how to use cast in vb.net?

Select column1 ,column2 case when column3=''
then column1+ ':' + CAST(column2 as varchar(max))
else column2 end as description
from Tablename


How to change this query to linq?
Half of my linq query as follows..Struck in using cast
dim a= From r In objectname.Tablename Select r.column1, r.column2, r.column3 Where id= "E" (Here how to use cast)(This code in vb.net)