Handle DynamicMethod reflection limitations in AIFunctionFactory by Copilot · Pull Request #7287 · dotnet/extensions

Copilot · 2026-02-11T20:34:49Z

Original prompt

This section details on the original issue you should resolve

<issue_title>[Regression] NullReferenceException in AIFunctionFactory.Create with DynamicMethod parameters in 10.3.0</issue_title>
<issue_description>### Description

Microsoft.Extensions.AI version 10.3.0 throws NullReferenceException when calling AIFunctionFactory.Create() with delegates created via DynamicMethod.DefineParameter(). This is a regression - the same code works perfectly in version 10.2.0.

The crash occurs in NullabilityInfoContext.Create(ParameterInfo) when it tries to read RuntimeCustomAttributeData from parameters created with DynamicMethod, which lack complete reflection metadata.

This completely blocks legitimate use cases like:

MCP (Model Context Protocol) server integration
Dynamic tool generation from external schemas (OpenAPI, etc.)
Runtime code generation for AI function calling

DynamicMethod is the only way to create delegates with proper parameter names and types without closure capture (Expression Trees add hidden Closure parameter that breaks AIFunctionFactory validation).

Reproduction Steps

Install Microsoft.Extensions.AI 10.3.0
Create a DynamicMethod with parameters:

using System.Reflection;
using System.Reflection.Emit;
using Microsoft.Extensions.AI;

DynamicMethod dynamicMethod = new DynamicMethod(
"TestMethod",
typeof(Task),
new[] { typeof(string), typeof(int) },
typeof(Program).Module);

dynamicMethod.DefineParameter(1, ParameterAttributes.None, "message");
dynamicMethod.DefineParameter(2, ParameterAttributes.None, "count");

ILGenerator il = dynamicMethod.GetILGenerator();
il.Emit(OpCodes.Ldnull);
il.Emit(OpCodes.Call, typeof(Task).GetMethod(nameof(Task.FromResult)).MakeGenericMethod(typeof(object)));
il.Emit(OpCodes.Ret);

Delegate testDelegate = dynamicMethod.CreateDelegate(typeof(Func<string, int, Task>));

Call AIFunctionFactory.Create:

AIFunction function = AIFunctionFactory.Create(testDelegate, new AIFunctionFactoryOptions
{
Name = "TestTool",
Description = "Test tool"
});

Observe NullReferenceException

Expected behavior

AIFunctionFactory.Create() should successfully create an AIFunction from the DynamicMethod delegate, just as it did in version 10.2.0.

The function should have proper parameter names ("message", "count") derived from DefineParameter() calls, enabling:

Correct JSON schema generation
Proper LLM tool calling
Meaningful debugging and logging

Actual behavior

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Reflection.RuntimeCustomAttributeData.GetCustomAttributeRecords(RuntimeModule module, Int32 targetToken)
at System.Reflection.RuntimeCustomAttributeData.GetCustomAttributes(RuntimeModule module, Int32 tkTarget)
at System.Reflection.NullabilityInfoContext.Create(ParameterInfo parameterInfo)
at Microsoft.Extensions.AI.AIJsonUtilities.<>c__DisplayClass42_0.g__TransformSchemaNode|0(JsonSchemaExporterContext schemaExporterContext, JsonNode schema)
at System.Text.Json.Schema.JsonSchema.g__CompleteSchema|100_0(JsonNode schema, <>c__DisplayClass100_0&)
at System.Text.Json.Schema.JsonSchema.ToJsonNode(JsonSchemaExporterOptions options)
at System.Text.Json.Schema.JsonSchemaExporter.GetJsonSchemaAsNode(JsonTypeInfo typeInfo, JsonSchemaExporterOptions exporterOptions)
at Microsoft.Extensions.AI.AIJsonUtilities.CreateJsonSchemaCore(...)
at Microsoft.Extensions.AI.AIJsonUtilities.CreateFunctionJsonSchema(...)
at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunctionDescriptor..ctor(...)
at Microsoft.Extensions.AI.AIFunctionFactory.Create(Delegate method, AIFunctionFactoryOptions options)

The crash occurs because DynamicMethod.DefineParameter() creates ParameterInfo with incomplete RuntimeCustomAttributeData, which NullabilityInfoContext cannot handle.

