Why System.Printing namespace are not supported for use within a Windows service or ASP.NET application or service.

Question

https://learn.microsoft.com/en-us/dotnet/api/system.printing?view=windowsdesktop-8.0

Why the System.Printing show this caution box in the documentation, any references or detailed information?

Answer 1

Hi @Jing Hong Lee,

Are you wondering why it is not recommended to use classes from the System.Printing namespace in Windows Services or ASP.NET applications or services?

Based on the warning message, it's clear that trying to use classes in the System.Printing namespace from a Windows service or ASP.NET application or service may cause unexpected problems, such as service degradation and runtime exceptions.

I didn't find any more information about the official documentation at the moment. But I have seen related explanations:

The System.Printing assembly is dependent upon WPF. WPF is not usable inside a service because a service does not have a UI. It also doesn't make sense in ASP.NET because a UI wouldn't be seen from the client. The documentation doesn't say it won't work, it just says that it may not work as expected and that is because of the WPF underpinning. For example under the hood some of the classes rely on the Dispatcher from WPF. You'll likely only have a single Dispatcher (because that is traditionally the UI thread) so your multi-threaded service/ASP.NET app will be serialized down to a single thread for some calls. This will kill performance and may (depending upon the thread) cause a failure.Therefore, System.Printing show this caution box in the documentation.

Links that may be helpful:

https://learn.microsoft.com/en-us/answers/questions/358550/c-printing-problem-with-system-drawing-printing-fr

Best regards,
Lan Huang

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Answer 2

many of the printer support classes make use of the user's profile data. to access known printers for example. a service does not have a user profile.