C# Class naming convention: Is it BaseClass or ClassBase or AbstractClass

There are examples in the Framework with the Base suffix, e.g. System.Configuration.Provider.ProviderBase, System.Web.SessionState.SessionStateStoreProviderBase.

But by no means all abstract base classes in the Framework follow this convention (e.g. System.Data.Common.DbParameter, System.Data.Common.DbCommand).

Personally I would avoid using the suffix unless I wanted to emphasize the fact that it's an abstract class and felt that otherwise users of the class might expect the name to indicate a concrete implementation.

None of the above. Consider what purpose your base class provides; name it that. For example, the base class of Automobile and Bicycle could be Vehicle.

If you're creating base classes just to have a base class of one class, and with no purpose or reason other than that, you're probably doing something wrong.

If you're talking about virtual base classes, Microsoft's standard is ClassnameBase (like CollectionBase.)

We use BaseEntity, but I think it your own preference. I frequently see the other.

Just be consistent within your context, be that your project, namespace or if possible, your team. Different conventions are worse than a bad convention IMHO.