Why is new String(“Hello”) invalid in C#?

Question

What is the logic/reason behind making

String s= new String(\"Hello World\");

Illegal in C#? The error is

The best over

Answer 1

A string is immutable until you start messing with unsafe code, so the language designers chose not to add a feature that isn't needed in normal usage. That is not to say it wouldn't be handy in certain situations.

If you do this:

string a = "foobar";
string b = a;
Mutate(a, "raboof");
Console.WriteLine("b={0}", b);

where:

unsafe void Mutate(string s, string newContents)
{
    System.Diagnostics.Debug.Assert(newContents.Length == s.Length);
    fixed (char* ps = s)
    {
        for (int i = 0; i < newContents.Length; ++i)
        {
            ps[i] = newContents[i];
        }
    }
}

You may be surprised to know that even though string 'a' is the one that was mutated the output will be:

b=raboof

In this situation one would like to write:

string a = "foobar";
string b = new String(a);
Mutate(a, "raboof");
Console.WriteLine("b={0}", b);

And expect to see output like:

b=foobar

But you can't, because it is not part of the implementation of System.String.

And I guess a reasonable justification for that design decision is that anyone comfortable writing something like the unsafe Mutate method is also capable of implementing a method to clone a string.