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 backendA framework that powers scripting in Unity. Unity supports three different scripting backends depending on target platform: Mono, .NET and IL2CPP. Universal Windows Platform, however, supports only two: .NET and IL2CPP. More info
See in Glossary:

Ahead-of-time compile (AOT)

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

Reflection

Unity supports reflection on AOT platforms. However, if this 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.

System.Reflection.Emit

An AOT platform cannot implement any of the methods in the System.Reflection.Emit namespace.

Serialization

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 cannot detect that it needs to generate the code needs for the type or method.

Generic Types and Methods

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 it is for List<double>. However IL2CPP will share code for usages for reference types, so the same code will be used for List<object> and List<string>.

It is possible to reference generic types and methods that 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 did not find a compile time in the following cases:

1. Creating a new generic instance at runtime: Activator.CreateInstance(typeof(SomeGenericType<>).MakeGenericType(someType));
2. Invoking a static method on a generic instance: typeof(SomeGenericType<>).MakeGenericType(someType)).GetMethod(“AMethod”).Invoke(null, null);
3. Invoking a static generic method: typeof(SomeType).GetMethod(“GenericMethod”).MakeGenericMethod(someType).Invoke(null, null);
4. Some calls to generic virtual functions that cannot be inferred at compile time.
5. Calls with deeply nested generic value types, such as Struct<Struct<Struct<...<Struct<int>>>>.

To support those cases IL2CPP generates generic code that will work with any type parameter. However this code is slower because it can make no assumptions on the size of the type or if it is a reference or value type. If you need to ensure that faster generic methods are generated, do the following:

• If the generic argument will always be a reference type, add the where: class constraint. Then IL2CPP will generate the fallback method using reference type sharing which causes no performance degradation.
• If the generic argument will always be a value type, add the where: struct constraint. This enables some optimizations, but the code will still be slower because the value types can be different sizes.
• Create a method named UsedOnlyForAOTCodeGeneration and add references to the generic types and methods you wish IL2CPP to generate. This method does not need (and probably shouldn’t) be called. The example below will ensure that a specialization for GenericType<MyStruct> will be 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 that when the “Faster (smaller) builds” setting is enabled only the single fully sharable version of generic code is compiled. This reduces the number of methods generated, reducing compile time and build size, but comes at the expense of runtime performance.

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
• If the managed method is generic, the [MonoPInvokeCallback(Type)] overload might need to be used to specify the generic specializations that need to 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 below:

// 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;
}

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 supports exception filters, however the execution order filter statements and catch blocks is different because IL2CPP uses C++ exceptions to implement managed exceptions. This will not be noticeable unless a filter blocks writes to a field.

MarshalAs and FieldOffset attributes

IL2CPP does not support reflection of the MarshalAs 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.

Marshal.Prelink

IL2CPP doesn’t support the Marshal.Prelink or Marshal.PrelinkAll API methods.

System.Diagnostics.Process API

IL2CPP doesn’t support the System.Diagnostics.Process API methods. For cases where this is required on desktop platforms, use the Mono scripting backend.