Dynamic lookup allows you a unified approach to invoking
things dynamically. With dynamic lookup, when you have an object in your hand
you do not need to worry about whether it comes from COM, IronPython, the HTML
DOM or reflection; you just apply operations to it and leave it to the runtime
to figure out what exactly those operations mean for that particular object.
This affords you enormous flexibility, and can greatly
simplify your code, but it does come with a significant drawback: Static typing
is not maintained for these operations. A dynamic object is assumed at compile
time to support any operation, and only at runtime will you get an error if it
wasn't so. Oftentimes this will be no loss, because the object wouldn't have a
static type anyway, in other cases it is a tradeoff between brevity and safety.
In order to facilitate this tradeoff, it is a design goal of C# to allow you to
opt in or opt out of dynamic behavior on every single call.
The dynamic type
C# 4.0 introduces a new static type called dynamic. When you have an object of type dynamic you can 'do things to it' that are
resolved only at runtime:
dynamic d = GetDynamicObject(var);
d.M(7);
The C# compiler allows you to call a method with any name
and any arguments on d because it is of
type dynamic. At runtime the actual object
that d refers to will be examined to
determine what it means to ' call M
with an int ' on it.
The type dynamic can be
thought of as a special version of the type object,
which signals that the object can be used dynamically. It is easy to opt in or
out of dynamic behavior: any object can be implicitly converted to dynamic, 'suspending belief' until runtime.
Conversely, there is an 'assignment conversion' from dynamic
to any other type, which allows implicit conversion in assignment-like constructs:
dynamic d = 7; // implicit conversion
int i = d; // assignment conversion
Dynamic operations
Not only method calls, but also field and property accesses,
indexer and operator calls and even delegate invocations can be dispatched
dynamically:
dynamic d = GetDynamicObject(var);
d.M(7); // calling methods
d.f = d.P; // getting and settings fields and properties
d['one'] = d['two']; // getting and setting thorugh indexers
int i = d + 3; // calling operators
string s = d(5,7); // invoking as a delegate
The role of the C# compiler here is simply to package up the
necessary information about 'what is being done to d',
so that the runtime can pick it up and determine what the exact meaning of it
is given an actual object d. Think of it as
deferring part of the compiler's job to runtime.
The result of any dynamic operation is itself of type dynamic.
Runtime lookup
At runtime a dynamic operation is dispatched according to
the nature of its target object :
COM objects
If d is a COM object,
the operation is dispatched dynamically through COM IDispatch.
This allows calling to COM types that don't have a Primary Interop Assembly
(PIA), and relying on COM features that don't have a counterpart in C#, such as
indexed properties and default properties.
Dynamic objects
If d implements the
interface IDynamicObject d
itself is asked to perform the operation. Thus by implementing IDynamicObject a type can completely redefine
the meaning of dynamic operations. This is used intensively by dynamic
languages such as IronPython and IronRuby to implement their own dynamic object
models. It will also be used by APIs, e.g. by the HTML DOM to allow direct
access to the object's properties using property syntax.
Plain objects
Otherwise d is a
standard .NET object, and the operation will be dispatched using reflection on
its type and a C# 'runtime binder' which implements C#'s lookup and overload
resolution semantics at runtime. This is essentially a part of the C# compiler
running as a runtime component to 'finish the work' on dynamic operations that
was deferred by the static compiler.
Example
Assume the following code:
dynamic d1 = new Foo();
dynamic d2 = new Bar();
string s;
d1.M(s, d2, 3, null);
Because the receiver of the call to M is dynamic,
the C# compiler does not try to resolve the meaning of the call. Instead it
stashes away information for the runtime about the call. This information
(often referred to as the 'payload') is essentially equivalent to:
'Perform an instance method call of M with the following arguments:
1.
a string
2.
a dynamic
3.
a literal int
3
4.
a literal object
null'
At runtime, assume that the actual type Foo of d1
is not a COM type and does not implement IDynamicObject.
In this case the C# runtime binder picks up to finish the overload resolution
job based on runtime type information, proceeding as follows:
1.
Reflection is used to obtain the actual runtime
types of the two objects, d1 and d2, that did not have a static type (or rather
had the static type dynamic). The result
is Foo for d1
and Bar for d2.
2.
Method lookup and overload resolution is
performed on the type Foo with the call M(string,Bar,3,null) using ordinary C#
semantics.
3.
If the method is found it is invoked; otherwise
a runtime exception is thrown.
Overload resolution with dynamic arguments
Even if the receiver of a method call is of a static type,
overload resolution can still happen at runtime. This can happen if one or more
of the arguments have the type dynamic:
Foo foo = new Foo();
dynamic d = new Bar();
var result = foo.M(d);
The C# runtime binder will choose between the statically
known overloads of M on Foo, based on the runtime type of d, namely Bar. The result is again of type dynamic.
The Dynamic Language Runtime
An important component in the underlying implementation of
dynamic lookup is the Dynamic Language Runtime (DLR), which is a new API in
.NET 4.0.
The DLR provides most of the infrastructure behind not only
C# dynamic lookup but also the implementation of several dynamic programming
languages on .NET, such as IronPython and IronRuby. Through this common
infrastructure a high degree of interoperability is ensured, but just as
importantly the DLR provides excellent caching mechanisms which serve to
greatly enhance the efficiency of runtime dispatch.
To the user of dynamic lookup in C#, the DLR is invisible
except for the improved efficiency. However, if you want to implement your own
dynamically dispatched objects, the IDynamicObject
interface allows you to interoperate with the DLR and plug in your own
behavior. This is a rather advanced task, which requires you to understand a
good deal more about the inner workings of the DLR. For API writers, however,
it can definitely be worth the trouble in order to vastly improve the usability
of e.g. a library representing an inherently dynamic domain.
Limitations
There are a few limitations and things that might work
differently than you would expect.
·
The DLR allows objects to be created from
objects that represent classes. However, the current implementation of C#
doesn't have syntax to support this.
·
Dynamic lookup will not be able to find
extension methods. Whether extension methods apply or not depends on the static
context of the call (i.e. which using clauses occur), and this context
information is not currently kept as part of the payload.
·
Anonymous functions (i.e. lambda expressions)
cannot appear as arguments to a dynamic method call. The compiler cannot bind
(i.e. 'understand') an anonymous function without knowing what type it is
converted to.
One consequence of these limitations is that you cannot easily
use LINQ queries over dynamic objects:
dynamic collection = var;
var result = collection.Select(e => e + 5);
If the Select method is an
extension method, dynamic lookup will not find it. Even if it is an instance
method, the above does not compile, because a lambda expression cannot be
passed as an argument to a dynamic operation.