I know the CallerMemberName-attribute, which replaces the null parameter with, for example, the property name you are calling the method from.
This is very useful for things like PropertyChanged-Notifications. Currently we have a different scenario, where we would like to have a parameter-attribute which replaces the null parameter with the method name you're calling.
Generally speaking, is it possible to do something like this?
To be honest, I havent dealt much with custom attribute yet, but in our case, it would be kinda interesting to have something like this.
Is there any helpful Information I can start with?
There is no such attribute, but you could use C# 6 nameof operator:
public void SomeMethod ()
{
Console.WriteLine(nameof(SomeMethod));
}
Of course this does not dynamically and automatically inserts the name of the method you are in, but requires you to have an actual reference to the method. However, it supports full IntelliSense and will also update automatically when you refactor the method name. And the name is inserted at compile time, so you don’t get any performance downside.
If you wanted to place this code in a more central place, like you do with e.g. INPC implementations in base view models, then your idea is a bit flawed anyway. If you had a common method you call to figure out the method name you’re in, then it would always report the method name of the common method:
public void SomeMethod ()
{
Console.WriteLine(GetMethodName());
}
// assuming that a CallingMemberNameAttribute existed
public string GetMethodName([CallingMemberName] string callingMember = null)
{
return callingMember; // would be always "GetMethodName"
}
But instead, you could use the CallerMemberNameAttribute here again, which will then correctly get the method name calling the GetMethodName function:
public void SomeMethod ()
{
Console.WriteLine(GetMethodName());
}
public string GetMethodName([CallerMemberNamed] string callerMember = null)
{
return callerMember;
}
Related
I've been struggling to find examples of how to write a custom attribute to validate method parameters, i.e., turn this form:
public void DoSomething(Client client)
{
if (client.HasAction("do_something"))
{
// ...
}
else
{
throw new RequiredActionException(client, "do_something");
}
}
into this:
public void DoSomething([RequiredAction(Action="some_action")] Client client)
{
// ...
}
As far as I can tell, I need to add this attribute to my custom attribute, but I'm at a loss on how to access the decorated parameter Client:
[AttributeUsageAttribute(AttributeTargets.Parameter)]
public class RequireActionAttribute : System.Attribute
{
public Type Action {get; set;}
public RequireActionAttribute()
{
// .. How do you access the decorated parameter?
Client client = ???
if (!client.HasAction(Action))
{
throw new RequiredActionException(client, Action);
}
}
}
You're applying it correctly - but an attribute basically doesn't know the member it refers to. This definitely makes life harder.
Not only does it not have access to the member that it refers to, but that member would be a ParameterInfo, not a Client - there's no easy way of accessing the value of a parameter externally. Your method would need to call some helper code, passing the value of client in order to handle it appropriately... or you need to hook into the code which is going to call your method to start with, in order to notice the attribute.
It's not clear exactly how you were hoping to use this, but it may well be that you need to change your design significantly.
Attributes are not enough for doing it.
If I understood you correctly you want to add an attribute on a parameter in order to validate it at run time and that is impossible only with attributes.
It is impossible because attributes are only "metadata" and not executed code.
You will need some "real" code to read it and act accordingly. That code can be injected at compile time or you can hook into the function execution.
Attributes probably should be put on the method itself. When I was searching for the solution I found the following link and the way it uses interceptor seems even better http://www.codinginstinct.com/2008/05/argument-validation-using-attributes.html
Consider this method and the description:
[Description("It must be called from a property, else it is a runtime error.")]
protected T Load<T>()
{
return InternalLoad<T>();
}
The design of this method requires that the caller must be a property, othewise InternalLoad throws an exception. It uses StackFrame to get the caller name, and if it is not of the form of get_<PropertyName> or set_<PropertyName>, it throws an exception. It all happens at runtime which I don't like.
I want to know if there is any way to make sure at compile-time that the caller is always a property. In other words, only properties should be allowed to call this method. Is there any way to check that at compile-time?
As a last resort, is it possible to extend the C# compiler which would use a custom attribute (such as CallableFromAttribute) to make sure that?
