[API Proposal]: Add API for getting memory for Objective-C object without creating GCHandle

Background and motivation

The current ObjectiveCMarshal.CreateReferenceTrackingHandle API does two things:

Creates a tracking GCHandle
Returns tagged memory for the object in question.

I'm trying to improve memory usage and simplify code in dotnet/macios, and as part of that I want to store some flags in the tagged memory. These flags must exist before we have a tracking GCHandle (and also after any such handles have been freed), so I think it would make sense to have a separate API to just get the tagged memory, without creating a tracking GCHandle.

API Proposal

namespace System.Runtime.InteropServices.ObjectiveC;

public static class ObjectiveCMarshal
{
    /// <summary>
    /// Request a pointer to memory tagged for the supplied object.
    /// </summary>
    /// <param name="obj">The object whose tagged memory to return.</param>
    /// <returns>A pointer to memory tagged to the object.</returns>
    /// <exception cref="InvalidOperationException">Thrown if the ObjectiveCMarshal API has not been initialized.</exception>
    /// <remarks>
    /// The Initialize() must be called prior to calling this function.
    ///
    /// The <paramref name="obj"/> must have a type in its hierarchy marked with
    /// <see cref="ObjectiveCTrackedTypeAttribute"/>.
    ///
    /// The "Is Referenced" callback passed to <see cref="Initialize" />
    /// will be passed the memory returned from this function.
    /// The memory it points at is defined by the length in the <see cref="Span{IntPtr}"/> and
    /// will be zeroed out. It will be available until <paramref name="obj"/> is collected by the GC.
    /// The returned memory can be used for any purpose by the caller of this function and usable
    /// during the "Is Referenced" callback.
    ///
    /// Calling this function multiple times with the same <paramref name="obj"/> will
    /// return the same tagged memory. It is only guaranteed to be zero initialized on
    /// the first call.
    ///
    /// The return value is the same as the tagged memory returned from <see cref="CreateReferenceTrackingHandle" />.
    /// </remarks>
    public static Span<IntPtr> GetOrCreateTaggedMemory(object obj);
}

API Usage

This would be used somewhat like this:

var memory = ObjectiveCMarshal.GetOrCreateTaggedMemory (myObject);
fixed (void *ptr = memory)
    ((NSObjectData *) ptr)->flags = MyFlag;

Alternative Designs

Calling ObjectiveCMarshal.CreateReferenceTrackingHandle and just immediately freeing the GCHandle works fine:

public static Span<IntPtr> GetOrCreateTaggedMemory (object obj)
{
    var handle = ObjectiveCMarshal.CreateReferenceTrackingHandle (obj, out var span);
    handle.Free ();
    return span;
}

but this feels rather wasteful.

Risks

None that I can think of.

@AaronRobinsonMSFT

CC @AaronRobinsonMSFT

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Oops, sorry. Please ignore #128476 (comment). I was looking at the JavaMarshal API not the ObjectiveCMarshal API surface area. I don't see a GetContext() on the ObjectiveCMarshal class. This does seem annoying. Let me review this again.

@rolfbjarne

@rolfbjarne Okay, this API makes some more sense now that I'm looking at the Objective-C APIs.

My only suggestion would be to update the name to be public Span<IntPtr> GetOrCreateTaggedMemory(object obj)

The implementation would be simple and I don't see any real issues with adding this behavior since it would just be part of the "prestep" for the existing API. No new semantics appear to be introduced in this case. I agree with your documnetation as well keeping all the same invariants for this API as for CreateReferenceTrackingHandle.

Apologies for the initial confusion.

Please update the API with the suggested name and then feel free to mark it with "api-ready-for-review".

@rolfbjarne

@rolfbjarne Airport time is "free" time, objcmarshal-get-or-create-tagged-memory.

Renamed to GetOrCreateReferenceTrackingMemory due to "TaggedMemory" being new nomenclature that didn't seem necessary.

namespace System.Runtime.InteropServices.ObjectiveC;

public static class ObjectiveCMarshal
{
    /// <summary>
    /// Request a pointer to referenced tracking memory for the supplied object.
    /// </summary>
    /// <param name="obj">The object whose tracking memory to return.</param>
    /// <returns>A pointer to tracking memory on the object.</returns>
    /// <exception cref="InvalidOperationException">Thrown if the ObjectiveCMarshal API has not been initialized.</exception>
    /// <remarks>
    /// The Initialize() must be called prior to calling this function.
    ///
    /// The <paramref name="obj"/> must have a type in its hierarchy marked with
    /// <see cref="ObjectiveCTrackedTypeAttribute"/>.
    ///
    /// The "Is Referenced" callback passed to <see cref="Initialize" />
    /// will be passed the memory returned from this function.
    /// The memory it points at is defined by the length in the <see cref="Span{IntPtr}"/> and
    /// will be zeroed out. It will be available until <paramref name="obj"/> is collected by the GC.
    /// The returned memory can be used for any purpose by the caller of this function and usable
    /// during the "Is Referenced" callback.
    ///
    /// Calling this function multiple times with the same <paramref name="obj"/> will
    /// return the same tracking memory. It is only guaranteed to be zero initialized on
    /// the first call of this or <see cref="CreateReferenceTrackingHandle" />.
    ///
    /// The return value is the same as the tracking memory returned from <see cref="CreateReferenceTrackingHandle" />.
    /// </remarks>
    public static Span<IntPtr> GetOrCreateReferenceTrackingMemory(object obj);
}

Add `ObjectiveCMarshal.GetOrCreateReferenceTrackingMemory` API (#128508)

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[API Proposal]: Add API for getting memory for Objective-C object without creating `GCHandle` #128476

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[API Proposal]: Add API for getting memory for Objective-C object without creating GCHandle #128476

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rolfbjarne commented on May 22, 2026

dotnet-policy-service commented on May 22, 2026

AaronRobinsonMSFT commented on May 22, 2026

AaronRobinsonMSFT commented on May 22, 2026

AaronRobinsonMSFT commented on May 22, 2026

AaronRobinsonMSFT commented on May 22, 2026

AaronRobinsonMSFT commented on May 22, 2026

AaronRobinsonMSFT commented on Jun 1, 2026

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[API Proposal]: Add API for getting memory for Objective-C object without creating `GCHandle` #128476