Recently, while working on a project for a client I was writing a .Net application using the Entity Framework backed onto a PostgreSQL database. All of a sudden I got a System.Runtime.Serialization.SerializationException when I was trying to run my app, and the stack trace (weirdly) didn’t help.
It turns out that while I had added the Npgsql dependency to the project, it wasn’t enough. Entity Framework it seems, needs to have access to Npgsql.dll assembly information. When EntityFramework tries to use the library, it won’t find it (despite being in the project) unless it is in the Global Assembly Cache (GAC).
The Global Assembly Cache (GAC) is a folder in Windows directory to store the .NET assemblies that are specifically designated to be shared by all applications executed on a system. Assemblies can be shared among multiple applications on the machine by registering them in global Assembly cache(GAC). Source
The best way to resolve this is to use the Npgsql installer that matches the version in your project from: https://github.com/npgsql/npgsql/releases. Those setups take care of registering Npgsql in GAC and set up the machine.config file to include the Npgsql db provider factory.
After I did this, everything worked as it should, and all was right with the world.