It is normal to find people who are new to BizTalk, writing expressions just like they do in C# programming language, well this works most of the time, it is when it doesn’t work that you start wondering what is wrong, and end up wasting a lot of time figuring out the problem and some more time finding the solution.
BizTalk uses a language called XLANG/s which is suitable for expressing the process flow (Just like BPEL) and resembles C# syntax when writing expressions, but it is not C# you are using there.
One does not usually get to learn the complete XLANG/s syntax as we construct the orchestration visually by dropping and configuring shapes on to the orchestration surface. You can however see the XLANG/s if you open the orchestration in a text editor like Notepad.
Here is a nice blog post by Charles Young mentioning list of things that are supported in BizTalk. http://geekswithblogs.net/cyoung/articles/3820.aspx