I come from C++ background and was wondering whether in C# exist any magic that would let me to do the following:
char[] buf = new char[128*1024*1024];
// filling the arr
buf = buf.ToString().Replace(oldstring, newstring).ToArray();
Is there a chance to do it fast (not write all things by hand) and efficient (have one copy of buffer)?
Not very clear what is really your char array is in code provided, but... Use String[] ctor overload to construct the string from your char array, and after call replace with desired parameters.
string strbuf = new string(buf) will allocate more memory, it this true?
If you need to stay with the array (and cannot use something else to start with, e.g. as others suggested a StringBuilder), then no. There is no built in, "zero-copy", way to "replace a (sub-)char-array with another (sub-)char-array in a given char-array".
Since strings are immutable in .NET, the most memory efficient way of manipulating them is by using the StringBuilder class, which internally treats them as mutable.
Here's an example:
var buffer = new StringBuilder(128*1024*1024);
// Fill the buffer using the 'StringBuilder.Append' method.
// Examples:
// buffer.Append('E');
// buffer.Append("foo");
// buffer.Append(new char[] { 'A', 'w', 'e', 's', 'o', 'm', 'e' });
// Alternatively you can access the elements of the underlying char array directly
// through 'StringBuilder.Chars' indexer.
// Example:
// buffer.Chars[9] = 'E';
buffer = buffer.Replace(oldstring, newstring);
StringBuilder class treats strings as mutable, so the manipulation itself is memory efficient.