How to fix the ThreadStateException (ActiveX control cannot be instantiated) exception when converting HTML to PDF?

In the HTML to PDF conversion process, the HTMLConverter throws an exception with the message "ActiveX control '8856f961-340a-11d0-a96b-00c04fd705a2' cannot be instantiated because the current thread is not in a single-threaded apartment."

The HTMLConverter internally makes use of the MSHTML.dll that can run only in the STA mode. Running the application in the MTA mode leads to this exception. 

This can be fixed by any of the following ways:

1. In the ASP.NET applications, add 'AspCompat=true' in the @page directive.
2. In ASP.NET MVC, windows or web services and console applications make use of the following code snippet:

   [C#]
    
   Thread t = new Thread(ConvertToPDF); 
   t.SetApartmentState(ApartmentState.STA); 
   t.Start(); 
   t.Join();
   private void ConvertToPDF() { // Convert PDF here }
   

[C#]
private void ConvertToPDF()
{
  // Convert PDF here using IE based converter
}

Note:

Starting with v16.2.0.x, if you reference Syncfusion® assemblies from a trial setup or from the NuGet feed, include a license key in your projects. Refer to the link to learn about generating and registering the Syncfusion® license key in your application to use the components without a trial message.

Conclusion

I hope you enjoyed learning about How to fix the ThreadStateException (ActiveX control cannot be instantiated) exception when converting HTML to PDF.

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