Dependency Injection with MEF

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I am trying to learn dependency injection.

Ive made a test app to try it out. The output should be "Testing Appended string", but it keeps giving me a NullReferenceException. Any help?

C#

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition.Hosting;
using System.Linq;
using System.Reflection;
using System.Text;
using System.ComponentModel.Composition;

namespace buildtest
{
    interface IAppend
    {
        string Append(string str);
    }

    [Export(typeof(IAppend))]
    class Appender : IAppend
    {

        public string Append(string str)
        {
            return str + "Appended string";
        }
    }

    class AppHelper
    {
        [Import(typeof(IAppend))]
        public IAppend App { get; set; }

    }

    class runner
    {
        public void Run()
        {
            AppHelper t = new AppHelper();
            //Composition
            CompositionContainer container;
            try
            {
                var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
                container = new CompositionContainer(catalog);
                container.ComposeParts(this);

            }
            catch (Exception ex)
            {
                throw ex;
            }
            t.App.Append("Testing ");

            Console.ReadLine();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            new runner().Run();
        }
    }
}

Posted 2-Apr-14 9:45am

stubobis1

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Comments

Sergey Alexandrovich Kryukov 2-Apr-14 16:00pm

In what line?
—SA

2 solutions

Solution 1

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

[EDIT]

After OP's clarification (thank you, stubobis1!):

The immediate problem is now obvious. In the line you pointed out, t.App is null. In the same line, you try to dereference the result of calculation of t.App (which is the property getter call) by calling Append(string) on the null value, which result in the exception you complained about.

Now, why t.App is null? Because you never assigned it a value. Solution 1 explains what you could have done.

Good luck,

—SA

Posted 2-Apr-14 10:01am

Sergey Alexandrovich Kryukov

Updated 3-Apr-14 3:39am

v4

Comments

Kornfeld Eliyahu Peter 2-Apr-14 16:13pm

It's much simpler, see my answer...

Sergey Alexandrovich Kryukov 2-Apr-14 16:18pm

Maybe you answer is correct, but I cannot agree that the answer should be "simpler" than mine. It's simple enough. OP needs to be able to figure out things by himself, instead of getting a concrete easy-to-use fix you offered. Next time, you may be asked the same question from the same inquirer. Do you call it help? I think saying "go ahead and use the debugger" would be even more useful.
—SA

Kornfeld Eliyahu Peter 2-Apr-14 16:24pm

Oh, I didn't mean by that, that what you are saying is wrong or useless, not at all - you are correct about that OP (or any other programmer) have to develop his abilities to a certain level. My thought was, that it is only a typo, as OP's code nothing more than the most basic MEF sample...

stubobis1 2-Apr-14 16:28pm

I forgot to say where the null ref was being thrown;
it was thrown @ t.App.Append, when it tried to access the app though AppHelper.

Sergey Alexandrovich Kryukov 2-Apr-14 17:41pm

Well, thank you, it helped me to complete the fix for you; please see the update to my answer, after [EDIT].
—SA

Maciej Los 2-Apr-14 16:49pm

Who's voted 1?
Useful explanation.
+5

Sergey Alexandrovich Kryukov 2-Apr-14 17:49pm

Thank you, Maciej.
Please see my recent comments to Solution 2 and this one, as well as my update to the answer.
—SA

Maciej Los 3-Apr-14 1:45am

What about this line: AppHelper t = new AppHelper();?
t shouldn't be Null. It has been initiated. Or am i missed something?

Kornfeld Eliyahu Peter 3-Apr-14 1:49am

Just a bit...
SA says well, what is null is t.App. It's newer initialized in the common way, but via the MEF framework. That may confused SA and you...

Sergey Alexandrovich Kryukov 3-Apr-14 9:27am

I understand. MEF via reflection can look at the attribute of this property and initialize it. This is how plugins always work. I think t.App is null, and OP's report confirms it. You really explained what could happen in your last comment to your own answer. I would advise to make the answer more clear (I up-voted it after I got your explanation). I really was confused.

I edited my answer and credited your explanation.

Thank you very much.
—SA

Kornfeld Eliyahu Peter 3-Apr-14 9:33am

See my answer - t.App is null because OP passed this to the function (ComposeParts) that should do the plugin initialization, and he should pass t...After passing t it works like expected...

Sergey Alexandrovich Kryukov 3-Apr-14 9:49am

As I say, I would only advise to explain this initialization in a bit more detail.
—SA

Sergey Alexandrovich Kryukov 3-Apr-14 9:28am

Not t, t.App...
—SA

Maciej Los 3-Apr-14 11:22am

I see it now ;)
I think that all was explained by Kornfeld Eliyahu Peter.

Sergey Alexandrovich Kryukov 3-Apr-14 12:26pm

I hope we're reconciled now.
—SA

Maciej Los 3-Apr-14 13:15pm

We were never conflicted ;)
:laugh:

Sergey Alexandrovich Kryukov 3-Apr-14 13:21pm

No we did not, but we had an argument. And I brought some invalid arguments and made some wrong conclusions, so I apologize for my mistake.
—SA

Maciej Los 3-Apr-14 13:33pm

Hey, it's not time for apologies. And there is no reason. You are very kindly person, Sergey - as always. Discussion on arguments (even if they are bad) is always a discussion only. Nothing else.
Head up!

