Reflection-free mode is a highly experimental mode of the NativeAOT compiler and runtime that greatly reduces the functionality of the reflection APIs and brings a couple interesting benefits as a result. The benefits of this mode are:
- Greatly reduced size of self contained deployments - a fully self-contained "Hello world" style app compiles to a 1 MB file (on x64) with no dependencies.
- Reduced working set and better code locality - parts of the program are more tightly packed together.
- Less metadata for people to reverse engineer - apps compiled in reflection-free mode are as hard to reverse engineer as apps written in e.g. C++.
Of course the benefits come with a drawback: not all .NET code can work in such environment. In fact, most of the existing code probably won't. Use this mode with caution. #67193 tracks potential improvements of this mode.
To enable reflection-free mode in a project that is already using NativeAOT, add the following property to a PropertyGroup
in your project file:
<PropertyGroup>
<IlcDisableReflection>true</IlcDisableReflection>
</PropertyGroup>
When reflection is disabled, the AOT compiler stops emitting data structures that are required to make reflection work at runtime and stops enforcing policies that makes the code more reflection friendly.
Think of:
- Names of methods, types, parameters are no longer generated into the executable.
- Additional metadata, like list of method parameter types, is no longer generated.
- The compiler no longer generates standalone method bodies that would be suitable for reflection invoke.
- Mapping tables that map method names to native code are no longer generated.
- The implementation of reflection is no longer compiled.
Reflection-free mode supports a limited set of reflection APIs that keep their expected semantics.
typeof(SomeType)
will return aSystem.Type
that can be compared with results of othertypeof
expressions or results ofObject.GetType()
calls. The patterns commonly used in perf optimizations of generic code (e.g.typeof(T) == typeof(byte)
) will work fine, and so willobj.GetType() == typeof(SomeType)
.- Following APIs on
System.Type
work:TypeHandle
,UnderlyingSystemType
,BaseType
,IsByRefLike
,IsValueType
,GetTypeCode
,GetHashCode
,GetElementType
,GetInterfaces
,HasElementType
,IsArray
,IsByRef
,IsPointer
,IsPrimitive
,IsAssignableFrom
,IsAssignableTo
,IsInstanceOfType
. Activator.CreateInstance<T>()
will work. The compiler statically analyzes and expands this to efficient code at compile time. No reflection is involved at runtime.Assembly.GetExecutingAssembly()
will return aSystem.Reflection.Assembly
that can be compared with other runtimeAssembly
instances. This is mostly to make it possible to use theNativeLibrary.SetDllImportResolver
API.
We might be able to add support for the following APIs without sacrificing too much of the goals of the reflection-free mode:
Enum.Parse
andEnum.ToString
. These methods currently only work with integers (ToString
returns an integer, andParse
can parse integers). Note thatEnum.GetValues
would still not work.- Type names (
Type.Name
,Type.Namespace
,Type.FullName
): We could probably add this back if there's good use cases in an otherwise reflection-free mode. Delegate.DynamicInvoke
can be made to work if there's a good use case too.
- APIs that require dynamic code generation:
Reflection.Emit
,Assembly.Load
and friends - Obvious program introspection APIs: APIs on
Type
andAssembly
not mentioned above,MethodBase
,MethodInfo
,ConstructorInfo
,FieldInfo
,PropertyInfo
,EventInfo
. These APIs will throw at runtime. - APIs building on top of reflection APIs. Too many to enumerate.
Sometimes reflection is used in non-critical paths to retrieve type names. In reflection-free mode, accessing type names throws an exception. To help moving such code to reflection-free mode, we have an AppContext switch to generate fake names and namespaces for types. With this mode enabled, accessing the Name
and Namespace
property will not throw, but won't return the actual name either.
To enable this mode, add following item to an ItemGroup
in your project file:
<ItemGroup>
<RuntimeHostConfigurationOption Include="Switch.System.Reflection.Disabled.DoNotThrowForNames" Value="true" />
</ItemGroup>
To achieve similar result for when querying for Assembly
(will instead give the ExecutingAssembly):
<ItemGroup>
<RuntimeHostConfigurationOption Include="Switch.System.Reflection.Disabled.DoNotThrowForAssembly" Value="true" />
</ItemGroup>
And here for CustomAttributes (will return an empty array):
<ItemGroup>
<RuntimeHostConfigurationOption Include="Switch.System.Reflection.Disabled.DoNotThrowForAttributes" Value="true" />
</ItemGroup>
The reflection-free mode is implemented as a collection of features that can be controlled individually via the AOT compiler command line options for experiments and troubleshooting, including:
--scanreflection
(also exposed asIlcScanReflection
build property): Infer reflection usage by code analysis. This feature is disabled for reflection-free mode.--reflectiondata:none
: Disables generation of reflection data.--feature:System.Collections.Generic.DefaultComparers=false
: DisablesEqualityComparer<T>.Default
andComparer<T>.Default
optimizations that are based on reflection.
The complete set of individual features that the reflection-free mode is composed from currently can be found by looking for IlcDisableReflection
in Microsoft.NETCore.Native.targets.