 = watchers.First();
var first2 = watchers.First();
Assert.AreNotEqual(first1, first2, "Should be different instances");
Assert.AreEqual(first1.Trigger, first2.Trigger);
As this test shows, we resolve the collection once, but every time we iterate it (.First() iterates the collection), a new instance is created, but both instances have the same #".\Triggers\TriggerWatch\SomeTrigger.txt" value.
So as you can see, there is not limitation that prevents you from doing this effectively. However, you might need to think differently.
I'm also curious why there is a need for RegisterAll to exist in the
first place.
This is a very explicit design decision. You are right that most other containers just allow you to do a bunch of registrations of the same type and when asked for a collection, all registrations are returned. Problem with this is that it is easy to accidentally register a type again and this is something I wanted to prevent.
Further more, all containers have different behavior of which registration is returned when requesting for a single instance instead of requesting the collection. Some return the first registration others return the last. I wanted to prevent this ambiguity as well.
Last but not least, please note that registering collections of items of the same type should usually be an exception. In my experience 90% of the time when developers want to register multiple types of the same abstraction, there is some ambiguity in their design. By making registering collections explicit, I hoped to let this stick out.
What I will not accept is that I need to change and adapt my
architecture to meet the limitations of some tool.
I do agree with this. Your architecture should be leading, not the tools. You should chose your tools accordingly.
But please do note that Simple Injector has many limitations and most of those limitations are chosen deliberately to stimulate users to have a clean design. For instance, every time you violate one of the SOLID principles in your code, you will have problems. You will have problems keeping your code flexible, your tests readable, and your Composition Root maintainable. This in fact holds for all DI containers, but perhaps even more for Simple Injector. This is deliberate and if the developers are not interested in applying the SOLID principles and want a DI container that just works in any given circumstance, perhaps Simple Injector is not the best tool for the job. For instance, applying Simple Injector to a legacy code base can be daunting.
I hope this gives some perspective on the design of Simple Injector.
UPDATE
If you need singletons instead, this is even simpler. You can register them as follows:
var fs = new RealFileSystem();
container.RegisterSingle<IFileSystem>(fs);
container.RegisterAll<IFileWatcher>(
new FileWatcher(#".\Triggers\TriggerWatch\SomeTrigger.txt", fs),
new FileWatcher(#".\Triggers\TriggerWatch\SomeOtherTrigger.txt", fs)
);
UPDATE 2
You explicitly asked for RegisterAll<T>(Func<T>) support to lazily create a collection. In fact there already is support for this, just by using RegisterSingle<IEnumerable<T>>(Func<IEnumerable<T>>), as you can see here:
container.RegisterSingle<IEnumerable<IFileWatcher>>(() =>
{
return
from
var list = new List<IFileWatcher>
{
new FileWatcher(#".\Triggers\TriggerWatch\SomeTrigger.txt", container.GetInstance<IFileSystem>()),
new FileWatcher(#".\Triggers\TriggerWatch\SomeOtherTrigger.txt", container.GetInstance<IFileSystem>())
};
return list.AsReadOnly();
});
The RegisterAll<T>(IEnumerable<T>) is in fact a convenient overload that eventually calls into RegisterSingle<IEnumerable<T>>(collection).
Note that I explicitly return a readonly list. This is optional, but is an extra safety mechanism that prevents the collection from being altered by any application code. When using RegisterAll<T> collections are automatically wrapped in a read-only iterator.
The only catch with using RegisterSingle<IEnumerable<T>> is that the container will not iterate the collection when you call container.Verify(). However, in your case this would not be a problem, since when an element of the collection fails to initialize the call to GetInstance<IEnumerable<IFileWatcher>> will fail as well and with that the call to Verify().
UPDATE 3
I apologize if I gave to the impression that I meant your design is wrong. I have no way of knowing this. Since you explicitly asked about why some features where missing, I tried my best to explain the rationale behind this. That doesn't mean however that I think your design is bad, since there is no way for me of knowing.
let's switch back to what that would look like with the maintainer's view of good design
I'm not sure why you think that this is my view on good design? Having a SomeClass with a constructor that need to be changed every time you add a task in the system is definitely not a good design. We can safely agree on this. That breaks OCP. I would never advice anyone to do such thing. Besides having a constructor with many arguments is a design smell at least. The next minor release of Simple Injector even adds a diagnostic warning concerning types with too many dependencies since this often is an indication of a SRP violation. But again see how Simple Injector tries to ‘help’ developers here by providing guidance.
Still however, I do promote the use of generic interfaces, and that’s a case that the Simple Injector design is optimized for especially. An ITask interface is a good example of this. In that case, the ITask<T> will often be an abstraction over some business behavior you wish to execute, and the T is a parameter object that holds all parameters of the operation to execute (you can see it as a message with a message handler). This however is only useful when a consumer needs to execute an operation with a specific set of parameters (a specific version of T), for instance it wants to execute ITask<ShipOrder>. Since you are executing a batch of all tasks without supplying parameter, a design based on ITask<T> would probably be awkward.
