[AdaptiveCards Templating] [Dot Net library] Backward compatibility of adaptive card template library

Feature request

Make the adaptive card template dot net library backward compatible so that templates containing both {} and ${} can be evaluated.

Problem statement

3rd party content is available for search across different canvases (ex: Bing, SharePoint, and Teams).
There is data that has been ingested for making it available for search using the previous versions of the adaptive card template library and it needs to be supported.

Issue faced

As of now, different UX teams (ex: SharePoint UX and Bing UX) are having some independent logic, by parsing the template and making it backward compatible (I guess they are using the java script library). This does not scale as we extend our scope across different clients (ex: Teams).

Hence, we are evaluating an approach to parse the adaptive card template at the server side using dot-net library (so that multiple clients need not be dependent on the backward incompatibility issue)

For this we need the dot-net templating library to be backward compatible.

Start by locating the Dot Net templating library and its template parsing and evaluation entry points. Compare how templates using {} and ${} are currently handled, then define compatibility tests for templates containing both forms. Done means the library evaluates previously ingested templates consistently for all clients.