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Bug and workaround in dotnet MEF — The export is not assignable to type .

Today, while writing an app use netfx 4.8 with MEF support based on System.ComponentModel.Composition 4.5 from nuget, a strange bug hit me.

Because I’m not treat MEF as essential, I’d like to write the main code in a project and create MEF wrap in another one like:

  • In Project A: class Real : InterfaceA
  • In Project B: class MEFWrap : Real, and marked with Export attribute.

Certainly, Project A is referenced by Project B.  While building the solution, both dlls generated from Project A and Project B are placed in the output folder of Project B.

While trying to use MEF to load the class from Project B, the funny thing happened:

  • If AssemblyCatalog is chosen to create the object, from the assembly object loaded by LoadFrom method, nothing is wrong, but
  • If AggregateCatalog is used with all assemblies loaded from the folder (Project A and Project B) at the same time, an exception raised while calling ComposesParts:

    The export InterfaceA is not assignable to type InterfaceA.

I don’t know why it’s happened but here are 2 ways to get avoid of it.

  • Only load assembly Project B, not both of them, if it’s possible, when you know the name or name pattern while searching files, or
  • Using AssemblyCatalog for each file, instead of using AggregateCatalog as a whole, will also works.

 

 

Multiple Low-end bar code scanners support for dotNet

Adds support for low end bar code scanners, which work as a keyboard only. By using this library, you can receive code from multiple scanners with the scanner handle id or name (seeing demo on Github).

Platform: dotNet Framework 4 on Windows x86 (need 32-bit due to WinAPI calling)

Package: Nuget

Source code: Github

License: Demo and Test projects are licensed under MIT. Main and other libraries are licensed under LGPLv3.

All I want, is write a lock…

Today, I need to use an instance of synced version of HashSet in C#, aka, ConcurrentHashSet<T> in tech language.

First, I dig some in GitHub and NuGet. I found 2 solutions contains this solution, as they described. One has the exact name of ConcurrentHashSet, but it’s not implemented the ISet<T>. Another is a big set which contains this class with the name, which is written using ConcurrentDictionary<T>, with no ISet<T> implemented either.

Hmm, it seem that if I really need ISet<T> support, I have to do it myself. Let me create class inherited HashSet<T> to write some overridden methods and properties with lock… Wait a minute, only 3 methods from Object is allowed for overriding? Wtf.

If I cannot stand on the shoulders of HashSet<T>, let me dig the HashSet<T> first. Luckily, Microsoft released the source code of the entire dotNet framework. HashSet<T> is included as well. All I need to copy the whole code into mine and… Uhh? What’s the license of the file?

By asking a lawyer friend, I’m told that I MAY have right to use this code in my program but I’m NOT GRANTED to publish the ConcurrentHashSet<T> I wrote separately. That maybe the reason why I failed in the first step — Microsoft didn’t finish their job and don’t allow anyone to fix it for them.

(Maybe) best practice: Use shared projects in Visual Studio involved with Nuget

I wrote many solutions in Visual Studio. Some of them includes components, which intend to be published to Nuget.

Previously, I wrote these projects directly and publish them to Nuget. This is not a smooth way. Consider this: You have project A need to be published to Nuget. Project B, as well. Project C need to reference project A and B, and also need to be published to Nuget. Project D need to reference C for testing. Obviously, following Nuget standard, the Project C need to reference A by using Nuget Packages instead of the projects of A and B directly. But it seems that there is no called testing mode publishing in Nuget supported. There is no easy way to test your project before publishing to Nuget. Maybe you need to change the references in Project C and D repeatedly, switching mode between testing and publishing.

Recently, Shared Project support added to Visual Studio. Now I use a better way, IMHO, in developing such a solution mentioned above.

  • Create a shared project for each project need to be published to Nuget.
  • Put all codes into shared project instead of original one.
  • Create a project for publishing to Nuget, reference the shared project related. — Project N
  • Create a project for testing, reference the shared project related. — Project T

If your Nuget projects need more references which will be published to Nuget from this solution, add them from Nuget to the Project N, and add them as reference from their Project T to this Project T.

All Project Ns will only be used for publishing to Nuget. As well as all Project Ts will be used for testing. They share the same code but the different source for referencing.

TimerFix

A replacement of NetFx Threading.Timer without cumulative error

This is a complete replacement of Threading.Timer. All methods of Threading.Timer are supported. You could use this TimerFix by simply replacing the Threading.Timer object.

Threading.Timer is encapsulated in this class.

In constructors, you could pass the interval setting value. Less the value set, more CPU time is required. Default value is 15 milliseconds.

 

Source code: Github

License: MIT

Nuget: SecretNest.TimerFix

Supported platform: NetCore 1, NetCore 2, NetFx 4, NetStd 1.3, NetStd 2.0

Hidden Lead Bytes in Thumbprint while copying from Windows

Microsoft digs lots of pits, and I keep jumping among them.

Recently, I wrote a program using HttpClient in dotNet to post some data to server through HTTP Post. The server is set with client certificate required.

My designing is simple:

  1. Open X509Store to query the certificate by using thumbprint.
  2. Attach the certificate found in step 1 into WebRequestHandler.
  3. Pass the handler created in step 2 to the instance of HttpClient and send the request.

Required by step 1, I need to type the thumbprint of the certificate into code. The steps I did:

  1. Open certificate from Windows. It’s shown up like this:
  2. Copy the text of Thumbprint and paste it into a notepad.
  3. Replace all spaces to nothing.
  4. Copy the new text into code.

When I run the code, it ended in a strange way. No certificate is found by using the thumbprint I provided. I dig a long time before I found this pit prepared by Microsoft:
The text, copied from the window above (in step 1), contains hidden lead bytes “0x200E”. These bytes won’t display in code view of Visual Studio, nor in Notepad.

After these removed, problem is gone as well.