But let's assume for a second that it is appropriate. Let's assume this, so I can explain how Simple Injector is optimized in this case. At the end of this update, I’ll show you how Simple Injector might still be able to help in your case, so hold your breath. In your code sample, you register your generic tasks as follows:
container.Register<ITask<SomeGeneric1>(() => new FileWatchTask(container.GetInstance<IFileSystem>(),container.GetInstance<IMessageSubscriptionManagerService>(),configuration,container.GetAllInstances<IFileWatcher>()));
container.Register<ITask<SomeGeneric2>(() => new DefaultFtpTask(container.GetInstance<IFtpClient>(),container.GetInstance<IFileSystem>()));
container.Register<ITask<SomeGeneric3>(() => new DefaultImportFilesTask(container.GetInstance<IFileSystem>()));
This is a rather painful way of registering all tasks in the system, since every time you change a constructor of a task implementation, you'll have to change this code. Simple Injector allows you to auto-wire types by looking at their constructor. In other words, Simple Injector allows you to simplify this code to the following:
container.Register<ITask<SomeGeneric1>, FileWatchTask>();
container.Register<ITask<SomeGeneric2>, DefaultFtpTask>();
container.Register<ITask<SomeGeneric3>, DefaultImportFilesTask>();
This already is much more maintainable, results in better performance and allows you to do add other interesting scenarios later on such as context based injection (since Simple Injector is in control of the whole object graph). This is the advised way of registering things in Simple Injector (prevent the use of a Func if possible).
Still, when having a architecture where a task is the center element, you would probably add new task implementations quite regularly. This will result in having dozens of registration lines and having to go back to this code to add a line every time you add a task. Simple Injector however has a batch registration feature that allows you to shrink this back to one single line of code:
// using SimpleInjector.Extensions;
container.RegisterManyForOpenGeneric(typeof(ITask<>), typeof(ITask<>).Assembly);
By calling this line, the container will search for all ITask<T> implementations that are located in the interface’s assembly and it will register them for you. Since this is done at runtime using reflection, the line does not have to be altered when new tasks are added to the system.
And since you're talking about the OCP, IMO Simple Injector has great support for the OCP. At some points it even beats all other frameworks out there. When I think about OCP, I particularly think about one specific pattern: the decorator pattern. The decorator pattern is a very important pattern to use when applying the OCP. Cross-cutting concerns for instance should not be added by changing some piece of business logic itself, but can best be added by wrapping classes with decorators. With Simple Injector, a decorator can be added with just a single line of code:
// using SimpleInjector.Extensions;
container.RegisterDecorator(typeof(ITask<>), typeof(TransactionTaskDecorator<>));
This ensures that a (transient) TransactionTaskDecorator<T> is wrapped around all ITask<T> implementations when they got resolved. Those decorators are integrated in the container’s pipeline, which means that they can have dependencies of their own, can have initializers, and can have a specific lifestyle. And decorators can be stacked easily:
container.RegisterDecorator(typeof(ITask<>), typeof(TransactionTaskDecorator<>));
container.RegisterDecorator(typeof(ITask<>), typeof(DeadlockRetryTaskDecorator<>));
This wraps all tasks in a transaction decorator and wraps that transaction decorator again in a deadlock retry decorator. And you can even apply decorators conditionally:
container.RegisterDecorator(typeof(ITask<>), typeof(ValidationTaskDecorator<>),
context => ShouldApplyValidator(context.ServiceType));
And if your decorator has a generic type constraint, Simple Injector would automatically apply the decorator when the generic type constraints match, nothing you have to do about this. And since Simple Injector generates expression trees and compiles them down to delegates, this is all a one-time cost. That doesn’t mean it’s for free, but you’ll pay only once and not per resolve.
There's no other DI library that makes adding decorators as easy and flexible as Simple Injector does.
So this is where Simple Injector really shines, but that doesn't help you much :-). Generic interfaces don't help you in this case, but still, even in your case, you might be able make your registration more maintainable. If you have many task implementations in the system (that is, much more than three), you might be able to automate things like this:
var taskTypes = (
from type in typeof(ITask).Assemby.GetTypes()
where typeof(ITask).IsAssignableFrom(type)
where !type.IsAbstract && !type.IsGenericTypeDefinition
select type)
.ToList();
// Register all as task types singleton
taskTypes.ForEach(type => container.Register(type, type, Lifestyle.Singleton));
// registers a list of all those (singleton) tasks.
container.RegisterAll<ITask>(taskTypes);
Alternatively, with Simple Injector 2.3 and up, you can pass in Registration instances directly into the RegisterAll method:
var taskTypes =
from type in typeof(ITask).Assemby.GetTypes()
where typeof(ITask).IsAssignableFrom(type)
where !type.IsAbstract && !type.IsGenericTypeDefinition
select type;
// registers a list of all those (singleton) tasks.
container.RegisterAll(typeof(ITask),
from type in taskTypes
select Lifestyle.Singleton.CreateRegistration(type, type, container));
This does assume however that all those task implementations have a single public constructor and all constructor arguments are resolvable (no configuration values such as int and string). If this is not the case, there are ways to change the default behavior of the framework, but if you want to know anything about this, it would be better to move that discussion to a new SO question.
Again, I’m sorry if I have annoyed you, but I rather annoy some developers than missing the opportunity in helping a lot others :-)