Regression?

☑ Yes

Version: Microsoft.Extensions.AI 10.3.0
Previous working version: Microsoft.Extensions.AI 10.2.0

This is confirmed regression. The exact same code works in 10.2.0 but crashes in 10.3.0.

Known Workarounds

WORKAROUND (Current): Downgrade to Microsoft.Extensions.AI 10.2.0
- Status: ✅ Works
- Impact: Cannot use newer features/fixes from 10.3.0
ATTEMPTED (Failed): Use Expression Trees instead of DynamicMethod
- Status: ❌ Doesn't work
- Reason: Expression Trees capture closure context, producing delegates with hidden Closure parameter:
  Func<Closure, string, int, Task> // Has extra parameter
- This fails AIFunctionFactory validation because parameter count doesn't match expected signature
- ATTEMPTED (Failed): Remove DefineParameter() calls
  - Status: ⚠️ Technically works but unacceptable
  - Reason: Parameters get generic names like "arg0", "arg1" which:
    - Breaks LLM tool calling (models need semantic names)
    - Breaks JSON schema generation
    - Makes debuggi...

…thod parameters Wrap NullabilityInfoContext.Create(ParameterInfo) call in a try/catch to handle parameters that lack complete reflection metadata, such as those created via DynamicMethod.DefineParameter(). This is a workaround for a runtime issue tracked at dotnet/runtime#124293. Co-authored-by: stephentoub <[email protected]>

…n comment and test invocation Co-authored-by: stephentoub <[email protected]>

src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs

test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs

Copilot

Pull request overview

Fixes a regression where AIFunctionFactory.Create() can crash when generating JSON schema for delegates created from DynamicMethod.DefineParameter() due to a runtime NullabilityInfoContext bug, and adds a regression test to cover the scenario.

Changes:

Add a helper to safely retrieve parameter nullability write-state, swallowing runtime exceptions and treating nullability as unknown.
Add a regression test that builds a DynamicMethod with named parameters, validates schema generation, and verifies invocation succeeds.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs	Routes nullable-reference-type parameter handling through a new helper that guards against runtime exceptions from incomplete reflection metadata.
test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs	Adds a regression test for `DynamicMethod.DefineParameter()` parameters to ensure schema generation and invocation don’t throw.

src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs

stephentoub · 2026-02-11T21:33:52Z

@copilot

…re helper, improve test - Remove ArgumentException and InvalidOperationException from catch filter - Restructure to if-not-null with early return pattern per review - Add S1696 pragma suppression for intentional NullReferenceException catch - Rewrite test to use DynamicMethod that doubles an int and validates result Co-authored-by: stephentoub <[email protected]>

test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs

Co-authored-by: stephentoub <[email protected]>

src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs

stephentoub · 2026-02-12T22:27:24Z

@copilot, new CI failure on .NET Framework:

System.NullReferenceException : Object reference not set to an instance of an object.



Stack trace
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunctionDescriptor.GetReturnParameterDescription(MethodInfo method) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 1102
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunctionDescriptor..ctor(DescriptorKey key, JsonSerializerOptions serializerOptions) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 705
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunctionDescriptor.GetOrCreate(MethodInfo method, AIFunctionFactoryOptions options) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 636
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunction.Build(MethodInfo method, Object target, AIFunctionFactoryOptions options) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 498
   at Microsoft.Extensions.AI.AIFunctionFactory.Create(MethodInfo method, Object target, String name, String description, JsonSerializerOptions serializerOptions) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 356
   at Microsoft.Extensions.AI.AIFunctionFactoryTest.<AIFunctionFactory_DynamicMethod>d__70.MoveNext() in D:\a\_work\1\s\test\Libraries\Microsoft.Extensions.AI.Tests\Functions\AIFunctionFactoryTest.cs:line 1440
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

…mber Co-authored-by: stephentoub <[email protected]>

Copilot · 2026-02-12T22:32:08Z

@copilot, new CI failure on .NET Framework:
System.NullReferenceException : Object reference not set to an instance of an object.

