stringbuilder - Increase the size of a string in c# -

i trying write method replaces blank space in %20 in c#. using stringbuilder build new string. running out of range exception, because size of array runs out. there way increase size of string build using string

static void main(string[] args) {     string add = "mr john smtih";     console.writeline(add);     stringbuilder sb = new stringbuilder(add);      int j = 0;     (int = 0; < add.length; i++)     {         if (add[i].equals(' '))         {             sb[j] = '%';             j++;             sb[j] = '2';             j++;             sb[j] = '0';         }         else         {             sb[j] = add[i];         }         j++;     }     console.writeline(add);     console.writeline(sb);     console.readline(); } 

the string class immutable, once initialize cannot change size of it.

you should use append() method in stringbuilder. code this:

static void main(string[] args) {     string add = "mr john smtih";     console.writeline(add);     stringbuilder sb = new stringbuilder();     (int = 0; < add.length; i++)     {         if (add[i].equals(' '))         {             sb.append("%20");         }         else         {             sb.append(add[i]);         }     }     console.writeline(add);     console.writeline(sb);     console.readline(); }