Delegates (Lambda expressions) Vs Interfaces and abstract classes

I have been looking for a neat answer to this design question with no success. I could not find help neither in the ".NET Framework design guidelines" nor in the "C# programing guidelines".
I basically have to expose a pattern as an API so the users can define and integrate their algorithms into my framework like this:
1)
// This what I provide
public abstract class AbstractDoSomething{
public abstract SomeThing DoSomething();
}
Users need to implementing this abstract class, they have to implement the DoSomething method (that I can call from within my framework and use it)
2)
I found out that this can also acheived by using delegates:
public sealed class DoSomething{
public String Id;
Func<SomeThing> DoSomething;
}
In this case, a user can only use DoSomething class this way:
DoSomething do = new DoSomething()
{
Id="ThisIsMyID",
DoSomething = (() => new Something())
}
Question
Which of these two options is best for an easy, usable and most importantly understandable to expose as an API?
EDIT
In case of 1 : The registration is done this way (assuming MyDoSomething extends AbstractDoSomething:
MyFramework.AddDoSomething("DoSomethingIdentifier", new MyDoSomething());
In case of 2 : The registration is done like this:
MyFramework.AddDoSomething(new DoSomething());

Which of these two options is best for an easy, usable and most importantly understandable to expose as an API?
The first is more "traditional" in terms of OOP, and may be more understandable to many developers. It also can have advantages in terms of allowing the user to manage lifetimes of the objects (ie: you can let the class implement IDisposable and dispose of instances on shutdown, etc), as well as being easy to extend in future versions in a way that doesn't break backwards compatibility, since adding virtual members to the base class won't break the API. Finally, it can be simpler to use if you want to use something like MEF to compose this automatically, which can simplify/remove the process of "registration" from the user's standpoint (as they can just create the subclass, and drop it in a folder, and have it discovered/used automatically).
The second is a more functional approach, and is simpler in many ways. This allows the user to implement your API with far fewer changes to their existing code, as they just need to wrap the necessary calls in a lambda with closures instead of creating a new type.
That being said, if you're going to take the approach of using a delegate, I wouldn't even make the user create a class - just use a method like:
MyFramework.AddOperation("ThisIsMyID", () => DoFoo());
This makes it a little bit more clear, in my opinion, that you're adding an operation to the system directly. It also completely eliminates the need for another type in your public API (DoSomething), which again simplifies the API.

