c# - String.Replace not replacing the string during runtime -

the following line in below code doesn't cause effect:

string1.replace(string1.substring(firstchar, lastchar - firstchar), "##"); 

string1 remains unchanged , same index returned when using indexof.

while (firstchar != string1.lastindexof("test")) {      firstchar = string1.indexof("test");     lastchar = string1.indexof(" ");     using (streamwriter writer = new streamwriter("c:\\textfile1.txt"))     {         writer.writeline(string1.substring(firstchar, lastchar - firstchar));         writer.writeline();         writer.dispose();     }     string1.replace(string1.substring(firstchar, lastchar - firstchar), "##");  } 

change

string1.replace(string1.substring(firstchar, lastchar - firstchar), "##"); 

to

string1 = string1.replace(string1.substring(firstchar, lastchar - firstchar), "##"); 

string.replace not alter original string, returns altered string it's return value.