For a somewhat limited number of bases there is
Convert.ToString(6, 2);
The problem is that the only bases it allows are 2, 8, 10 and 16.
<Edit>
I have just found
Converting numbers to another radix[
^] which may assist you.
</Edit>
<Edit 2>
The c# example on
this[
^] page on MSDN claims to convert to any base in the range 2,36.
</Edit 2>