I want to make it as much flexible as possible:
[CallableFrom(Caller.Property)] //Caller is an enum
protected T Load<T>()
{
return InternalLoad<T>();
}
[CallableFrom(Caller.Property | Caller.Method)]
protected T SomeOtherLoad<T>()
{
//code
}
and then
public string Method()
{
var x = this.SomeOtherLoad<string>(); //okay
var y = this.Load<string>(); //compilation error !!
}
Let me know if there is any confusion. I will clarify it. :-)
I need this feature because I'm trying to implement a class called PropertyManager which serves as base class for other classes which need to define properties. The typical use of this class would be this:
public sealed Vendor : PropertyManager
{
public string VendorName
{
get { return this.Load<string>(); }
set { this.Store(value); }
}
public DateTime Created
{
get { return this.Load<DateTime>(); }
set { this.Store(value); }
}
}
Here Load and Store methods, defined in the base class, discover the name of property from which they're invoked; treating that name as key, it reads the associated value from a dictionary (in case of Load), or writes to it (in case of Store). It raises property changing and changed events. It also supports undo, as ProperyMananger can easily keep track of all changes made to a property.
No, there is no such possibility at compile time. There cannot be: all such compile-time checks are based on whether a method is accessible from the method that accesses it, not the method that calls it. If a property setter exposes Load via a delegate, there is no possible way even in a theoretical modified compiler to check at compile time that nobody uses that delegate other than another property setter.
I'm not sure why you want this, so I'm assuming that your check is necessary and your runtime check verifies exactly what you need it to. If that assumption is wrong, there may be some options, but your question as asked is simply impossible.
I created a custom attribute class that will check the system security and throws an authentication exception if there is a security error.
public class EntityChecker: System.Attribute
{
public EntityChecker(int entityId)
{
// doing some logic to check if the entityId is allowed to be inserted
}
}
I want to use this custom attribute as a declaration to an entity addition function and I want to pass a variable from the function to the attribute constructor. can something like this be done?
[EntityChecker(entityId)]
public int AddNewEntity(entityId)
{
// logic of entity addition
}
Can something like this be done ?!
No. Constructor parameters in attributes must be resolved at compile time. They are intended as metadata on the type or method itself, not something that would be used per call or per instance.
Given your description, an attribute is likely not an appropriate way to handle this. Since you want to run extra code that happens per call, you will need a different technique. For example, you could pass a delegate, ie:
public int CheckedAddEntity(int entityId, Func<int, int> funcToAdd)
{
// Perform your checking on entityId here
return funcToAdd();
}
This would let you then call via something like:
int result = CheckedAddEntity(entityId, AddNewEntity);
In this case, I recommend looking at Aspect-Oriented programming. It is a different way of doing code, but one that allows you to re-use the boilerplate logic (e.g. authentication) throughout. You might have to design your attribute a little bit differently, but all of the logic can be put into an "aspect" which then gets compiled automatically into the code when you build the project.
I personally use PostSharp, although I know there are others out there. They have a free license available for development; as long as you don't require advanced functionality, it's very cost-effective.
http://www.postsharp.net/blog/post/5-Ways-That-Postsharp-Can-SOLIDify-Your-Code-Authorization
Thanks for looking!
Background
I have an extension method that is used to wrap a given method in a try/catch and I am adding code for logging any caught exceptions:
public static T HandleServerError<T>(this Func<T> func)
{
T result = default(T);
try
{
result = func();
}
catch (Exception ex)
{
//******************************
//Code for logging will go here.
//******************************
ErrorHandlers.ThrowServerErrorException(ex);
}
return result;
}
Here is how the method is called:
var result = new Func<SomeClass.SomeType>(() => SomeClass.SomeMethod(id, name, color, quantity)).HandleServerError();
return result;
As you can see, whatever method I am calling is injected into the extension method and executed inside the try/catch.
We will be using NLog or ELMAH for logging, but that is largely irrelevant to this question.
Problem
If something goes wrong, I need to log as much information about the delegated method as possible since things like "Object reference not set to an instance of an object" is not in itself helpful.
I would like to log the class and name of the method being called as well as the parameters in the method signature along with their values. If possible, I would even like to log which line failed, and finally the actual stack trace.
I am guessing that I need to use reflection for this and maybe catch the binding flags somehow as the injected method executes but I am not entirely sure if that is the best approach or if it is even feasible.
