Reuse legacy COM component in ASP .NET core Web API

Rajesh Subramanian 0

I have a legacy COM dll which exposes business logic via COM objects. Its implemented using VC++. I am in process of modernising my application for web. Initially I wanted reuse this COM component without any change. A single instance of COM object only can be created in one process. It doesn't support multiple instances in a process. How to consume this COM component from my ASP.NET web API?

2 answers

Bruce (SqlWork.com) 61,731 Reputation points

2024-04-26T18:40:24.8666667+00:00

if you com object is single threaded, then in asp.net core you would run it a dedicated thread (see background service). then you would use messages to send a request / response to the com object.

to implement messages, you could use blocking collections. you could also just have static request/response variables, and use locking and a mutex.

another option is create a node app that hosts the com object. the you new asp.net app would use httpclient to call the api. you could write a simple c++ wrapper, but node is single threaded, so this is nothing special. or try:

https://github.com/idobatter/node-win32ole

if you might also look at updating your com object to a single threaded apartment model and create on a STA thread.

https://learn.microsoft.com/en-us/windows/win32/com/single-threaded-apartments
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1 person found this answer helpful.
Rajesh Subramanian 0 Reputation points

2024-04-26T23:45:21.0766667+00:00

Is it possible to accomplish concurrent access if COM objects are accessed from node App?
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Michael Taylor 51,346 Reputation points

2024-04-26T16:20:02+00:00
Same way you do it in your legacy app.

Add a COM reference in the project by right-clicking the Dependencies node and selecting the COM object.

Create an instance of the COM client in your app and use accordingly.

Of course using COM immediately makes your app Windows only but that is probably OK for now.
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Rajesh Subramanian 0 Reputation points

2024-04-26T18:01:03.8133333+00:00

My COM component is created in unmanaged C++ and it supports only single threaded.

Is there any way to address this?

Michael Taylor 51,346 Reputation points

2024-04-26T18:28:10.5+00:00

It's a COM object so what language you used doesn't really matter. You can load COM objects from C++ without any issue.

The single threaded is a larger issue but that just means it has to be hosted on the one and only STA for your process. Since a web app doesn't have an STA you'll need to create a standalone thread, mark it as STA and then let it run for the life of your app. Any calls to the COM object will be handled by the STA thread. Some sample code for doing this can be seen in this older post. Note however that the performance of your app is going to be negatively impacted because all calls to the COM object will get serialized to this single thread. So if you have 100 calls come in at once they will be serialized and the request will stall. This should be used only as a temporary solution.

Rajesh Subramanian 0 Reputation points

2024-04-26T18:31:26.4966667+00:00

Thanks.is there any way to handle concurrent access in better way ?

Michael Taylor 51,346 Reputation points

2024-04-26T19:09:36.1833333+00:00

Not really. Moving away from the COM object would be your best option. Creating a single COM object per request would still cause all the requests to go through the same STA thread. Using a single COM object which locked itself for concurrency would have the same affect.

Technically you could have each web request that needed the COM object spawn a new STA that then created the COM object but performance would be horrible and resource usage would be insane. Having more than 1 STA in a process just seems inherently wrong.

Maybe you need to consider not using the COM object in your app at all. Not sure what it does so providing suggestions would be hard. If it just handles work that could be done async then consider moving it into a completely separate worker process that runs periodically and does the work. The web app could "add" data to whatever the worker process uses to determine when to do work and then wait for it to complete. Of course if this needs to happen before a web request can be sent back to the browser then it really isn't doable at all.
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Reuse legacy COM component in ASP .NET core Web API

2 answers