协变泛型接口内符合变体的C#事件

Question

我有以下接口和委托：

// Base devices, non-generic
public interface IDevice
{
    event DeviceVarChange VarChange;
}

public interface IVar
{
    // ...
}

public delegate void DeviceVarChange(IVar theVar);

我希望能够用更具体的事件参数创建更具体的接口，同时，让它们与上面的基本接口兼容并“兼容”。例如：

// Specific devices, non-generic
public interface ISpecificDevice : IDevice
{
    // Problem: I have my "VarChange" event, but it uses IVar!
    // I want it to use "ISpecificVar"!
}

public interface ISpecificVar
{
    // ...
}

所以我选择了一个通用的解决方案。因为我希望我的子接口是“兼容变量的”，并且客户端只“消费”我设备中的变量，所以我尝试使用协方差：

// Base devices, generic
public interface IDevice<out TVar> where TVar : IVar
{
    // Compilation error: TVar must be invariant but is covariant in this context
    event DeviceVarChange<TVar> VarChange;
}

public interface IVar
{
    // ...
}

// Problem: Delegate arguments can be contravariant, but not covariant
public delegate void DeviceVarChange<TVar>(TVar theVar) where TVar : IVar;

但是我有一个问题，委托参数不能是协变的。将委托参数设置为逆变量可以解决编译错误，但这样我就不能再像IDevice<IVar>那样使用事件的ISpecificDevice (运行时错误，委托必须是相同类型的)。

public interface IDevice<out TVar> where TVar : IVar
{
    // No more compilation error, "DeviceVarChange" is contravariant
    event DeviceVarChange<TVar> VarChange;
}

public interface IVar
{
    // ...
}

public delegate void DeviceVarChange<in TVar>(TVar theVar) where TVar : IVar;

public interface ISpecificDevice : IDevice<ISpecificVar>
{
    // The event is using "ISpecificVar" here, as wanted
}

public interface ISpecificVar : IVar
{
    // ...
}

// + a "SpecificDevice" class implementing "ISpecificDevice"

// Try to use a specific instance as a base one
public class Test
{
    public void DoTest()
    {
        var genericDevice = GetSpecificDeviceAsBase();

        // Runtime error: delegates must be of the same type
        genericDevice.VarChange += OnVarChange;
    }

    public IDevice<IVar> GetSpecificDeviceAsBase()
    {
        return new SpecificDevice();
    }

    public void OnVarChange(IVar theVar)
    {
        // ...
    }
}

有没有办法解决这个问题？

或者使用第一种非通用解决方案，并在“特定”事件中强制显式转换是唯一的方法？

我也可以使用一个不变的泛型DeviceVarChange<TVar>委托，非泛型接口，并在每个具有特定接口的子接口中添加一个新事件，但如果我可以避免这个问题，欢迎提出建议。

Answer 1

这是我目前的解决办法，作为第一个，希望是暂时的答案。这是我将使用的，同时我希望有人建议我使用一些更好的东西。

public interface IDevice
{
    event DeviceVarChange<IVar> VarChange;
}

public interface IVar
{
    // ...
}

public delegate void DeviceVarChange<TVar>(TVar theVar) where TVar : IVar;

public interface ISpecificDevice : IDevice
{
    // Overload the event inside the interface with the specific class
    new event DeviceVarChange<ISpecificVar> VarChange;
}

public interface ISpecificVar : IVar
{
    // ...
}

public class SpecificDevice : ISpecificDevice
{
    // Dear maintainer,
    // Forgive me.
    // Regards
    private void FireVarChange(ISpecificVar theVar)
    {
        lock (delegatesForVarChange)
        {
            foreach (var theDelegate in delegatesForVarChange)
            {
                theDelegate.Invoke(theVar);
            }
        }
    }

    private List<dynamic> delegatesForVarChange = new List<dynamic>();

    event DeviceVarChange<IVar> IDevice.VarChange
    {
        add
        {
            lock (delegatesForVarChange)
            {
                delegatesForVarChange.Add(value);
            }
        }
        remove
        {
            lock (delegatesForVarChange)
            {
                delegatesForVarChange.Remove(value);
            }
        }
    }

    public event DeviceVarChange<ISpecificVar> VarChange
    {
        add
        {
            lock (delegatesForVarChange)
            {
                delegatesForVarChange.Add(value);
            }
        }
        remove
        {
            lock (delegatesForVarChange)
            {
                delegatesForVarChange.Remove(value);
            }
        }
    }
}

// Try to use a specific instance as a base one
public class Test
{
    public void DoTest()
    {
        var genericDevice = GetSpecificDeviceAsBase();

        // Success
        genericDevice.VarChange += OnVarChange;
    }

    public IDevice GetSpecificDeviceAsBase()
    {
        return new SpecificDevice();
    }

    public void OnVarChange(IVar theVar)
    {
        // ...
    }
}