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Fix flaky NullReferenceException in MultiThreadedGetExportsWorkWithImportingConstructor on Mono/ARM #125804

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Fix flaky NullReferenceException in MultiThreadedGetExportsWorkWithImportingConstructor on Mono/ARM #125804

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e9453ef
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Copilot Mar 19, 2026
190244a
Fix NullReferenceException in MultiThreadedGetExportsWorkWithImportin…
Copilot Mar 19, 2026
5d93104
Changes before error encountered
Copilot Mar 20, 2026
b60ceee
Fix IDE0200: use method group in GetActivator for LazyInitializer call
Copilot Mar 20, 2026
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11 changes: 9 additions & 2 deletions ...stem.Composition.TypedParts/src/System/Composition/TypedParts/Discovery/DiscoveredPart.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
using System.Linq.Expressions;
using System.Numerics.Hashing;
using System.Reflection;
using System.Threading;

namespace System.Composition.TypedParts.Discovery
{
Expand All @@ -31,6 +32,7 @@ internal sealed class DiscoveredPart

// Lazily initialised among potentially many exports
private ConstructorInfo _constructor;
private ParameterInfo[] _constructorParameters;
private CompositeActivator _partActivator;

private static readonly IDictionary<string, object> s_noMetadata = new Dictionary<string, object>();
Expand Down Expand Up @@ -112,7 +114,12 @@ private IEnumerable<CompositionDependency> GetPartActivatorDependencies(Dependen
}
}

var cps = _constructor.GetParameters();
// Use LazyInitializer to ensure GetParameters() is called at most once per
// DiscoveredPart instance and the result is safely published to all threads.
// On some runtimes (e.g. Mono on ARM), concurrent calls to GetParameters()
// on the same ConstructorInfo can return different ParameterInfo instances
// due to lazy initialization, causing lookup failures in GetActivator.
var cps = LazyInitializer.EnsureInitialized(ref _constructorParameters, () => _constructor.GetParameters());
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@stephentoub stephentoub Mar 20, 2026

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This is going to allocate a new delegate on every call, regardless of whether _constructorParameters is initialized or not.

Same for the similar change below.


for (var i = 0; i < cps.Length; ++i)
{
Expand Down Expand Up @@ -168,7 +175,7 @@ public CompositeActivator GetActivator(IEnumerable<CompositionDependency> depend
var contextParam = Expression.Parameter(typeof(LifetimeContext), "cc");
var operationParm = Expression.Parameter(typeof(CompositionOperation), "op");

var cps = _constructor.GetParameters();
var cps = LazyInitializer.EnsureInitialized(ref _constructorParameters, _constructor.GetParameters);
Expression[] paramActivatorCalls = new Expression[cps.Length];

var partActivatorDependencies = dependencies
Expand Down
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