What's the use of System.String.Copy in .NET?

Question

I'm afraid that this is a very silly question, but I must be missing something.

Why might one want to use String.Copy(string)?

The documentation says the method

Creates a new instance of String with the same value as a specified String.

Since strings are immutable in .NET, I'm not sure what's the benefit of using this method, as I'd think that

 string copy = String.Copy(otherString); 

would for all practical purposes seem to yield the same result as

 string copy = otherString; 

That is, except for whatever internal bookkeeping that's going on, and the fact that copy is not ReferenceEquals to otherString, there are no observable differences - String being an immutable class whose equality is based on value, not identity. (Thanks to @Andrew Hare for pointing out that my original phrasing was not precise enough to indicate that I realized there was a difference between Copying and not, but was concerned about the perceived lack of useful difference.)

Of course when passed a null argument, Copy throws an ArgumentNullException, and the "new instance" might consume more memory. The latter hardly seems like a benefit, and I'm not sure that the null check is a big enough bonus to warrant a whole Copy method.

Thanks.

Answer 1

With String.Copy you are actually allocating new memory and copying the characters from one string to another; you get a completely new instance as opposed to having both variables being the same instance. This may matter if you use the string with unmanaged code which deals with the memory locations directly and can mutate the string.

Answer 2

Here's one piece of the puzzle. It doesn't explain why you would want to do it, but it does help explain a functional difference.

If you pin the string using the fixed keyword, the contents would be mutable. Off the top of my head, I can't think of a situation in which you would want to do this, but it is possible.

string original = "Hello World"; string refCopy = original; string deepCopy = String.Copy(original);  fixed(char* pStr = original) {    *pStr = 'J'; }  Console.WriteLine(original); Console.WriteLine(refCopy); Console.WriteLine(deepCopy); 

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Output:

Jello World Jello World Hello World