Is the assembly dll signed when we create key and hit 'build'?

Has VS already signed this DLL?

Yes, unless you specifically request delay signing.

Note that this means that a configuration which specifies signing requires the key to be present, which doesn't mesh well with the key being a private resource with limited access. Delay signing is one option, but another is to have multiple configurations where some produce signed assemblies and some produce unsigned assemblies. See the Noda Time project file for example, where we have "Release", "Signed Release", "Release Portable" and "Signed Release Portable" (as well as debug configurations). The signed builds are only used when actually releasing a version publicly; most development is performed using the unsignged configurations.