How do I create an x64 platform solution in visual studio when the configuration manager does not list x64 as an option?

I am currently converting some visual studio solutions from Win32 to x64 and I am unable to find the x64 option in configuration manager for some solution files. Other solutions list x64 and work as expected. The output for each of th开发者_运维技巧ese solutions is a windows dll files.

Is there a reason why Visual Studio will provide x64 as an option for some solution files but not for others?

Can x64 be added when it is not in the list of platforms?

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It's hard to give an answer without knowing what language you're refering to. Is it a .net language (VB/C#) or unmanaged C++?

A good starting point would probably be using something like Beyond Compare to diff a solution file that has an x64 target against one that doesn't, this should show you what's missing and probably allow you to "fill in the blanks" amd re-enable x64 for the solutions that don't currently offer it.