Sergey Alexandrovich Kryukov 3-Apr-14 13:43pm

I wish other CodeProject members understood things the way you do. :-)
—SA

Nelek 2-Apr-14 17:56pm

I countered as well

Sergey Alexandrovich Kryukov 2-Apr-14 18:27pm

Thank you, Nelek.
—SA

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Kornfeld Eliyahu Peter · Accepted Answer · 2014-04-02T10:12:00

Solution 2

C#

container.ComposeParts(this);

With this line you try to put the composable parts into this!!!

C#

container.ComposeParts(t);

And now you put it into your new AppHelper!!!

[As advised by SA more explanation why that works]
MEF is a plugin engine that can load classes from different assemblies, create instances from the classes in the loaded assembly and assign that object to some property in the hosting assembly.
The pairs are made according the Export/Import attributes.
For instance in our code we have an external (almost) class signed with Export[IAppend] and we have a class - AppHelper - that has property signed with Import[IAppend].
When calling the MEF engine via ComposeParts, the engine looks for such pairs and make the connection. If we pass some object that has no properties with the matching sign (like this) MEF engine will return without any action. However if we pass an object - like t - to the engine it will assign an instance of the external class to the matching property - in our case App.

Posted 2-Apr-14 10:12am

Kornfeld Eliyahu Peter

Updated 3-Apr-14 3:55am

v3

Comments

stubobis1 2-Apr-14 16:29pm

Thank you SO MUCH!

Can you explain why it works like that?

Kornfeld Eliyahu Peter 2-Apr-14 16:37pm

The heart of MEF is pair Export to Import. this (your runner class) was not signed as an Import point, so MEF couldn't add the composable parts to it. But t - which of type of AppHelper - is signed with the proper Import...
You have to read more about how MEF is build to understand the Export-Import schema.
(I have to little article that talk about implementations using MEF. After you did some basic reading you may be interested in them
http://www.codeproject.com/Tips/622400/Windows-Service-to-Host-Multiple-WCF-Services
http://www.codeproject.com/Articles/668153/Extensible-Web-Application)

stubobis1 2-Apr-14 16:56pm

Again, Thank you!

Maciej Los 2-Apr-14 16:47pm

Hawk eye!
+5!

Kornfeld Eliyahu Peter 2-Apr-14 16:51pm

Oh, not really, but I worked with MEF over the last year very intensively...

Sergey Alexandrovich Kryukov 2-Apr-14 17:48pm

Not so fast, Maciej.
This is not exactly so. To see why, please see my comment below, further comments to my answer and my update to my answer. I hope I wasn't unjust.
—SA

Maciej Los 3-Apr-14 1:46am

Not so fast, Sergey ;)
Please, see my cooment to your comment ;)

Sergey Alexandrovich Kryukov 2-Apr-14 17:47pm

Sorry, but after OP's clarification I found that the answer is not an answer. Please see his comments to my answer and my update.

Here is your problem: you was too fast to post an answer. Did you pay attention for what I did? I first asked the OP "in what" line". Put general answer, and later added more pin-pointed fix, exactly per OP's information. Don't get me wrong: if you found a real problem in my answer, I would say you thank you. Everyone can see that this is what I always do. But, instead, you've been fast second time: criticized my answer without sufficient grounds. It's perfectly okay: we all do mistakes, only you should not be offended by my criticism, because I made it with accurate argument and advice.

Good luck,
—SA

Kornfeld Eliyahu Peter 2-Apr-14 17:56pm

Your update to your answer is show that you have no the slightest idea what we are talking about. It's MEF. t.App should not been assigned in the classic way, as you suggest, and it have no connection to the Application object...
t.App will be assigned by the MEF engine when you are loading the composable parts found in the assemblies you examine for that purpose...So while the first part of your answer is good and helpful for any programmer, the second part is totally misleading...
(I do not understand why you are angry with me - if it's that 1 vote on your answer, I only can tell you (without the power to prove it) it's not me...)

Sergey Alexandrovich Kryukov 2-Apr-14 18:30pm

Please look at the code and tell me how t.App can be non-null, okay? Of you find out it is not null, I'll gladly admit my mistake. Everything else is just irrelevant.
Actually, thank you for pointing out this is not the application; I'll remove it; it was just a wrong shot off the hips.
—SA

Kornfeld Eliyahu Peter 2-Apr-14 18:35pm

AppHelper t = new AppHelper();
CompositionContainer container;

var catalog = new AssemblyCatalog(Assembly.GetExecutingAssembly());
container = new CompositionContainer(catalog);
container.ComposeParts(t); // OP write here 'this' and not 't', that's why his code wasn't work, but this line will set t.App to an instance the Appender class

Sergey Alexandrovich Kryukov 2-Apr-14 19:12pm

Exception means that the code compiles. If it does not, the question is incorrect.. "this" is apparently not null. What is null then?

I asked you to tell me how t.App can be non-null. I thin it is null. Can you disprove that?

—SA

Kornfeld Eliyahu Peter 3-Apr-14 0:00am

Again, this is the line assigns an object to t.App
container.ComposeParts(t); // OP write here 'this' and not 't', that's why his code wasn't work, but this line will set t.App to an instance the Appender class

Sergey Alexandrovich Kryukov 3-Apr-14 9:35am

Ah... it could be possible.
All right, at least you've proven wrong. I can see what I was missing. I'll remove part of my answer and re-vote on yours (5).
I would advise you to edit your answer and make the explanation more clear. Such mechanism of indirect assignment of a property are based on the attributes and reflection.

Thank you very much.

—SA