Question

How I set new value for an string by index value?

I tried:

string a = "abc"; 
a[0] = "A"; 

not works for strings, but yes for chars. Why?

Answer 1

Strings in C# (and other .NET languages which use System.String in the base class library) are immutable. That is, you can’t modify a string character by character that way (or for that matter, can you modify a string ever).

If you want to modify a string based on the index, you have to convert it to an array using System.String.ToCharArray() first. You convert it back to a string using System.String‘s constructor, passing in the modified array.

Your example would have to be changed to look like:

string a = "abc";
char[] array = a.ToCharArray();
array[0] = 'A'; //Note single quotes, not double quotes
a = new string(array);