I want add dll reference (unity 5.5.3)

I want add dll reference. (using SuperSocket.ClientEngine;)

I can’t add dll file because I meet error code.

108455-7.png

How can I solve it?

thank you.

You can’t. If you have access to the DLL source, then load it up in Visual Studio and target “Unity .NET 3.5 Subset” or something similar, to guarantee compatibility. You’ll have to fix any problems that may occur from not having access to .NET 4.x functionality, if possible, then build that to a new DLL and drag that into Unity instead.

Alternatively, use Unity 2017.1 or higher and switch the Scripting Runtime Version in Edit → Project Settings → Player to “Experimental: .NET 4.6 Equivalent”, and you will likely be able to use that DLL as-is.