16.7 Tie Classes

The mapping to skeleton classes we saw in Section 16.4 requires the servant class to inherit from its skeleton class. Occasionally, this creates a problem: some class libraries require you to inherit from a base class in order to access functionality provided by the library; because C# does not support multiple implementation inheritance, this means that you cannot use such a class library to implement your servants because your servants cannot inherit from both the library class and the skeleton class simultaneously.

To allow you to still use such class libraries, Ice provides a way to write servants that replaces inheritance with delegation. This approach is supported by tie classes. The idea is that, instead of inheriting from the skeleton class, you simply create a class (known as an implementation class or delegate class) that contains methods corresponding to the operations of an interface. You use the ‑‑tie option with the slice2cs compiler to create a tie class. For example, for the Node interface we saw in Section 16.4.1, the ‑‑tie option causes the compiler to create exactly the same code as we saw previously, but to also emit an additional tie class. For an interface <interface‑name>, the generated tie class has the name <interface‑name>Tie_:

public class NodeTie_ : NodeDisp_, Ice.TieBase
{
    public NodeTie_()
    {
    }

    public NodeTie_(NodeOperations_ del)
    {
        _ice_delegate = del;
    }

    public object ice_delegate()
    {
        return _ice_delegate;
    }

    public void ice_delegate(object del)
    {
        _ice_delegate = (NodeOperations_)del;
    }

    public override int ice_hash()
    {
        return GetHashCode();
    }

    public override int GetHashCode()
    {
        return _ice_delegate == null
                                ? 0
                                : _ice_delegate.GetHashCode();
    }

    public override bool Equals(object rhs)
    {
       if (object.ReferenceEquals(this, rhs))
       {
           return true;
       }
       if (!(rhs is NodeTie_))
       {
           return false;
       }
       if (_ice_delegate == null)
       {
           return ((NodeTie_)rhs)._ice_delegate == null;
       }
       return _ice_delegate.Equals(((NodeTie_)rhs)._ice_delegate);
    }

    public override string name(Ice.Current __current)
    {
        return _ice_delegate.name(__current);
    }

    private NodeOperations_ _ice_delegate;
}

This looks a lot worse than it is: in essence, the generated tie class is simply a servant class (it extends NodeDisp_) that delegates each invocation of a method that corresponds to a Slice operation to your implementation class (see Figure 16.1).

Figure 16.1. A skeleton class, tie class, and implementation class.

The Ice.TieBase interface defines the ice_delegate methods that allow you to get and set the delegate.

Given this machinery, we can create an implementation class for our Node interface as follows:

public class NodeI : NodeOperations_
{
    public NodeI(string name)
    {
        _name = name;
    }

    public override string name(Ice.Current current)
    {
        return _name;
    }

    private string _name;
}

Note that this class is identical to our previous implementation, except that it implements the NodeOperations_ interface and does not extend NodeDisp_ (which means that you are now free to extend any other class to support your implementation).

To create a servant, you instantiate your implementation class and the tie class, passing a reference to the implementation instance to the tie constructor:

NodeI fred = new NodeI("Fred");         // Create implementation
NodeTie_ servant = new NodeTie_(fred);  // Create tie

Alternatively, you can also default-construct the tie class and later set its delegate instance by calling ice_delegate:

NodeTie_ servant = new NodeTie_();      // Create tie
// ...
NodeI fred = new NodeI("Fred");         // Create implementation
// ...
servant.ice_delegate(fred);             // Set delegate

When using tie classes, it is important to remember that the tie instance is the servant, not your delegate. Furthermore, you must not use a tie instance to incarnate (see Section 16.8) an Ice object until the tie has a delegate. Once you have set the delegate, you must not change it for the lifetime of the tie; otherwise, undefined behavior results.

You should use the tie approach only if you need to, that is, if you need to extend some base class in order to implement your servants: using the tie approach is more costly in terms of memory because each Ice object is incarnated by two C# objects instead of one, the tie and the delegate. In addition, call dispatch for ties is marginally slower than for ordinary servants because the tie forwards each operation to the delegate, that is, each operation invocation requires two function calls instead of one.

Also note that, unless you arrange for it, there is no way to get from the delegate back to the tie. If you need to navigate back to the tie from the delegate, you can store a reference to the tie in a member of the delegate. (The reference can, for example, be initialized by the constructor of the delegate.)