Azure Function : Could not load file or assembly 'Microsoft.CodeAnalysis, Version=3.7.0.0

 Azure Function :  Could not load file or assembly 'Microsoft.CodeAnalysis, Version=3.7.0.0

Reason :

 SDK versions will "clean" your output folder for assemblies that they don't think you need. 

Solution : 

Turn it off the clean operation completely by adding the property 
<_FunctionsSkipCleanOutput>true</_FunctionsSkipCleanOutput> to your csproj file