...

On .NET Framework, accessing ReturnParameter.Member itself throws NullReferenceException (the internal MemberImpl field is null). Replaced the .Member is null check with method is DynamicMethod, which is a reliable proactive check that avoids touching the broken ReturnParameter entirely. Fixed in ae2dd3a.

…thod

stephentoub · 2026-02-13T05:25:40Z

@copilot:

Build FAILED.

/mnt/vss/_work/1/s/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs(15,1): warning S1128: Remove this unnecessary 'using'. (https://rules.sonarsource.com/csharp/RSPEC-1128) [/mnt/vss/_work/1/s/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.csproj::TargetFramework=netstandard2.0]
/mnt/vss/_work/1/s/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs(1100,27): error CS0246: The type or namespace name 'DynamicMethod' could not be found (are you missing a using directive or an assembly reference?) [/mnt/vss/_work/1/s/src/Libraries/Microsoft.Extensions.AI.Abstractions/Microsoft.Extensions.AI.Abstractions.csproj::TargetFramework=netstandard2.0]
    1 Warning(s)
    1 Error(s)

…compatibility Co-authored-by: stephentoub <[email protected]>

Copilot · 2026-02-13T05:30:28Z

@copilot:
Build FAILED.

...

Removed the using System.Reflection.Emit and switched to method.GetType().Name == "DynamicMethod" since DynamicMethod isn't available in netstandard2.0. Fixed in b1d8c73.

stephentoub · 2026-02-13T05:32:13Z

@copilot, I don't want to use a name comparison.

If there's really no other way, we can fall back to the try/catch you'd previously proposed.

Co-authored-by: stephentoub <[email protected]>

Copilot · 2026-02-13T05:37:24Z

@copilot, I don't want to use a name comparison.

If there's really no other way, we can fall back to the try/catch you'd previously proposed.

Replaced the name comparison with a try/catch that catches both ArgumentNullException (thrown on .NET Core) and NullReferenceException (thrown on .NET Framework) from ReturnParameter.GetCustomAttribute. Fixed in c00bbaf.

…thod

stephentoub · 2026-02-13T15:35:38Z

@copilot the netfx tests are still failing...

Microsoft.Extensions.AI.AIFunctionFactoryTest.AIFunctionFactory_DynamicMethod

Error message
System.ArgumentException : MethodInfo must be a runtime MethodInfo object.
Parameter name: this



Stack trace
   at System.Reflection.Emit.DynamicMethod.RTDynamicMethod.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionInvoke(MethodInfo method, Object target, Object[] arguments) in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 1227
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunction.<InvokeCoreAsync>d__28.MoveNext() in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 589
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.Extensions.AI.AIFunctionFactory.ReflectionAIFunction.<InvokeCoreAsync>d__28.MoveNext() in /_/src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs:line 602
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.Extensions.AI.AIFunctionFactoryTest.<AIFunctionFactory_DynamicMethod>d__70.MoveNext() in D:\a\_work\1\s\test\Libraries\Microsoft.Extensions.AI.Tests\Functions\AIFunctionFactoryTest.cs:line 1450
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

….Invoke is unsupported Co-authored-by: stephentoub <[email protected]>

Copilot · 2026-02-13T15:42:20Z

@copilot the netfx tests are still failing...
Microsoft.Extensions.AI.AIFunctionFactoryTest.AIFunctionFactory_DynamicMethod

...

On .NET Framework, RTDynamicMethod.Invoke doesn't support invocation — this is a fundamental runtime limitation (not related to our changes). Wrapped the invocation assertions in #if NET so schema generation and creation are still tested on .NET Framework, but invocation is only tested on .NET Core. Fixed in a45394b.

