Sitecore 9 is the beginning of the end for WFFM, thank God. Unfortunately, I am not out of the woods with my upgrade task as a new version was released. A new forms solution called Sitecore Forms looks very promising, and my money is on WFFM moving into maintenance sooner rather than later.
IFieldItem assembly not imported
The type 'IFieldItem' is defined in an assembly that is not referenced. You must add a reference to assembly 'Sitecore.WFFM.Abstractions, Version=188.8.131.52, Culture=neutral, PublicKeyToken=null'
Sitecore.WFFM.Abstractions.NoReferences NuGet package.
Cannot implicitly convert type IFieldItem to FieldItem
Cannot implicitly convert type 'Sitecore.WFFM.Abstractions.Data.IFieldItem' to 'Sitecore.Forms.Core.Data.FieldItem'. An explicit conversion exists (are you missing a cast?)
Use the var keyword or
IFieldItem as the type.
Data does not exist
There was a significant change in how Sitecore stores form data as of version 8.1. I assume it was to bring the module in line with the xDB worldview. It changed again in Sitecore 8.2. The
Sitecore.Forms.Core assembly no longer contains the
The type or namespace name 'Data' does not exist in the namespace 'Sitecore.Forms' (are you missing an assembly reference?)
You must use analytics to get data from forms, installation
Sitecore.WFFM.Abstractions.NoReferences NuGet package and a reworking of code is required.