According to your example, I suggest that you will use these lines to help reset some numbers. In this case, it would be easier to use the method String.PadLeft()to complete your addition. You can also use a similar function for python, rjust().
eg.
var str = "5";
var padded = str.PadLeft(8, '0');
, , String.Concat() Enumerable.Repeat(). .
.
var chr = '0';
var repeatedChr = new String(chr, 8);
var str = "ha";
var repeated = String.Concat(Enumerable.Repeat(str, 5));