This error will occur if the %DXROOT%\Data\Reflection folder does not exist.

The files in that folder are required to document members inherited from base .NET Framework classes. If you get this error, check to be sure that the DXROOT environment variable points to the latest copy of Sandcastle (usually in the C:\Program Files\Sandcastle) folder and that the .\Data\Reflection folder exists beneath it. If it does not, you should reinstall Sandcastle.

Note
If you do need to recreate the files for some reason (i.e. you install a new .NET Framework release), you can run Start | Programs | Sandcastle Help File Builder | Rebuild Reflection Data.

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