Net5 should support something like Windows.Foundation.Metadata.GCPressure

[Scottj1s]

This attribute, when attached to lightweight RCWs, provides the GC with a t-shirt size description of native payload backing the RCW. This prevents pathological heap freeing of heavy objects. With the removal of WinRT from the runtime and adoption of C#/WinRT, the WinUI effort still requires this capability. The name of the attribute and supporting types can certainly be changed.

[AaronRobinsonMSFT]

/cc @jkoritzinsky @elinor-fung

[AaronRobinsonMSFT]

We can build this in or alternatively CsWinRT can implement this manually using GC.AddMemoryPressure().

Internally that is how the built-in WinRT support worked. Below are the relevant values to match the built-in semantics.

Pressure options:

runtime/src/coreclr/src/vm/runtimecallablewrapper.h

Lines 396 to 411 in 9d9008e

	//-----------------------------------------------------
	// The amount of GC pressure we apply has one of a few possible values.
	// We save space in the RCW structure by tracking this instead of the
	// actual value.
	enum GCPressureSize
	{
	GCPressureSize_None = 0,
	GCPressureSize_ProcessLocal = 1,
	GCPressureSize_MachineLocal = 2,
	GCPressureSize_Remote = 3,
	GCPressureSize_WinRT_Base = 4,
	GCPressureSize_WinRT_Low = 5,
	GCPressureSize_WinRT_Medium = 6,
	GCPressureSize_WinRT_High = 7,
	GCPressureSize_COUNT = 8
	};

Values:

runtime/src/coreclr/src/vm/runtimecallablewrapper.h

Lines 81 to 91 in 9d9008e

	#ifdef HOST_64BIT
	#define GC_PRESSURE_WINRT_BASE 1000
	#define GC_PRESSURE_WINRT_LOW 12000
	#define GC_PRESSURE_WINRT_MEDIUM 120000
	#define GC_PRESSURE_WINRT_HIGH 1200000
	#else // HOST_64BIT
	#define GC_PRESSURE_WINRT_BASE 750
	#define GC_PRESSURE_WINRT_LOW 8000
	#define GC_PRESSURE_WINRT_MEDIUM 80000
	#define GC_PRESSURE_WINRT_HIGH 800000
	#endif // HOST_64BIT

Usage:

runtime/src/coreclr/src/vm/runtimecallablewrapper.cpp

Lines 2050 to 2060 in 9d9008e

	const int RCW::s_rGCPressureTable[GCPressureSize_COUNT] =
	{
	0, // GCPressureSize_None
	GC_PRESSURE_PROCESS_LOCAL, // GCPressureSize_ProcessLocal
	GC_PRESSURE_MACHINE_LOCAL, // GCPressureSize_MachineLocal
	GC_PRESSURE_REMOTE, // GCPressureSize_Remote
	GC_PRESSURE_WINRT_BASE, // GCPressureSize_WinRT_Base
	GC_PRESSURE_WINRT_LOW, // GCPressureSize_WinRT_Low
	GC_PRESSURE_WINRT_MEDIUM, // GCPressureSize_WinRT_Medium
	GC_PRESSURE_WINRT_HIGH, // GCPressureSize_WinRT_High
	};

runtime/src/coreclr/src/vm/runtimecallablewrapper.cpp

Lines 2424 to 2449 in 9d9008e

	void RCW::AddMemoryPressure(GCPressureSize pressureSize)
	{
	CONTRACTL
	{
	THROWS;
	GC_TRIGGERS;
	MODE_PREEMPTIVE;
	}
	CONTRACTL_END;
	int pressure = s_rGCPressureTable[pressureSize];
	if (pressureSize >= GCPressureSize_WinRT_Base)
	{
	// use the new implementation for WinRT RCWs
	GCInterface::NewAddMemoryPressure(pressure);
	}
	else
	{
	// use the old implementation for classic COM interop
	GCInterface::AddMemoryPressure(pressure);
	}
	// Remember the pressure we set.
	m_Flags.m_GCPressure = pressureSize;
	}

[AaronRobinsonMSFT]

Spoke offline. CsWinRT has all the needed APIs to implement this themselves and has decided to do that. See microsoft/CsWinRT#282.