char* members in value types marshalled as string result in a memory leak

[Saalvage]

The following type

CS_VALUE_TYPE class DLL_API ValueType {
public:
    const char* char_ptr_member;
};

results in a memory leak when the following code is executed:

while (true) {
    var test = new ValueType();
    test.CharPtrMember = "Looooooooooooong string";
}

The set accessor allocates memory which is never freed outside of the set accessor.

At this time I see three options to solve this, listed in presumed increasing level of complexity:

1. Disabling char* to string marshalling for value type members
2. Using IDisposable
3. Wrapping the unmanaged memory in a managed class that cleans up the memory when it gets finalized and having that class be an additional member of the value type.

[tritao]

Can you post the generated code for the string setter? I wonder if we have a 4th option, which is to optimize the string marshaling so it does not allocate at all.

Edit: Or is the issue that native destructor for the value type object is never called? If that's the case, then I think it should be IDisposable like classes are I think?

[Saalvage]

Here's the setter.

set
{
    if (__char_ptr_member_OwnsNativeMemory)
        Marshal.FreeHGlobal(__instance.char_ptr_member);
    __char_ptr_member_OwnsNativeMemory = true;
    if (value == null)
    {
        __instance.char_ptr_member = global::System.IntPtr.Zero;
        return;
    }
    var __bytes0 = global::System.Text.Encoding.UTF8.GetBytes(value);
    var __bytePtr0 = Marshal.AllocHGlobal(__bytes0.Length + 1);
    Marshal.Copy(__bytes0, 0, __bytePtr0, __bytes0.Length);
    Marshal.WriteByte(__bytePtr0 + __bytes0.Length, 0);
    __instance.char_ptr_member = (__IntPtr) __bytePtr0;
}

Seems like IDisposable seems most sensible here!

[Saalvage]

After some more testing, if the string is long enough the same issues presents itself with std::string members, when the assigned string is long enough (longer than the short-string buffer). So the dtor needs to be called as well as the unmanaged memory freed up.

[tritao]

Ok, this helps. I was mistakenly thinking you were assigning to a native std::string for some reason, which I thought would deal itself with keeping ownership of the string data. In this case, I think we need to have IDisposable indeed.

After some more testing, if the string is long enough the same issues presents itself with std::string members, when the assigned string is long enough (longer than the short-string buffer). So the dtor needs to be called as well as the unmanaged memory freed up.

Damn, more bugs! 😱