Scripting restrictions

Unity provides a common scripting API and experience across all platforms it supports. However, some platforms have inherent restrictions. To help you understand these restrictions, the following table describes which restrictions apply to each platform and scripting backend:

.NET 4.x equivalent scripting runtime

Ahead-of-time compile

Some platforms do not allow runtime code generation. Therefore, any managed code which depends upon just-in-time (JIT) compilation on the target device will fail. Instead, you must compile all of the managed code ahead-of-time (AOT). Often, this distinction doesn’t matter, but in a few specific cases, AOT platforms require additional consideration.

System.Reflection.Emit

An AOT platform cannot implement any of the methods in the System.Reflection.Emit namespace. The rest of System.Reflection is acceptable, as long as the compiler can infer that the code used via reflection needs to exist at runtime.

Serialization

AOT platforms might encounter issues with serialization and deserlization 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 cannot detect that it needs to generate the code needs for the type or method.

Generic virtual methods

If you use generic methods, the compiler must do some additional work to expand your written code to the code executed on the device. For example, you need different code for List with an int or a double. If you use virtual methods, where the behavior is determined at runtime rather than compile time, the compiler can easily require runtime code generation in places where it is not entirely obvious from the source code.

The following code example works exactly as expected on a JIT platform (it prints “Message value: Zero” to the console once):

using UnityEngine;
using System;

public class AOTProblemExample : MonoBehaviour, IReceiver
{
    public enum AnyEnum 
    {
        Zero,
        One,
    }

    void Start() 
    {
        // Subtle trigger: The type of manager *must* be
        // IManager, not Manager, to trigger the AOT problem.
        IManager manager = new Manager();
        manager.SendMessage(this, AnyEnum.Zero);
    }

    public void OnMessage<T>(T value) 
    {
        Debug.LogFormat("Message value: {0}", value);
    }
}

public class Manager : IManager 
{
    public void SendMessage<T>(IReceiver target, T value) {
        target.OnMessage(value);
    }
}

public interface IReceiver
{
    void OnMessage<T>(T value);
}

public interface IManager 
{
    void SendMessage<T>(IReceiver target, T value);
}

However, when you execute this code on an AOT platform with the IL2CPP scripting backend, this exception occurs:

ExecutionEngineException: Attempting to call method 'AOTProblemExample::OnMessage<AOTProblemExample+AnyEnum>' for which no ahead of time (AOT) code was generated.
  at Manager.SendMessage[T] (IReceiver target, .T value) [0x00000] in <filename unknown>:0 
  at AOTProblemExample.Start () [0x00000] in <filename unknown>:0 

Likewise, the Mono scripting backend provides this similar exception:

  ExecutionEngineException: Attempting to JIT compile method 'Manager:SendMessage<AOTProblemExample/AnyEnum> (IReceiver,AOTProblemExample/AnyEnum)' while running with --aot-only.
  at AOTProblemExample.Start () [0x00000] in <filename unknown>:0 

The AOT compiler does not recognize that it should generate code for the generic method OnMessage with a T of AnyEnum, so it continues, skipping this method. When that method is called, and the runtime can’t find the proper code to execute, it returns this error message.

To work around an AOT issue like this, you can force the compiler to generate the proper code. To do this, add the following example method to the AOTProblemExample class:

public void UsedOnlyForAOTCodeGeneration() 
{
    // IL2CPP needs only this line.
    OnMessage(AnyEnum.Zero);

    // Mono also needs this line. Note that we are
    // calling directly on the Manager, not the IManager interface.
    new Manager().SendMessage(null, AnyEnum.Zero);

    // 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.");
}

When the compiler encounters the explicit call to OnMessage with a T of AnyEnum, it generates the proper code for the runtime to execute. The method UsedOnlyForAOTCodeGeneration does not need to be called; it just needs to exist for the compiler to recognize it.

Calling managed methods from native code

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:

• The managed method must be a static method
• The managed method must have the [MonoPInvokeCallback] attribute

No threads

Some platforms do not support the use of threads, so any managed code that uses the System.Threading namespace fails at runtime. Also, some parts of the .NET class libraries implicitly depend upon threads. An often-used example is the System.Timers.Timer class, which depends on support for threads.

Exception filters

IL2CPP does not support C# exception filters. You should modify the code that depends on exception filters into the proper catch blocks.

TypedReference

IL2CPP does not support the System.TypedReference type or the __makeref C# keyword.

MarshalAs and FieldOffset attributes

IL2CPP does not support reflection of the MarhsalAs and FieldOffset attributes at runtime. It does support these attributes at compile time. You should use these for proper platform invoke marshaling.

The dynamic keyword

IL2CPP does not support the C# dynamic keyword. This keyword requires JIT compilation, which is not possible with IL2CPP.