The IL2CPPA Unity-developed scripting back-end which you can use as an alternative to Mono when building projects for some platforms. More info
See in Glossary back end has some notable limitations, due partly to the inherent characteristics of ahead-of-time (AOT) compilation and partly to platform-specific restrictions.
Threads aren’t supported on the Web platform, so any managed code that uses the System.Threading namespace fails at runtime. Some parts of the .NET class libraries also implicitly depend on threads. A common example is the System.Timers.Timer class, which depends on support for threads.
Unity supports reflection on AOT platforms. However, if the compiler can’t infer that the code is used via reflection, the code might not exist at runtime. For more information, refer to Managed code stripping.
An AOT platform can’t implement any of the methods in the System.Reflection.Emit namespace.
IL2CPP supports exception filters. However, the execution order of filter statements and catch blocks is different because IL2CPP uses C++ exceptions to implement managed exceptions. This isn’t noticeable unless a filter block writes to a field.
On Mono the field write occurs at filter evaluation time, before entering the catch block, and in a defined order relative to other filters. On IL2CPP the field write might occur at a different time because the filter is simulated. This can lead to observable differences, such as:
catch or finally blocks.catch or finally sees stale or differently ordered values.Best practice is to avoid side effects in filters by moving any state changes into the catch block (or a helper function called from catch) to ensure consistent behavior across IL2CPP and Mono.
AOT platforms might encounter issues with serialization and deserialization because of the use of reflection. If a type or method is only used via reflection as part of serialization or deserialization, the AOT compiler can’t detect that it needs to generate the code required for the type or method.
For generic types and methods, the compiler must determine which generic instances are used because different generic instances might require different code. For example, the code for List<int> is different than the code for List<double>. However, IL2CPP shares code for usages of reference types, so the same code is used for List<object> and List<string>.
It’s possible to reference generic types and methods that IL2CPP did not find at compile time in the following cases:
Activator.CreateInstance(typeof(SomeGenericType<>).MakeGenericType(someType));
typeof(SomeGenericType<>).MakeGenericType(someType).GetMethod("AMethod").Invoke(null, null);
typeof(SomeType).GetMethod("GenericMethod").MakeGenericMethod(someType).Invoke(null, null);
Struct<Struct<Struct<...<Struct<int>>>>.To support those cases, IL2CPP generates generic code that works with any type parameter. However, this code is slower because it can’t determine a type’s size, or whether it’s a reference or value type. If you need to ensure that faster generic methods are generated, do the following:
where: class constraint. IL2CPP then generates the fallback method using reference type sharing, which causes no performance degradation.where: struct constraint. This enables some optimizations, but the code will still be slower because the value types can be different sizes.UsedOnlyForAOTCodeGeneration and add references to the generic types and methods you want IL2CPP to generate. This method never needs to be called. The following example ensures that a specialization for GenericType<MyStruct> is generated:public void UsedOnlyForAOTCodeGeneration()
{
// Ensure that IL2CPP will create code for MyGenericStruct
// using MyStruct as an argument.
new GenericType<MyStruct>();
// Ensure that IL2CPP will create code for SomeType.GenericMethod
// using MyStruct as an argument.
new SomeType().GenericMethod<MyStruct>();
public void OnMessage<T>(T value)
{
Debug.LogFormat("Message value: {0}", value);
}
// Include an exception so we can be sure to know if this
// method is ever called.
throw new InvalidOperationException(
"This method is used for AOT code generation only. " +
"Do not call it at runtime.");
}
Note: To compile only the single, fully-sharable version of generic code, set the IL2CPP Code Generation Player setting to Optimize for code size and build time. This reduces the number of methods generated, reducing compile time and build size, but comes at the expense of runtime performance.
Managed methods that need to be marshaled to a C function pointer so that they can be called from native code have a few restrictions on AOT platforms:
[MonoPInvokeCallback] attribute.[MonoPInvokeCallback(Type)] overload to specify the generic specializations that must be supported. If so, the type must be a generic instance with the correct number of generic arguments. It’s possible to have multiple [MonoPInvokeCallback] attributes on a method as follows:// Generates reverse P/Invoke wrappers for NameOf<long> and NameOf<int>
// Note that the types are only used to indicate the generic arguments.
[MonoPInvokeCallback(typeof(Action<long>))]
[MonoPInvokeCallback(typeof(Action<int>))]
private static string NameOfT<T>(T item)
{
return typeof(T).Name;
}
IL2CPP doesn’t support the following .NET APIs and features:
MarshalAs and FieldOffset attributes. It does support these attributes at compile time. You should use these for proper platform invoke marshaling.dynamic keyword. This keyword requires JIT compilation, which is not possible with IL2CPP.Marshal.Prelink or Marshal.PrelinkAll API methods.System.Diagnostics.Process API methods.For cases where these are required on desktop platforms, use the Mono scripting back end.