mono-api-gchandle.html

# GC Handles

### Synopsys

GC handles are wrappers that are used to keep references to managed objects in the unmanaged space and preventing the object from being disposed.

These are the C equivalents of the `System.GCHandle` structure.

There are two kinds of GCHandles that can be created:

- Handles to objects (use `mono_gchandle_new`).
- Weak handles to objects (use `mono_gchandle_new_weakref`). Weak handles can have the objects reclaimed by the garbage collector.

To retrieve the target address of an object pointed to by a `GCHandle` you should use `mono_gchandle_get_target`.

For example, consider the following C code:
static MonoObject* o = NULL;

The object in `o' will *NOT* be scanned.

If you need to store an object in a C variable and prevent it from being collected, you need to acquire a GC handle for it.
guint32 handle = mono_gchandle_new (my_object, TRUE);

TRUE means the object will be pinned, so it won't move in memory when we'll use a moving GC. You can access the MonoObject* referenced by a handle with:
MonoObject* obj = mono_gchandle_get_target (handle);

When you don't need the handle anymore you need to call:
mono_gchandle_free (handle);

Note that if you assign a new object to the C var, you need to get a new handle, it's not enough to store a new object in the C var.

So code that looked like this:
static MonoObject* obj = NULL; ... obj = mono_object_new (...); // use obj ... // when done to allow the GC to collect obj obj = NULL;

should now be changed to:
static guint32 o_handle; ... MonoObject *obj = mono_object_new (...); o_handle = mono_gchandle_new (obj, TRUE); // use o or mono_gchandle_get_target (o_handle) ... // when done to allow the GC to collect obj mono_gchandle_free (o_handle);
`mono_gchandle_new`
Syntax
guint32 mono_gchandle_new_internal (MonoObject *obj, gboolean pinned)

Parameters

obj | managed object to get a handle for
pinned | whether the object should be pinned

Return value
a handle that can be used to access the object from unmanaged code.
Description

This returns a handle that wraps the object, this is used to keep a reference to a managed object from the unmanaged world and preventing the object from being disposed.

If pinned is false the address of the object can not be obtained, if it is true the address of the object can be obtained. This will also pin the object so it will not be possible by a moving garbage collector to move the object.

`mono_gchandle_new_weakref`
Syntax
uint32_t mono_gchandle_new_weakref (MonoObject *obj, mono_bool track_resurrection)

Parameters

obj | managed object to get a handle for
track_resurrection | Determines how long to track the object, if this is set to `TRUE`, the object is tracked after finalization, if `FALSE`, the object is only tracked up until the point of finalization.

Return value
a handle that can be used to access the object from unmanaged code.
Description

This returns a weak handle that wraps the object, this is used to keep a reference to a managed object from the unmanaged world. Unlike the `mono_gchandle_new_internal` the object can be reclaimed by the garbage collector. In this case the value of the GCHandle will be set to zero.

If track_resurrection is `TRUE` the object will be tracked through finalization and if the object is resurrected during the execution of the finalizer, then the returned weakref will continue to hold a reference to the object. If track_resurrection is `FALSE`, then the weak reference's target will become `NULL` as soon as the object is passed on to the finalizer.

`mono_gchandle_get_target`
Syntax
MonoObject* mono_gchandle_get_target_internal (guint32 gchandle)

Parameters

gchandle | a GCHandle's handle.

Return value
A pointer to the `MonoObject*` represented by the handle or `NULL` for a collected object if using a weakref handle.
Description

The handle was previously created by calling `mono_gchandle_new_internal` or `mono_gchandle_new_weakref`.