I would go with the abstract class / interface if:
DoSomething is required
DoSomething will normally get really big (so DoSomething's implementation can be splited into several private / protected methods)
I would go with delegates if:
DoSomething can be treated as an event (OnDoingSomething)
DoSomething is optional (so you default it to a no-op delegate)

Though personally, if in my hand, I would always go by Delegate Model. I just love the simplicity and elegance of higher order functions. But while implementing the model, be careful about memory leaks. Subscribed events are one of the most common reasons of memory leaks in .Net. This means, suppose if you have an object that has some events exposed, the original object would never be disposed until all events are unsubscribed since event creates a strong reference.

As is typical for most of these types of questions, I would say "it depends". :)
But I think the reason for using the abstract class versus the lambda really comes down to behavior. Usually, I think of the lambda being used as a callback type of functionality -- where you'd like something custom happen when something else happens. I do this a lot in my client-side code:
- make a service call
- get some data back
- now invoke my callback to handle that data accordingly
You can do the same with the lambdas -- they are specific and are targeted for very specific situations.
Using the abstract class (or interface) really comes down to where your class' behavior is driven by the environment around it. What's happening, what client am I dealing with, etc.? These larger questions could suggest that you should define a set of behaviors and then allow your developers (or consumers of your API) to create their own sets of behavior based upon their requirements. Granted, you could do the same with lambdas, but I think it would be more complex to develop and also more complex to clearly communicate to your users.
So, I guess my rough rule of thumb is:
- use lambdas for specific callback or side-effect customized behaviors;
- use abstract classes or interfaces to provide a mechanism for object behavior customization (or at least the majority of the object's primary behavior).
Sorry I can't give you a clear definition, but I hope this helps. Good luck!

A few things to consider :
How many different functions/delegates would need to be over-ridden? If may functions, inheretance will group "sets" of overrides in an easier to understand way. If you have a single "registration" function, but many sub-portions can be delegated out to the implementor, this is a classic case of the "Template" pattern, which makes the most sense to be inherited.
How many different implementations of the same function will be needed? If just one, then inheretance is good, but if you have many implementations a delegate might save overhead.
If there are multiple implementations, will the program need to switch between them? Or will it only use a single implementation. If switching is required, delegates might be easier, but I would caution this, especially depending on the answer to #1. See the Strategy Pattern.
If the override needs access to any protected members, then inheretance. If it can rely only on publics, then delegate.
Other choices would be events, and extension methods as well.

Is object creation in getters bad practice?

Let's have an object created in a getter like this :
public class Class1
{
public string Id { get; set; }
public string Oz { get; set; }
public string Poznamka { get; set; }
public Object object
{
get
{
// maybe some more code
return new Object { Id = Id, poznamla = Poznamka, Oz = OZ };
}
}
}
Or should I rather create a Method that will create and return the object ?

Yes, it is bad practice.
Ideally, a getter should not be changing or creating anything (aside from lazy loading, and even then I think it leads to less clear code...). That way you minimise the risk of unintentional side effects.

Properties look like fields but they are methods. This has been known to cause a phenomenal amount of confusion. When a programmer sees code that appears to be accessing a field, there are many assumptions that the programmer makes that may not be true for a property.So there are some common properties design guidelines.
Avoid returning different values from the property getter. If called multiple times in a row, a property method may return a different value each time; a field returns the same value each time.
A property method may require additional memory or return a reference to something that is not actually part of the object's state, so modifying the returned object has no effect on the original object; querying a field always returns a reference to an object that is guaranteed to be part of the original object's state. Working with a property that returns a copy can be very confusing to developers, and this characteristic is frequently not documented.
Consider that a property cannot be passed as an out or ref parameter to a method; a field can.
Avoid long running property getters. A property method can take a long time to execute; field access always completes immediately.
Avoid throwing exceptions from getters.
Do preserve previous values if a property setter throws an exception
Avoid observable side effects.
Allow properties to be set in any order even if this results in a temporary invalid state of objects.
Sources
"CLR via C#", Jeffrey Richter. Chapter 9. Defining Properties Intelligently
"Framework Design Guidelines" 2nd edition, Brad Abrams, Krzysztof Cwalina, Chapter 5.2 Property Design

