I'm not sure of any client API implementations using C# (other than just using a C# REST framework as you mentioned), but I might be able to address the remaining questions. The vCloud API...
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I'm not sure of any client API implementations using C# (other than just using a C# REST framework as you mentioned), but I might be able to address the remaining questions. The vCloud API doesn't necessarily have to be delivered alongside of Express, however that's most commonly the case. The vCloud API covers things not conventionally available in a vanilla vSphere install; many of the deployment and administration features (especially for isolation) are features available in Express but not necessarily available through vCenter alone. Express providers don't necessarily have to expose the vCloud API, but that's most often the case. For example, our instance at BlueLock exposes the vCloud API in nearly the same context as the web admin portal itself, so they often march together with bugfixes, feature enhancements, etc. So far, whenever I've seen the "Express API" mentioned in the forums the poster is mentioning the vCloud API. It's pretty much the only user-facing API I've seen implemented for Express. Hope that helps!