Creating C# Classes at runtime

Question

I have been curious about dynamically create class at runtime in C# and stumbled across this article. http://olondono.blogspot.com/2008/02/creating-code-at-runtime.html I am curious to hear some pros and cons regarding construction of a class at runtime.

Any opinions?

Answer 1

Meta-programming has all the advantages of build-time code-gneneration, but without the additional code step. This is very common in library code, such as ORMs, serializers, some types of AOP, DI/IoC containers, etc.

• + avoids the need for extra build steps or writing mundane code
• + such code can handle what actually is the case at runtime, rather than having to handle any uncommon edge-cases or lots of wrappers around wrappers (decorator pattern)
• + allows codegen in scenarios where the metadata is only known at runtime
• + runtime IL can have more access to private fields etc, thanks to how DynamicMethod can be associated with a type; fully generated (dll) code would require [InternalsVisibleTo] or similar, which may be impossible
• - not all systems support runtime code-gen; it is disabled on some server setups, compact framework, iPhone, etc
• - it is bug ugly to do. Regardless of how you do it, this is not normal code.
• - it needs a really good understanding of how things actually work under the covers
• + if forces you to get a really good understanding of how things actually work under the covers

I'm currently re-writing an existing library to use runtime IL generation; it is very rewarding and I'm happy with it; but it is unlike anything I've written before.

Answer 2

This is not a matter of pros and cons.

Sometimes it is convenient to create a class based on information that is tedious to convert to code in another way or on information that is not available until runtime.

The example in the linked article is not something you (as an application programmer) would normally do. But it is useful in tools that, for instance, generate classes based on a database or XML schema.