Summary

RuntimeParameterInfo.GetCustomAttributesData() throws a NullReferenceException when called on a parameter that has no metadata (e.g., parameters of delegates pointing to DynamicMethods, or runtime-generated methods for arrays/pointers/byrefs).

Root Cause

In src/coreclr/System.Private.CoreLib/src/System/Reflection/RuntimeParameterInfo.cs, the ICustomAttributeProvider region has four methods. Three of them (GetCustomAttributes(bool), GetCustomAttributes(Type, bool), and IsDefined(Type, bool)) all guard against the no-metadata case by checking MdToken.IsNullToken(m_tkParamDef) and returning early. However, GetCustomAttributesData() lacks this guard:

public override object[] GetCustomAttributes(bool inherit)
{
    if (MdToken.IsNullToken(m_tkParamDef))  // ✅ guarded
        return [];
    return CustomAttribute.GetCustomAttributes(this, (typeof(object) as RuntimeType)!);
}

// ... other guarded methods ...

public override IList<CustomAttributeData> GetCustomAttributesData()
{
    return RuntimeCustomAttributeData.GetCustomAttributesInternal(this);  // ❌ NOT guarded
}

When GetCustomAttributesData() is called on a no-metadata parameter, it flows into RuntimeCustomAttributeData.GetCustomAttributesInternal(RuntimeParameterInfo), which calls target.GetRuntimeModule()! — but GetRuntimeModule() can return null for no-metadata parameters. The null module is then dereferenced in GetCustomAttributeRecords() at module.MetadataImport, causing the NullReferenceException.

This is triggered in practice when NullabilityInfoContext.Create(ParameterInfo) is called on parameters obtained from delegates (e.g., via AIFunctionFactory.Create(Delegate, ...)), because NullabilityInfoContext.Create calls parameterInfo.GetCustomAttributesData().

Repro stack trace

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.RuntimeCustomAttributeData.GetCustomAttributesInternal(RuntimeParameterInfo target)
   at System.Reflection.NullabilityInfoContext.Create(ParameterInfo parameterInfo)

Fix

Add the same MdToken.IsNullToken(m_tkParamDef) guard to GetCustomAttributesData() that the other three methods in the same region already have, returning an empty list when there is no metadata:

public override IList<CustomAttributeData> GetCustomAttributesData()
{
    if (MdToken.IsNullToken(m_tkParamDef))
        return Array.Empty<CustomAttributeData>();

    return RuntimeCustomAttributeData.GetCustomAttributesInternal(this);
}

Tests should be added to verify that GetCustomAttributesData() returns an empty list (and does not throw) when called on parameters of methods that lack metadata, such as those obtained from DynamicMethod or runtime-generated array methods. Tests should be added to the existing custom attribute test files in the repository.