The exception is thrown at: https://github.com/mongodb/mongo-csharp-driver/blob/660658d4253f06506c1d60aae7ef68745ab609b2/src/MongoDB.Bson/Serialization/BsonClassMap.cs#L40

It used to work up to and including dotnet core 3.0 preview3.

Today I upgraded to preview4, causing this breaking change.

I researched the issue briefly, apparently the new core version does not include the System.Runtime.Serialization.Formatters reference in CoreLib (so the System.Runtime.Serialization.FormatterServices type cannot be loaded this way).

Assembly.Load("System.Runtime.Serialization.Formatters") will work instead of  typeof(string).GetTypeInfo().Assembly as a method to load the assembly, but it will not work with .net framework (so there is some versioning code required probably).