IQueryable LINQ Providers
LINQ providers that implement IQueryable<(Of <(T>)>) can vary widely in their complexity. This section discusses the different levels of complexity.
A less complex IQueryable provider might interface with a single method of a Web service, such as the provider that is created in the Walkthrough: Creating an IQueryable LINQ Provider topic. This type of provider is very specific because it expects specific information in the queries that it handles. It has a closed type system, perhaps exposing a single result type. Most of the execution of the query occurs locally, for example by using the Enumerable implementations of the standard query operators. A less complex provider might examine only one method call expression in the expression tree that represents the query, and let the remaining logic of the query be handled elsewhere.
An IQueryable provider of medium complexity might target a data source that has a partially expressive query language. If it targets a Web service, it might interface with more than one method of the Web service and select the method to call based on the question that the query poses. A provider of medium complexity would have a richer type system than a simple provider, but it would still be a fixed type system. For example, the provider might expose types that have one-to-many relationships that can be traversed, but it would not provide mapping technology for user-defined types.
A complex IQueryable provider, such as the LINQ to SQL provider, might translate complete LINQ queries to an expressive query language, such as SQL. A complex provider is more general than a less complex provider, because it can handle a wider variety of questions in the query. It also has an open type system and therefore must contain extensive infrastructure to map user-defined types. Developing a complex provider requires a significant amount of effort.