If you want your getter to create a new object every time it is accessed, that's the way to do it. This pattern is normally refered to as a Factory Method.
However, this is not normally needed on properties (ie. getters and setters), and as such is considered bad practice.

yes, it is ... from the outside, it should be transparent, whether you access a property or a field ...
when reading twice from field, or a property, you expect two things:
there is no impact on the object's (external) behaviour
you get identical results
I have no real knowledge of C#, but I hope, the following makes my point clear. let's start like this:
Object o1 = myInst.object;
Object o2 = myInst.object;
o1.poznamka = "some note";
in the case of a field, conditions like the following will be true:
o1 == o2;
o2.poznamka == "some note";
if you use a property with a getter, that returns a new object every time called, both conditions will be false ...
your getter seems to be meant to produce a temporary snapshot of your instance ... if that is what you want to do, than make it a plain method ... it avoids any ambiguities ...

A property should, to all intents and purposes, act like a field. That means no exceptions should be thrown, and no new objects should be created (so you don't create lots of unneccessary objects if the property is used in a loop)
Use a wrapper class or similar instead.

According to me if something is 'property' the getter should return you a property (basically a data that is already existing) relevant to the object.
In your case, you are returning something that is not a property of that object at that moment. You are not returning a property of your object but a product of some action.
I would go with a method something like GetMyObject() instead. Especially if there is an 'action' will take place, I think it is most of the time best to have a method than a property name.
And try to imagine what would other developers who are not familiar with your code expect after seeing your property.

A property is just a convenient way to express a calculated field.
It should still represent something about an object, regardless of how the value itself is arrived at. For example, if the object in question is an invoice, you might have to add up the cost of each line item, and return the total.
What's written in the question breaks that rule, because returning a copy of the object isn't something that describes the object. If the return value changes between calls to the property without an explicit change of object state, then the object model is broken.
Speaking in generalities, returning a new object like this will just about always break the rule (I can't think of a counter-example right now), so I would say that it's bad practice.
There's also the gotcha of properties where you can so easily and innocently call on a property multiple times and end up running the same code (which hopefully isn't slow!).

For writing code that is easily tested, you have to maintain separation of Object initialization.
i.e while in test cases you do not have hold on test some specific items.
like in House object you dont want to test anything related to kitchen object.
and you wana test only the garden. so while you initiate a house class and initiate object in some constructors or in getters you wont be coding good that will support testing.

As an aside to the comments already made, you can run into some real debugging headaches when lazy loading fields via a property.
I had a class with
private Collection<int> moo;
public Collection<int> Moo
{
get
{
if (this.moo == null) this.moo = new Collection<int>();
return this.moo;
}
}
Then somewhere else in the class there was a public method that referenced
this.moo.Add(baa);
without checking it was instantiated.
It threw a null reference exception, as expected. But the exception was on a UI thread so not immediately obvious where it was coming from. I started tracing through, and everytime I traced through, the error dissapeared.
For a while I have to admit I thought I was going crazy. Debugger - no error. Runtime, error. Much scratching of head later I spotted the error, and realised that the Visual Studio debugger was instantiating the Collection as it displayed the public properties of the class.

It's maybe at most acceptable for structs. For reference types, I would only create a new object in a getter when it's only done once using some lazy-load pattern.

It depends on the use of the getter. It's a great place to include this kind of code for lazy loading.

It is a bad practice. In your example, you should be able to expect the same Object every time you access the object property.

