A simple dynamic proxy targeting .net standard 2.0
Wraptor allows you to extend, intercept, modify and redirect calls to interface defined methods on any object. This is achieved by generating a proxy object at run-time that redirects calls to an object of type IInterceptor.
IInterceptor currently provides three hooks:
void PreInvoke(object implementation, MethodInfo methodInfo, object[] arguments)object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)void PostInvoke(object implementation, MethodInfo methodInfo, object[] arguments, object returnValue)
Wraptor's IL-Generation and proxy source was inspired by John Mikhail's article Dynamic Proxy Creation Using C# Emit
Install the nuget package either via GUI in Visual Studio or enter the following command in the package manager console:
Install-Package WraptorFirst, you need to define an implementation of Wraptor.IInterceptor. Wraptor provides a handy Wraptor.InterceptorBase class you can extend:
public class MyInterceptor : InterceptorBase
{
public override void PreInvoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
Console.WriteLine($"PreInvoke: {methodInfo.Name} on {implementation.GetType().Name}");
base.PreInvoke(implementation, methodInfo, arguments);
}
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
Console.WriteLine($"Invoke: {methodInfo.Name} on {implementation.GetType().Name}");
return base.Invoke(implementation, methodInfo, arguments);
}
public override void PostInvoke(object implementation, MethodInfo methodInfo, object[] arguments, object returnValue)
{
Console.WriteLine($"PostInvoke: {methodInfo.Name} on {implementation.GetType().Name}");
base.PostInvoke(implementation, methodInfo, arguments, returnValue);
}
}This implementation can now be used on an instance of the Wraptor.Generator class to create the proxy using the Generate<T> method:
public interface IPerson
{
void SayName();
}
public class Person : IPerson
{
public string Name { get; set; }
public void SayName() => Console.WriteLine($"Hi, my name is {this.Name}");
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var interceptor = new MyInterceptor();
var alice = new Person { Name = "Alice" };
var proxy = generator.Generate<IPerson>(alice, interceptor);
proxy.SayName();
}
}Which will result in the following output:
PreInvoke: SayName on Person
Invoke: SayName on Person
Hi, my name is Alice
PostInvoke: SayName on Person
Please note the explicit definition of IPerson in the Generate<T> call. Wraptor uses the here defined interface to generate the proxy methods. Defining anything other than an interface will result in an ArgumentException.
One way is to start a Stopwatch in the Invoke method, wait for the actual method to complete and then process the ellapsed time.
public class MeasureInterceptor : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
var stopwatch = Stopwatch.StartNew();
var result = base.Invoke(implementation, methodInfo, arguments);
Console.WriteLine($"{methodInfo.Name} took {stopwatch.ElapsedMilliseconds}ms");
return result;
}
}Another approach could be to have a timer at instance level of the Interceptor and toggle the measuring in the PreInvoke and PostInvoke methods.
The arguments that will or have been used will be passed to all hooks as boxed objects in an array through the arguments parameter. To have an effect on the execution of the method, you will have to modify them in either the PreInvoke or Invoke method.
The following example illustrates the change of the first and only parameter passed to the Deposit method:
public interface IPerson
{
void Deposit(decimal moneys);
}
public class Person : IPerson
{
public decimal Balance { get; set; }
public void Deposit(decimal moneys) => this.Balance += moneys;
}
public class ArgumentInterceptor : InterceptorBase
{
public override void PreInvoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
if (methodInfo.Equals(typeof(IPerson).GetMethod(nameof(IPerson.Deposit))))
{
arguments[0] = 90001m;
}
}
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var interceptor = new ArgumentInterceptor();
var alice = new Person { Balance = 0 };
var proxy = generator.Generate<IPerson>(alice, interceptor);
proxy.Deposit(42);
Console.WriteLine(alice.Balance);
}
}Which then produces the following output:
90001
The Invoke method provides the result of the original method as a boxed object. You can modify this object before returning which will affect every caller:
public interface IPerson
{
int Age { get; set; }
}
public class Person : IPerson
{
public int Age { get; set; }
}
public class ReturnValueInterceptor : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
var result = base.Invoke(implementation, methodInfo, arguments);
if (methodInfo.Name.Equals($"get_{nameof(IPerson.Age)}", StringComparison.InvariantCultureIgnoreCase))
{
result = (int)result + 17;
}
return result;
}
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var interceptor = new ReturnValueInterceptor();
var alice = new Person { Age = 25 };
var proxy = generator.Generate<IPerson>(alice, interceptor);
Console.WriteLine(proxy.Age);
}
}Which will result in the following output:
42
Since the Invoke method controls the flow of execution, the call to the proxied method can be avoided by simply returning early:
public interface IProcessor
{
void BroadcastMessage(string message);
}
public class Processor : IProcessor
{
public void BroadcastMessage(string message) => Console.WriteLine(message);
}
public class Suppressor : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments) => null;
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var interceptor = new Suppressor();
var processor = new Processor();
var proxy = generator.Generate<IProcessor>(processor, interceptor);
proxy.BroadcastMessage("Hello World");
}
}The example above will not produce any output at all, as the Console.WriteLine statement will never be reached.
We can therefore also redirect calls to completely different methods, as long as the underlying type of the return value is the same as on the originally called method:
public interface IProcessor
{
string ProcessMessage(string message);
}
public class Processor : IProcessor
{
public string ProcessMessage(string message) => $"{message} - normally processed";
}
public class OtherProcessor
{
public string ProcessMessage(string message) => $"{message} - processed by {this.GetType().Name}";
}
public class Redirector : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
var otherProcessor = new OtherProcessor();
return otherProcessor.ProcessMessage((string)arguments[0]);
}
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var interceptor = new Redirector();
var processor = new Processor();
var proxy = generator.Generate<IProcessor>(processor, interceptor);
var result = proxy.ProcessMessage("Hello World");
Console.WriteLine(result);
}
}Which will result in the following output:
Hello World - processed by OtherProcessor
Since the generated proxies implement the defined interface, it is possible to chain interceptors:
public interface IProcessor
{
void ProcessMessage(string message);
}
public class Processor : IProcessor
{
public void ProcessMessage(string message) => Console.WriteLine(message);
}
public class InterceptorOne : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
arguments[0] = arguments[0] + " modified by InterceptorOne";
return base.Invoke(implementation, methodInfo, arguments);
}
}
public class InterceptorTwo : InterceptorBase
{
public override object Invoke(object implementation, MethodInfo methodInfo, object[] arguments)
{
arguments[0] = arguments[0] + " modified by InterceptorTwo";
return base.Invoke(implementation, methodInfo, arguments);
}
}
public class Program
{
static void Main(string[] args)
{
var generator = new Generator();
var processor = new Processor();
var proxy = generator.Generate<IProcessor>(processor, new InterceptorOne());
proxy = generator.Generate<IProcessor>(proxy, new InterceptorTwo());
proxy.ProcessMessage("Hello World");
}
}Which generates following output:
Hello World modified by InterceptorTwo modified by InterceptorOne
- Support for non-interface public methods
- Interception for private method calls (not sure if this is even possible)
- Support for extracting and saving generated proxies
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