Question
Using C#, how do I get the meta information (i.e. method name, class of origin, parameters, parameter values) about an injected/delegated method?
Thanks in advance.
It seems to me that there is a possibility for you to improve the way you are adding this logging cross-cutting concern to your application.
The main issue here is that although your solution prevents you from making any changes to SomeClass.SomeMethod (or any called method), you still need to make changes to the consuming code. In other words you are breaking the Open/closed principle, which tells us that it must be possible to make these kinds of changes without changing any existing code.
You might think I'm exaggerating, but you probably already have over a hundred calls to HandleServerError in your application, and the number of calls will only be growing. And you'll soon add even more of those 'functional decorators' to the system pretty soon. Did you ever think about doing any authorization checks, method argument validation, instrumentation, or audit trailing? And you must admit that doing new Func<T>(() => someCall).HandleServerError() just feels messy, doesn't it?
You can resolve all these problems, including the problem of your actual question, by introducing the right abstraction to the system.
First step is to promote the given method arguments into a Parameter Object:
public SomeMethodParameters
{
public int Id { get; set; }
public string Name { get; set; }
public Color Color { get; set; }
public decimal Quantity { get; set; }
public decimal Result { get; set; }
}
Instead of passing all the individual arguments into a method, we can pass them all together as one single object. What's the use of that, you may say? Read on.
Second step is to introduce a generic interface to hide the actual logic of the SomeClass.SomeMethod (or in fact any method) behind:
public interface IMethodHandler<TParameter>
{
void Handle(TParameter parameter);
}
For each (business) operation in the system, you can write an IMethodHandler<TParameter> implementation. In your case you could simply create an implementation that wraps the call to SomeClass.SomeMethod, like this:
public class SomeMethodHandler
: IMethodHandler<SomeMethodParameters>
{
void Handle(SomeMethodParameters parameter)
{
parameter.Result = SomeClass.SomeMethod(
parameter.id,
parameter.Name,
parameter.Color,
parameter.Quantity);
}
}
It might look a bit silly to do things like this, but it allows you to implement this design quickly, and move the logic of the static SomeClass.SomeMethod inside of the SomeMethodHandler.
Third step is let consumers depend on a IMethodHandler<SomeMethodParameters> interface, instead of letting them depend on some static method in the system (in your case again the SomeClass.SomeMethod). Think for a moment what the benefits are of depending on such abstraction.
One interesting result of this is that it makes it much easier to unit test the consumer. But perhaps you're not interested in unit testing. But you are interested in loosely coupling. When consumers depend on such abstraction instead of a real implementation (especially static methods), you can do all kinds of crazy things, such as adding cross-cutting concerns such as logging.
A nice way to do this is to wrap IMethodHandler<T> implementations with a decorator. Here is a decorator for your use case:
public class LoggingMethodHandlerDecorator<T>
: IMethodHandler<T>
{
private readonly IMethodHandler<T> handler;
public LoggingMethodHandlerDecorator(
IMethodHandler<T> handler)
{
this.handler = handler;
}
public void Handle(T parameters)
{
try
{
this.handler.Handle(parameters);
}
catch (Exception ex)
{
//******************************
//Code for logging will go here.
//******************************
ErrorHandlers.ThrowServerErrorException(ex);
throw;
}
}
}
See how the Handle method of this decorator contains the code of your original HandleServerError<T> method? It's in fact not that much different from what you were already doing, since the HandleServerError 'decorated' (or 'extended') the behavior of the original method with new behavior. But instead of using method calls now, we're using objects.
The nice thing about all this is, is that this single generic LoggingMethodHandlerDecorator<T> can be wrapped around every single IMethodHandler<T> implementation and can be used by every consumer. This way we can add cross-cutting concerns such as logging, etc, without both the consumer and the method to know about this. This is the Open/closed principle.
