CODEWITHSUNDEEP
c#.netunsafefixed-statement
martavoi
martavoi

Reputation: 7092

Unsafe string pointer statement

As I understood, according to MSDN C# fixed statement should work like:

fixed (char* p = str) ... // equivalent to p = &str[0]

so, why I can`t do this?

    const string str = "1234";
    fixed (char* c = &str[0])
    {
/// .....
    }

How can I get pointer to str[1], for an example?

Upvotes: 9

Views: 7810

Answers (4)

Moha Dehghan
Moha Dehghan

Reputation: 18463

This is because the [] operator on the string is a method that returns a value. Return value from a method when it is a primitive value type does not have an address.

[] operator in C# is not the same as [] in C. In C, arrays and character strings are just pointers, and applying [] operator on a pointer, is equivalent to moving the pointer and dereferencing it. This does not hold in C#.

Actually there was an error in the MSDN documentation you linked, that is fixed in the latest version.

See here for more explanation on this exact matter.

Upvotes: 4

CodesInChaos
CodesInChaos

Reputation: 108810

Since obtaining a pointer to a later element directly works with arrays, but not with strings it seems like the only reason is that MS didn't implement it. It would have been easily possible to design it like that, following the semantics of arrays.

But you can easily compute another pointer that points at other array elements. So it's not a big issue in practice:

fixed (char* p = str)
{
   char* p1 = p+1;
}

Upvotes: 7

Piotr Stapp
Piotr Stapp

Reputation: 19830

Change your code to this:

char[] strC = "1234".ToArray();
fixed (char* c = &strC[0])
{
    /// .....
}

Upvotes: -1

Jirka Hanika
Jirka Hanika

Reputation: 13529

The point of the fixed statement is to prevent a memory allocation from moving across virtual memory. The memory allocation is identified by its beginning, not by any other address.

So to keep &str[1] fixed, you really need the whole str fixed; you can then use pointer arithmetic to derive other pointers within the single allocation from the fixed pointer to str as you wish.

Upvotes: 1

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