[API Proposal]: Provide API for freeing Variant memory

[lowleveldesign]

Background and motivation

It is currently quite hard to reclaim memory allocated internally by the Marshal.GetNativeVariantForObject method. Let's take the following code as an example:

var variantPtr = Marshal.AllocHGlobal(VariantTypeSize); // VariantTypeSize = 24 (x64) or 16 (x86)

Marshal.GetNativeVariantForObject("test string", variantPtr);

// pass the variantPtr to native code and do the work

Marshal.FreeHGlobal(variantPtr);

We are leaking memory because GetNativeVariantForObject assigns the string to the AsBstr property of the internal Variant type. The AsBstr property internally allocates memory using Marshal.StringToBSTR. One solution could be to provide the Marshal.FreeNativeVariant method, which would reclaim the internal memory.

API Proposal

namespace System.Runtime.InteropServices;

public static class Marshal
{
    public static void FreeNativeVariant(nint addr);
}

API Usage

var variantPtr = Marshal.AllocHGlobal(VariantTypeSize); // VariantTypeSize = 24 (x64) or 16 (x86)

Marshal.GetNativeVariantForObject("test string", variantPtr);

// pass the variantPtr to native code and do the work

Marshal.FreeNativeVariant(variantPtr);
Marshal.FreeHGlobal(variantPtr);

Alternative Designs

Another approach that comes to my mind is to modify the BStrWrapper class so it will be responsible for allocating and freeing memory for the wrapped BSTR string. Then we would need to instruct the users to use the wrapper when calling GetNativeVariantForObject as using raw string leads to a leak. GetNativeVariantForObject already accepts BStrWrapper, so we won't need to modify the public API.

Risks

No response

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Issue Details

---

Background and motivation

It is currently quite hard to reclaim memory allocated internally by the Marshal.GetNativeVariantForObject method. Let's take the following code as an example:

var variantPtr = Marshal.AllocHGlobal(VariantTypeSize); // VariantTypeSize = 24 (x64) or 16 (x86)

Marshal.GetNativeVariantForObject("test string", variantPtr);

// pass the variantPtr to native code and do the work

Marshal.FreeHGlobal(variantPtr);

We are leaking memory because GetNativeVariantForObject assigns the string to the AsBstr property of the internal Variant type. The AsBstr property internally allocates memory using Marshal.StringToBSTR. One solution could be to provide the Marshal.FreeNativeVariant method, which would reclaim the internal memory.

API Proposal

namespace System.Runtime.InteropServices;

public static class Marshal
{
    public static void FreeNativeVariant(nint addr);
}

API Usage

var variantPtr = Marshal.AllocHGlobal(VariantTypeSize); // VariantTypeSize = 24 (x64) or 16 (x86)

Marshal.GetNativeVariantForObject("test string", variantPtr);

// pass the variantPtr to native code and do the work

Marshal.FreeNativeVariant(variantPtr);
Marshal.FreeHGlobal(variantPtr);

Alternative Designs

Another approach that comes to my mind is to modify the BStrWrapper class so it will be responsible for allocating and freeing memory for the wrapped BSTR string. Then we would need to instruct the users to use the wrapper when calling GetNativeVariantForObject as using raw string leads to a leak. GetNativeVariantForObject already accepts BStrWrapper, so we won't need to modify the public API.

Risks

No response

Author:	lowleveldesign
Assignees:	-
Labels:	api-suggestion, area-System.Runtime.InteropServices, untriaged
Milestone:	-

[AaronRobinsonMSFT]

This is handled in Win32 via VariantClear(). When marshaling through the IL Stubs this is handled by the VM itself. It does seem like a missing low-level API. I'm not sure it is worth adding given how long it has been missing, but we can see how the community responds.

[huoyaoyuan]

System.Runtime.InteropServices.Marshalling.ComVariant is the "better" type to work with VARIANT interop now. Its Dispose method will correctly dispose referenced memory. The new work flow should be:

using var variant = ComVariant.Create("test string"); // The variant struct is allocated on stack, no need for manual AllocHGlobal/FreeHGlobal
{
    DoPInvoke(&variant); // Take the address of variant struct directly
}
// The using scope disposes referenced memory

To work with a pointer from old flow, you can cast a pointer from it:

((ComVariant*)variantPtr)->Dispose();