[BUG] Unity 6000.5 compilation fails because GetInstanceID is an obsolete error #1134

Describe the bug

The latest stable MRTK packages do not compile with Unity 6000.5 because Unity changed UnityEngine.Object.GetInstanceID() from an obsolete warning to an obsolete error.

Confirmed affected packages:

• org.mixedrealitytoolkit.input 3.3.0
• org.mixedrealitytoolkit.spatialmanipulation 3.4.0

The same calls are also present in Input 4.0.0-pre.2 and the current main branch.

This blocks Editor compilation before entering Play Mode or building the project.

To reproduce

1. Open a project with Unity 6000.5.5f1.
2. Install MRTK Input 3.3.0 and Spatial Manipulation 3.4.0.
3. Allow Unity to compile the packages.
4. Observe the CS0619 errors.

Actual behavior

Spatial Manipulation 3.4.0:

Editor/Constraints/ConstraintManagerEditor.cs(291,66): error CS0619: 'Object.GetInstanceID()' is obsolete: 'GetInstanceID is deprecated. Use GetEntityId instead. This will be removed in a future version.'
Editor/Constraints/ConstraintManagerEditor.cs(373,76): error CS0619: 'Object.GetInstanceID()' is obsolete: 'GetInstanceID is deprecated. Use GetEntityId instead. This will be removed in a future version.'

Input 3.3.0:

Editor/Inspectors/InteractionModeManagerEditor.cs(91,25): error CS0619: 'Object.GetInstanceID()' is obsolete: 'GetInstanceID is deprecated. Use GetEntityId instead. This will be removed in a future version.'

Affected patterns include:

constraintManager.GetInstanceID()
selectedConstraintManager.GetInstanceID()

HashSet<int> seenInstanceIDs = new HashSet<int>();
int instanceID = key.objectReferenceValue.GetInstanceID();

Expected behavior

MRTK should compile on Unity 6000.5 using EntityId while retaining compatibility with the packages' older supported Unity versions.

For example, the duplicate mapping logic can use HashSet<EntityId>, GetEntityId(), and EntityId.None on supported Unity versions. Conditional compilation or a compatibility helper may be needed because the packages currently support Unity versions where EntityId is unavailable.

Screenshots

Unity Console reports the CS0619 errors above during package compilation.

Your setup

• Unity Version: 6000.5.5f1
• MRTK Input: 3.3.0
• MRTK Spatial Manipulation: 3.4.0
• Editor OS: macOS
• Regression check: Unity 6000.4.6f1 reports GetInstanceID() as an obsolete warning; Unity 6000.5.5f1 reports it as an obsolete error.

Target platform

Editor compilation issue; platform-independent. Observed in an iOS/OpenXR project.

Additional context

• The latest stable Input 3.3.0 package contains the obsolete call.
• Input 4.0.0-pre.2 and the current main branch also contain the same call.
• Related Graphics Tools issue: [Bug] Unity 6000.5 compilation fails on GetInstanceID and int AssetDatabase APIs microsoft/MixedReality-GraphicsTools-Unity#245