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Issues adding Toolbox items in the Designer

Adding toolbox items in the Designer is normally very simple but there are potential scenarios where toolbox items may not show up in the list of available controls (despite the relevant dlls being correctly located in the plugins directory or a sub-directory thereof).

In most cases this is due to one of two reasons:

1. The dll in question has dependencies that are not available.
In most cases the fix is simple in that any dependencies need to be copied into the plugins directory (or the relevant sub-directory) where the dll to be loaded exists.

2. The dll is being explicitly prevented from loading.
In various versions of Windows going back a number of years there has been a possibility for the operating system to mark copied / downloaded files as being “foreign” since they originate from an unknown source.
This can often occur with custom and / or user controls and the issue can be noted by viewing the properties of the dll itself:

If the file is marked as “blocked” then it will be prevented from being loaded as a usable library / control etc. within the toolbox in the Designer.
The file needs to “unblocked” and in most cases the operating system may need to be restarted to take effect since this security safeguard appears to remain for that session. This can be circumvented by moving the file elsewhere (outside of Program Files…) and then back into the plugins directory since this clears any cached security restrictions.