Immutability of strings

Strings are immutable. You just cannot change existing string. Any operation on the string crates a new instance of the string having new value. It means that if you need to replace a single character in a very long string, memory will be allocated for a new value.

string veryLongString = ...
// memory is allocated
string newString = veryLongString.Remove(0,1); // removes first character of the string.

If you need to perform many operations with string value, use StringBuilder class which is designed for efficient strings manipulation:

var sb = new StringBuilder(someInitialString);
foreach(var str in manyManyStrings)
{
    sb.Append(str);
} 
var finalString = sb.ToString();