How to get installed Windows SDK version?

The current version of the Windows SDK is stored in the CurrentVersion value of the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SDKs\Windows

and it can be retrieved using this PowerShell one-liner:

$(Get-Item "hklm:\SOFTWARE\Microsoft\Microsoft SDKs\Windows").GetValue("CurrentVersion")

On English locale at least:

dir "%ProgramFiles%\Microsoft SDKs\Windows"

should work. It is quite likely that there will be multiple versions installed, which is the right one for an one build can only be specified by that project.

If you need to determine, while compiling, what major OS version of the Windows SDK is being used then you can use the VER_PRODUCTBUILD macro, which is defined in ntverp.h. For instance:

#include <ntverp.h>
#if VER_PRODUCTBUILD > 9600
// Windows 10+ SDK code goes here
#else
// Windows 8.1- SDK code goes here
#endif

In most cases this should not be necessary because a product should be designed to build with a particular platform SDK. But for some large products there may be a desired to support multiple platform SDKs. This can be particularly useful when migrating from one to another. If there is a bug in a header file (such as the bogus "#pragma pop" in the Windows 8.1 SDK version of bthledef.h) then you may need to workaround this bug, but not include the workaround when using the Windows 10 SDK or higher. This technique can also be used to give helpful error messages if the required SDK version is not installed.

Note that VER_PRODUCTBUILD only gives major OS version information, such as 8.1 versus 10. That means that VER_PRODUCTBUILD is increasingly useless as it doesn't change with the updates to Windows 10. Therefore the more likely thing to look at is sdkddkver.h and the NTDDI_WIN10_* macros. As of the Windows 10.0.17763.0 SDK NTDDI_WIN10_RS5 is now defined. So, write code like this:

#include <sdkddkver.h>
#if !defined(NTDDI_WIN10_RS5)
    #error Windows 10.0.17763.0 SDK is required
#endif