string class Replace and Stringbuilder replace

Question

string s = "value_test_this";
string m = s.Replace('e','E');

StringBuilder strBuilder  = new StringBuilder("value_test_this");
strBuilder.Replace('e','E');

since strings are immutable, how does the Replace works in string class,

Answer 1

It creates another string in the memory and then points the m to that new string. The old string also stays in the memory.

And that is exactly why StringBuilder should be used if frequent modifications have to be made to the string.

If you want to know why Strings are immutable in C#, look at this SO discussion

Answer 2

If you do a string.Replace it's simply going to create a new string (since, as you said, it is immutable). In StringBuilder there's no new string being created, the one you have gets modified.