We hear a lot about those technologies to build API. It is not crystal clear how to decide the one to pick. REST is a software architectural style. gRPC is a cross-platform open-source Remote Procedure Call framework. SignalR is an open-source software library. GraphQL is an open-source data query and manipulation language.
We thus discuss their concepts and the problems they solve. We showcase how to bootstrap an API in C# with all four. Finally, we propose a decision tree to help you decide.