There are some commercial products that can meet your requirement. However, if you are looking for open source solutions, like most chemists do, here are some recommendations based on my own experiences.
Since you are using .NET, Indigo toolkit from GGA software is a good choice. http://www.ggasoftware.com/opensource/indigo
It has a very easy-to-read API documentation with a lot of examples. The rendering module of Indigo can render pictures of molecules and reactions from various file formats (SMILES, mol, sdf, rdf, rxn, etc...) with very good quality. I believe it can meet most of your requirements on rendering a chemical reaction.
I myself have been using OpenBabel for quite awhile. It is also a very good development tool for cheminformatics. It is written in C++ and has a .NET binding too. Some of the advanced functions are not included in the NET version of library but I am pretty sure that the rendering module is included. The quality of the picture is as good as Indigo but the API document is not so user friendly. In addition, you need to install the whole package before you can use it. If you are developing a Webform application using ASP.NET, you may have authorization problem accessing local codes on server side. I have to say, it is a pain in the *.
There is also Chemaxon software can do the rendering work for you too. It is commercial software, but some basic function such as rendering a reaction is free. It is based on Java but has a NET binding. The API documentation is OK. Of course, you need to install java on you computer.