How to make forward-word, backward-word, treat underscore as part of a word?

Question

In my C# mode, M-b and M-f are bound to {backward,forward}-word.

But these things stop at underscores, which I use sometimes as a prefix on member variables.

Answer 1

As huaiyuan mentioned, you ned to modify your syntax table. That said, what he posted modifies the current table...

Each language has a syntax table, e.g. For c++, you could do

(modify-syntax-entry ?_ "w" c++-mode-syntax-table)

I don't know what "your c# mode" uses, but a quick M-x apropos with a look up of syntax-table should help you find out...