But there is something else really nice about this. Your initial question was about how to get the information about the method name and the parameters. Well, all this information is easily available now, because we've wrapped all arguments in an object instead of calling some custom method wrapped inside a Func delegate. We could implement the catch clause like this:
string messageInfo = string.Format("<{0}>{1}</{0}>",
parameters.GetType().Name, string.Join("",
from property in parameters.GetType().GetProperties()
where property.CanRead
select string.Format("<{0}>{1}</{0}>",
property.Name, property.GetValue(parameters, null)));
This serializes the name of the TParameter object with its values to an XML format. Or you can of course use .NET’s XmlSerializer to serialize the object to XML or use any other serialization you need. All the information if available in the metadata, which is quite nice. When you give the parameter object a good and unique name, it allows you to identify it in the log file right away. And together with the actual parameters and perhaps some context information (such as datetime, current user, etc) you will have all the information you need to fix a bug.
There is one difference between this LoggingMethodHandlerDecorator<T> and your original HandleServerError<T>, and that is the last throw statement. Your implementation implements some sort of ON ERROR RESUME NEXT which might not be the best thing to do. Is it actually safe to continue (and return the default value) when the method failed? In my experience it usually isn't, and continuing at this point, might make the developer writing the consuming class think that everything works as expected, or might even make the user of the application think that everything worked out as expected (that his changes were saved for instance, while in fact they weren't). There's usually not much you can do about this, and wrapping everything in catch statements only makes it worse, although I can imagine that you want to log this information. Don’t be fooled by user requirements such as “the application must always work” or “we don’t want to see any error pages”. Implementing those requirements by suppressing all errors will not help and will not fix the root cause. But nonetheless, if you really need to catch-and-continue, just remove the throw statement`, and you'll be back at the original behavior.
If you want to read more about this way of designing your system: start here.
You can simply access its Method and Target properties as it's basically any other delegate.
Just use func.Method and func.Target.
I would like to be able to keep a C# API the same as it is now, but simply deprecate one of the parameters in a method call. Is it possible to do so, or do I need to create a new method without the parameter and mark the original one as Obsolete?
Short answer:
You will need to create a new nethod with the new signature, and mark the current as obsolete.
Longer answer
What you want to avoid at all cost is a code break! Then, particularly in a company framework, you want to advertise that your method will no longer be supported, for example, but you do not want to be responsible for depending solutions to crash because of an architecture or design decision or your side, right?
The ObsoleteAttribute class will do the trick for you.
Once a class member marked as obsolete, a warning will be raised on the client-side, the ones who use your framework, to continue that way, or even one of your colleague under the same project.
public class MyClass {
[Obsolete("This method should no longer be used, please use MyNewMethod() instead.")]
public void MyMethod(string name, long phoneNumber, long faxNumber) {
}
public void MyNewMethod(string name, long phoneNumber, long faxNumber, string email) {
}
}
This will advertise that MyMethod is no longer supported throughout your code users.
After a certain period of time, reasonable enough to allow everyone to change his/her code, you may tell this attribute to throw an error message when your obsolete method is still used in the code.
public class MyClass {
[Obsolete("This method should no longer be used, please use MyNewMethod() instead.", true)]
public void MyMethod(string name, long phoneNumber, long faxNumber) {
}
public void MyNewMethod(string name, long phoneNumber, long faxNumber, string email) {
}
}
By setting the second ObsoleteAttribute class constructor parameter to true, you tel the compiler to advertise the use of this method as an error.
After some time only, you can completely remove your method from your code to clean it up a little. This is part of the refactoring methods encouraged by the Agile Software Development methodology.
Does this help?
Yes, I think the only way is to create a new method without the parameter and mark the original one with ObsoleteAttribute.
With the Obsolete attribute:
[Obsolete("Please use void AnotherMethod() instead", false)]
void SomeMethod(Int32 data){
}
The Boolean in the attribute tells the compiler to generate a warning, if you change it to true the compiler will generate an error. See here for the documentation on it.
Yes. You can only mark types and members of types as obsolete, not individual parameters. You could, however, overload the method, thereby keeping the same name and mark the old one as obsolete:
class Act
{
[Obsolete("DoSomething(int, int) is obsolete", false /*warn only*/)]
public void DoSomething(int i, int j)
{
}
public void DoSomething(int i)
{
}
}
I think it would be best to create a new method.
What do you mean by making the parameter obsolete? Does this depend on a specific value for that argument? When you call a method, you always have to specify the argument anyway.