Limitations

Generic Virtual Methods

Generic virtual methods aren't supported, as it isn't possible to determine statically what method will be called in all circumstances. (Which is why C++ doesn't support virtual template methods, either.)

class HasGenericVirtualMethod {
    public virtual PrintValues<T>(params T[] values)
    {
        // ...
    }
}
 
// ...
var a = new HasGenericVirtualMethod ();
a.PrintValues (new[]{1, 2, 3, 4});

P/Invokes in Generic Types

P/Invokes in generic classes aren't supported:

class GenericType<T> {
    [DllImport ("System")]
    public static extern int getpid ();
}

Property.SetInfo on a Nullable Type is not supported

Using Reflection's Property.SetInfo to set the value on a Nullable<T> is not currently supported.

Value types as Dictionary Keys

Using a value type as a Dictionary<TKey, TValue> key is problematic, as the default Dictionary constructor attempts to use EqualityComparer<TKey>.Default. EqualityComparer<TKey>.Default, in turn, attempts to use Reflection to instantiate a new type which implements the IEqualityComparer<TKey> interface.

This works for reference types (as the reflection+create a new type step is skipped), but for value types it crashes and burns rather quickly once you attempt to use it on the device.

Workaround: Manually implement the IEqualityComparer<TKey> (http://www.go-mono.com/docs/index.aspx?link=T%3aSystem.Collections.Generic.IEqualityComparer%601) interface in a new type and provide an instance of that type to the Dictionary<TKey, TValue>(IEqualityComparer<TKey>) constructor (http://www.go-mono.com/docs/monodoc.ashx?link=C%3aSystem.Collections.Generic.Dictionary%602(System.Collections.Generic.IEqualityComparer%7b%600%7d)).

System.Reflection.Emit

The lack of System.Reflection.Emit means that no code that depends on runtime code generation will work. This includes things like:

• The Dynamic Language Runtime.
• Any languages built on top of the Dynamic Language Runtime.
• Remoting's TransparentProxy or anything else that would cause the runtime to generate code dynamically.

 Important: Do not confuse Reflection.Emit with Reflection.   Reflection.Emit is about generating code dynamically and have that code JITed and compiled to native code.   Due to the limitations on the iPhone (no JIT compilation) this is not supported.

But the entire Reflection API, including Type.GetType ("someClass"), listing methods, listing properties, fetching attributes and values works just fine.

Reverse Callbacks

In standard Mono it is possible to pass C# delegate instances to unmanaged code in lieu of a function pointer. The runtime would typically transform those function pointers into a small thunk that allows unmanaged code to call back into managed code.

In Mono these bridges are implemented by the Just-in-Time compiler. When using the ahead-of-time compiler required by the iPhone there are two important limitations at this point:

• You must flag all of your callback methods with the MonoPInvokeCallbackAttribute 
• The methods have to be static methods, there is no support for instance methods.