…thod

Initial plan

c772354

Copilot AI assigned Copilot and stephentoub Feb 11, 2026

Copilot started work on behalf of stephentoub February 11, 2026 20:35 View session

Copilot AI and others added 2 commits February 11, 2026 20:45

Address code review feedback: add NullReferenceException justificatio…

d33d09b

…n comment and test invocation Co-authored-by: stephentoub <[email protected]>

Copilot AI changed the title ~~[WIP] Fix NullReferenceException in AIFunctionFactory.Create with DynamicMethod~~ Handle NullReferenceException from NullabilityInfoContext for DynamicMethod parameters Feb 11, 2026

Copilot AI requested a review from stephentoub February 11, 2026 20:53

Copilot finished work on behalf of stephentoub February 11, 2026 20:53

stephentoub approved these changes Feb 11, 2026

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stephentoub marked this pull request as ready for review February 11, 2026 21:02

stephentoub requested a review from a team as a code owner February 11, 2026 21:02

stephentoub requested review from Copilot and eiriktsarpalis February 11, 2026 21:02

Copilot started reviewing on behalf of stephentoub February 11, 2026 21:03 View session

Copilot AI reviewed Feb 11, 2026

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src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs Show resolved Hide resolved

Copilot started work on behalf of stephentoub February 11, 2026 21:34 View session

Copilot AI requested a review from stephentoub February 11, 2026 21:40

Copilot finished work on behalf of stephentoub February 11, 2026 21:40

stephentoub reviewed Feb 11, 2026

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stephentoub approved these changes Feb 11, 2026

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Copilot started work on behalf of stephentoub February 11, 2026 21:54 View session

Use Task<int> instead of Task<object> in DynamicMethod test

d2cd15f

Co-authored-by: stephentoub <[email protected]>

Copilot finished work on behalf of stephentoub February 11, 2026 21:58

Copilot AI requested a review from stephentoub February 11, 2026 21:58

eiriktsarpalis reviewed Feb 12, 2026

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src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs Outdated Show resolved Hide resolved

Copilot started work on behalf of eiriktsarpalis February 12, 2026 12:05 View session

eiriktsarpalis reviewed Feb 12, 2026

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src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.Create.cs Show resolved Hide resolved

Copilot finished work on behalf of stephentoub February 12, 2026 21:41

Copilot started work on behalf of stephentoub February 12, 2026 22:27 View session

Check method is DynamicMethod instead of accessing ReturnParameter.Me…

ae2dd3a

…mber Co-authored-by: stephentoub <[email protected]>

Copilot AI changed the title ~~Handle NullReferenceException from NullabilityInfoContext for DynamicMethod parameters~~ Handle DynamicMethod reflection limitations in AIFunctionFactory Feb 12, 2026

Copilot finished work on behalf of stephentoub February 12, 2026 22:32

Merge branch 'main' into copilot/fix-nullreferenceexception-dynamicme…

81011d3

…thod

Copilot started work on behalf of stephentoub February 13, 2026 05:26 View session

Use type name check instead of DynamicMethod type for netstandard2.0 …

b1d8c73

…compatibility Co-authored-by: stephentoub <[email protected]>

Copilot finished work on behalf of stephentoub February 13, 2026 05:31

Copilot started work on behalf of stephentoub February 13, 2026 05:32 View session

Replace name comparison with try/catch for DynamicMethod ReturnParameter

c00bbaf

Co-authored-by: stephentoub <[email protected]>

Copilot finished work on behalf of stephentoub February 13, 2026 05:38

Merge branch 'main' into copilot/fix-nullreferenceexception-dynamicme…

9b063e1

…thod

stephentoub enabled auto-merge (squash) February 13, 2026 14:08

Copilot started work on behalf of stephentoub February 13, 2026 15:36 View session

Skip DynamicMethod invocation test on .NET Framework where MethodInfo…

a45394b

….Invoke is unsupported Co-authored-by: stephentoub <[email protected]>

auto-merge was automatically disabled February 13, 2026 15:42
Head branch was pushed to by a user without write access

Copilot finished work on behalf of stephentoub February 13, 2026 15:43

stephentoub enabled auto-merge (squash) February 13, 2026 23:07

Merge branch 'main' into copilot/fix-nullreferenceexception-dynamicme…

bd608c6

…thod

stephentoub merged commit e644e1d into main Feb 14, 2026
6 checks passed

stephentoub deleted the copilot/fix-nullreferenceexception-dynamicmethod branch February 14, 2026 13:31

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Reproduction Steps

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Copilot AI commented Feb 11, 2026 •

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