streamwriter for txt output file

im using streamwriter to output to text file fixed length

i have:

L="{0,-7}{1,-10}"

and the text with spaces

R="12345 1234567 "

when i run

streamwriter.writeline (L,R)

i get error INDEX ZERO MUST BE GREATER....ERROR

but when i run just block is ok

L="{0,-7}"

and the text with spaces

R="12345 "