Castle DynamicProxy

Introduction¶

Castle DynamicProxy is a library for generating lightweight .NET proxies on the fly at runtime. Proxy objects allow calls to members of an object to be intercepted without modifying the code of the class.

DynamicProxy differs from the proxy implementation built into the CLR which requires the proxied class to extend MarshalByRefObject. Extending MashalByRefObject to proxy an object can be too intrusive because it does not allow the class to extend another class and it does not allow transparent proxying of classes. Additionally Castle DynamicProxy provides capabilities beyond what standard CLR proxies can do, for example it lets you mix in multiple objects.

Requirements¶

To use Castle DynamicProxy you need the following environment:

• one of the following runtimes installed
  • .NET version 3.5 sp1 or newer
  • Silverlight version 4 or newer
• Castle.Core.dll (assembly where DynamicProxy lives)

At this time Mono is not supported

 

 

DynamicProxy assembly

In previous versions (up to v2.2) DynamicProxy used to live in its own assembly Castle.DynamicProxy.dll. It was later moved to Castle.Core.dll and now no other assembly is required to use it.

 

Interception pipeline¶

Another way, and it's how DP is used mostly, is by adding behavior to the proxied objects. That's what the IInterceptor interface you'll find in DynamicProxy is for. You use interceptors to inject behavior into the proxy.

The picture above shows schematically how that works.

• The blue rectangle is the proxy. Someone calls a method on the proxy (denoted by yellow arrow). Before the method reaches the target object it goes through a pipeline of interceptors.
• Each interceptor gets a IInvocation object (which is another important interface from Dynamic Proxy), that holds all the information about current request, like the MethodInfo of the method called, along with its parameters and returned value, reference to the proxy, as well as proxied object, and few others. Each interceptor gets its chance to inspect and change those values before the actual method on the target object is called.
  So for example at this stage you can log debug information about what parameters were passed to the method, or validate them. Then, the interceptor has to call invocation.Proceed(), to pass control further down the pipeline. An interceptor can call Proceed at most once, otherwise an exception is thrown.
• After last interceptor calls Proceed, the actual method on proxied object is invoked, and then the call travels back, up the pipeline (green arrow) giving each interceptor chance to inspect and act on, returned value, or thrown exceptions.
• Finally the proxy returns the value held by invocation.ReturnValue as the return value of called method.

 

Interceptor example¶

If this was not clear enough, here's a sample interceptor, that shows how it works:

[Serializable]

public class Interceptor : IInterceptor

{

public void Intercept(IInvocation invocation)

{

Console.WriteLine("Before target call");

try

{

invocation.Proceed();

}

catch(Exception)

{

Console.WriteLine("Target threw an exception!");

throw;

}

finally

{

Console.WriteLine("After target call");

}

}

}

Hopefully, at this stage you have a pretty good idea about what DynamicProxy is, how it works, and what it's good for. In the next chapter we'll dive into some more advanced capabilities, plugging into, and influencing the process of generating proxy class.

See also¶

Kinds of proxy objects