As you have it it is bad but not dis similar to an acceptable practice called lazy loading which can be read about here.
http://www.aspcode.net/Lazy-loading-of-structures-in-C-howto-part-8.aspx

It is a bad practice. But if you are thinking of objects as a bunch of getters & setters you should check the classical discussions about the topic.
As some folks mentioned, lazy loading could be a reason to do so. Depends on the actual business logic you are modeling here. You should create a separate method if it is better for legibility purposes, but if the code to create the object is simple you could avoid the indirection.

Declaring variables - best practices

I found a while ago (and I want to confirm again) that if you declare a class level variable, you should not call its constructor until the class constructor or load has been called. The reason was performance - but are there other reasons to do or not do this? Are there exceptions to this rule?
ie: this is what I do based on what I think the best practice is:
public class SomeClass
{
private PersonObject _person;
public SomeClass()
{
_person = new PersonObject("Smitface");
}
}
opposed to:
public class SomeClass
{
private PersonObject _person = new PersonObject("Smitface");
public SomeClass()
{
}
}

If you set your variable outside of the constructor then there is no error handling (handeling) available. While in your example it makes no difference, but there are many cases that you may want to have some sort of error handling. In that case using your first option would be correct.
Nescio talked about what implication this would have on your applicaiton if there were some constructor failures.
For that reason, I always use Option #1.

Honestly, if you look at the IL, all that happens in the second case is the compiler moves the initialization to the constructor for you.
Personally, I like to see all initialization done in the constructor. If I'm working on a throwaway prototype project, I don't mind having the initialization and declaration in the same spot, but for my "I want to keep this" projects I do it all in the constructor.

Actually, in spite of what others have said, it can be important whether your initialization is inside or outside the constructor, as there is different behaviour during object construction if the object is in a hierarchy (i.e. the order in which things get run is different).
See this post and this post from Eric Lippert which explains the semantic difference between the two in more detail.
So the answer is that in the majority of cases it doesn't make any difference, and it certainly doesn't make any difference in terms of performance, but in a minority of cases it could make a difference, and you should know why, and make a decision based on that.

There's a common pattern called Dependency Injection or Inversion of Control (IOC) that offers exactly these two mechanisms for "injecting" a dependant object (like a DAL class) into a class that's furthur up the dependency chain (furthur from the database)
In this pattern, using a ctor, you would
public class SomeClass
{
private PersonObject per;
public SomeClass(PersonObject person)
{
per = person;
}
}
private PersonObject Joe = new PersonObject("Smitface");
SomeClass MyObj = new SomeClass(Joe);
Now you could for example, pass in a real DAL class for production call
or a test DAL class in a unit test method...

It depends on the context of how the variable will be used. Naturally constants and static or readonly should be initialized on declaration, otherwise they typically should be initialized in the constructor. That way you can swap out design patterns for how your objects are instatiated fairly easy without having to worry about when the variables will be initialized.

You should generally prefer the second variant. It's more robust to changes in your code. Suppose you add a constructor. Now you have to remember to initialize your variables there as well, unless you use the second variant.
Of course, this only counts if there are no compelling reasons to use in-constructor initialization (as mentioned by discorax).

I prefer the latter, but only becuase I find it neater.
It's personal preference really, they both do the same thing.

I like to initialize in the constructor because that way it all happens in one place, and also because it makes it easier if you decide to create an overloaded constructor later on.
Also, it helps to remind me of things that I'll want to clean up in a deconstructor.

the latter can take advantage of lazy instantiation, i.e. it won't initialize the variable until it is referenced

i think this type of question is stylistic only, so who cares what way you do it. the language allows both, so other people are going to do both. don't make bad assumptions.

The first declaration is actually cleaner. The second conceals the fact the constructor initializes the class in the static constructor. If for any reason the constructor fails, the whole type is unusable for the rest of the applicaiton.

I prefer to initialize variables as soon as possible, since it avoids (some) null errors.
Edit: Obviously in this simplified example there is no difference, however in the general case I believe it is good practice to initialize class variables when they are declared, if possible. This makes it impossible to refer to the variable before it it initialized, which eliminates some errors which would be possible when you initialize fields in the constructor.
As you get more class variables and the initialization sequence in the constructor gets more complex, it gets easier to introduce bugs where one initialization depends on another initialization which haven't happend yet.