[C# 4.0] 2. Dynamic (2)

.NET 4.0 通过嵌入 DLR Runtime 来实现对动态编程的支持。DLR Runtime 包含 Expression Trees、CallSite Caching、Dynamic Object Interoperability 几个子系统。某些实现部分，我们在前一章分析 dynamic 关键字时已有所接触。

动态编程行为主要两个部分：

• 动态访问对象 : 这也就是 dynamic 所要做的主要工作，我们可以在运行时 "动态" 访问对象成员，而不是编译期。
• 创建动态对象 : 我们可以在运行期定义对象的行为，包括增删对象成员等。想必大家对动态语言的 OpenClass 已经神往已久了。

我们试着创建一些 .NET 4.0 动态调用的例子。和前一章 "动态" 调用 "静态对象成员" 不同，下面这些例子着重于 "动态对象"。

class MyClass : DynamicObject
{
    private Dictionary<string, object> values = new Dictionary<string, object>();

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        result = null;
        if (!values.ContainsKey(binder.Name)) return false;

        result = values[binder.Name];
        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        values[binder.Name] = value;
        return true;
    }

    public override bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result)
    {
        switch (binder.Name)
        {
            case "Test":
                var sb = new StringBuilder();
                for (int i = 0; i < binder.CallInfo.ArgumentCount; i++)
                {
                    sb.AppendFormat("{0}={1}/r/n", binder.CallInfo.ArgumentNames[i], args[i]);
                }

                result = sb.ToString();
                return true;
            default:
                result = null;
                return false;
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        dynamic o = new MyClass();
        o.X = 123;
        o.Y = "Hello, World!";
        var s = o.Test(A: 123, Y: "abc");

        Console.WriteLine(o.X);
        Console.WriteLine(o.Y);
        Console.WriteLine(s);
    }
}

输出:

123
Hello, World!
A=123
Y=abc

由于 DynamicObject 默认没有实现操作，因此在上面例子中我们得自己折腾。

public class DynamicObject : IDynamicMetaObjectProvider
{
    public virtual bool TryGetMember(GetMemberBinder binder, out object result)
    {
        result = null;
        return false;
    } 

    ... ...
}

当然，如果嫌麻烦，可以考虑用下面这种方式。

dynamic o = new ExpandoObject();
o.Test = new Func<int, string, string>((a, y) => String.Format("A={0}/r/nY={1}", a, y));

o.X = 123;
o.Y = "Hello, World!";
var s = o.Test(123, "abc");

Console.WriteLine(o.X);
Console.WriteLine(o.Y);
Console.WriteLine(s);

和 JavaScript、Python 之类的动态语言很像吧。我们还可以对其成员进行增删和遍历操作，还记得 Python 中的 dir() 吧。

dynamic o = new ExpandoObject();
o.Test = new Func<int, string, string>((a, y) => String.Format("A={0}/r/nY={1}", a, y));
o.X = 123;
o.Y = "Hello, World!";

// 遍历成员
var dict = o as IDictionary<string, object>;
Console.WriteLine(String.Join(", ", dict.Keys));

// 移除成员
dict.Remove("Y");
try
{
    Console.WriteLine(o.Y);
}
catch (RuntimeBinderException ex)
{
    Console.WriteLine(ex.Message);
}

DynamicObject 和 ExpandoObject 都实现了 IDynamicMetaObjectProvider 接口，这是 DLR 进行动态操作所必须的。

public class DynamicObject : IDynamicMetaObjectProvider
{
    // Methods
    protected DynamicObject();
    public virtual IEnumerable<string> GetDynamicMemberNames();
    public virtual DynamicMetaObject GetMetaObject(Expression parameter);
    public virtual bool TryBinaryOperation(BinaryOperationBinder binder, object arg, out object result);
    public virtual bool TryConvert(ConvertBinder binder, out object result);
    public virtual bool TryCreateInstance(CreateInstanceBinder binder, object[] args, out object result);
    public virtual bool TryDeleteIndex(DeleteIndexBinder binder, object[] indexes);
    public virtual bool TryDeleteMember(DeleteMemberBinder binder);
    public virtual bool TryGetIndex(GetIndexBinder binder, object[] indexes, out object result);
    public virtual bool TryGetMember(GetMemberBinder binder, out object result);
    public virtual bool TryInvoke(InvokeBinder binder, object[] args, out object result);
    public virtual bool TryInvokeMember(InvokeMemberBinder binder, object[] args, out object result);
    public virtual bool TrySetIndex(SetIndexBinder binder, object[] indexes, object value);
    public virtual bool TrySetMember(SetMemberBinder binder, object value);
    public virtual bool TryUnaryOperation(UnaryOperationBinder binder, out object result);
}
 
public sealed class ExpandoObject : IDynamicMetaObjectProvider, IDictionary<string, object>, 
    ICollection<KeyValuePair<string, object>>, IEnumerable<KeyValuePair<string, object>>, 
    IEnumerable, INotifyPropertyChanged
{
    // Events
    event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged;

    // Methods
    public ExpandoObject();
    void ICollection<KeyValuePair<string, object>>.Add(KeyValuePair<string, object> item);
    void ICollection<KeyValuePair<string, object>>.Clear();
    bool ICollection<KeyValuePair<string, object>>.Contains(KeyValuePair<string, object> item);
    void ICollection<KeyValuePair<string, object>>.CopyTo(KeyValuePair<string, object>[] array, int arrayIndex);
    bool ICollection<KeyValuePair<string, object>>.Remove(KeyValuePair<string, object> item);

    void IDictionary<string, object>.Add(string key, object value);
    bool IDictionary<string, object>.ContainsKey(string key);
    bool IDictionary<string, object>.Remove(string key);
    bool IDictionary<string, object>.TryGetValue(string key, out object value);

    IEnumerator<KeyValuePair<string, object>> IEnumerable<KeyValuePair<string, object>>.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator();
    
    DynamicMetaObject IDynamicMetaObjectProvider.GetMetaObject(Expression parameter);

    // Properties
    int ICollection<KeyValuePair<string, object>>.Count { get; }
    bool ICollection<KeyValuePair<string, object>>.IsReadOnly { get; }
    object IDictionary<string, object>.this[string key] { get; set; }

    ICollection<string> IDictionary<string, object>.Keys { get; }
    ICollection<object> IDictionary<string, object>.Values { get; }
}

DynamicObject 集中实现成员调用，可以更灵活地调度。而 ExpandoObject 则更轻便，更接近于动态语言编程习惯。