var in class level members

The error is pretty self describing, that var can be used as local variable declaration and this is field not local variabe which you are declaring in your class so it would not compile.

 

Eric Lippert has wrote a complete blog post on this, which you can read here:
http://blogs.msdn.com/b/ericlippert/archive/2009/01/26/why-no-var-on-fields.aspx 

 

There are couple of posts on this here as well:

 

http://stackoverflow.com/questions/4461597/why-class-fields-cannot-be-var

 

http://stackoverflow.com/questions/6522127/why-cant-i-declare-a-field-using-var?answertab=active#tab-top