I have a number of data classes that have over 25 properties of different value types (and this may change in the future as requirements change). I would like to override equals, mostly for unit testing purposes.
Currently, the only way I know how to do this is to actually test for equality of each property hard coded. This seems bad for two reasons - first, I will have to write a lot of code to test 25 properties for equality - second, if a property in one of the classes is added at a later point in time, the Equals method will not check that, and most likely this will go unnoticed and lead to problems down the road.
Since Equals usually checks for the properties of classes, there should be a way to dynamically compare the properties of the classes being compared, which ensures that property changes to a class don't result in an incorrect implementation of Equals. Is there a way to do this?
you could write something like this using reflection - but this would be very slow.
I would stick with overriding equals but think about which part you really need for equal. I usually only check the immutable parts (like Id) for equality and just ignore the mutable fields and I think this is a good practice.
Try using reflection to compare the properties. See Comparing object properties in c# for more info!
If your class is an entity, it should have a property which allows you to uniquely identify each instance.
If your class is implemented as a value type, you'll have to check for equality by checking each property.
In the latter case, in order to prevent tedious work, you could make use of reflection to get all properties of the class at runtime, retrieve the value and use the TypeDescriptor classes to compare the values.
You can use some AOP Frameworks. If the properties you're goona to compare are much more then those ones you gonna to avoid, mark the properties to skip with special custom attribute.
Maybe T4 can help you out. With it you can generate code at a click. Within this function you can then use the slow reflection mechanism to create a hard-coded GetHashCode() function that will be called at runtime. For a first look into T4 take a look at Scotts blog about it. Or simply try searching for Text Template Transformation Toolkit with your favorite search engine.
Related
I have a class with lots of, lots of, lots of properties.
In my programm i have to log heavily. Often I have to manually
build log strings like
string log = "Current state of object:" + "Property1" + myObj.Property1 + ...
I just thought what if override ToString and provide logging of whatever I need. It is considered goog practice?
How can I provide formatter control string? Say I want my ToString to operate in two modes
one is complete ouput of all properties and another light version wheere only relevant properties are output
Something like MyObj.ToString("full") and MyObj.ToString("basic")
Overriding ToString is indeed good practice, so long as you provide good information.
As for having different types of ToString - this is possible, but then you will not be overriding ToString, but providing an overload (that any using class will need to know about).
I would implement the lightweight version as the override and create a VerboseToString function for the full set of properties.
If you have many such objects, you can create a IVerboseString interface with a VerboseToString method and implement it in your objects. This way you can simply use the interface reference in your logging.
That's an excellent practice
You could, of course, parametrize the ToString method, but it would no longer mean an implicit conversion from object to string, so you can't write, say, string x = myObject; you will have to call ToString explicitly.
Why are you overriding ToString but not creating a method that takes Enum as parameter and does what you want inside?
I can't think of any particular problem associated with it, and myself I often appreciate it for spitting out info in unit tests or trace etc. but I'd take issue with your implementation for verbose and concise outputs - don't use voodoo strings or any kind of arbitrary input - I'd prefer it here id you either took an input argument bool verbose or you explicitly created a ToStringVerbose() method. It's also good practice IMHO for your overridden method to use base.ToString() as part of it's implementation.
should all c# classes override Equals and GetHashCode? For correctness
No, they already do.
Whether you have to override them, is up to how it will be used. In most cases, it is not needed.
All classes already inherit these methods from the base class, System.Object.
You can choose to override the methods in derived classes if you need to be able to compare two instances of an object beyond simple reference equality, otherwise it's not necessary.
Remember, however, that if you choose to override one of them, you also need to override the other in order to ensure that Hashtables and dictionary keys, among other things, work properly with you derived class. The GetHashCode method needs to reflect the same logic as the Equals method. See here for more explanations and examples:
http://msdn.microsoft.com/en-us/library/bsc2ak47.aspx
and
http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx
All classes to inherit this from the System.Object.
If you need to provide a specific Equals or GetHashCode for a class then you should override the methods in your classes. Otherwise just leave them..
http://msdn.microsoft.com/en-us/library/system.object.gethashcode(v=VS.71).aspx
Maybe not all, but all classes that will be put into a some kind of bag (IList, ICollection, IDictionary, Hashset, etc.) and need some simple method to differentiate them (just think about Sort(), Contains(), BinarySearch(), etc.).
If you use a class that way you should definitely implement them correct.
When you override Equals, basically.
When you want to provide a different
idea of equality than simple reference
equality.
String is a good example of this - two
strings are equal (under a simple
Equals call) if they represent the
same sequence of characters. The hash
code reflects this, such that if two
strings are equal they will have the
same hash code. (The reverse isn't
necessarily true - two unequal strings
can have the same hash code, but it's
unlikely.)
(Strings are tricky in other ways,
mind you - there are lots of different
ideas of equality based on culture and
casing, but String.Equals just looks
at the UTF-16 code points which make
up the string, and compares them in
the simplest conceivable fashion.)
by Jon Skeet
Before you start pointing me to duplicates just know that I have read nearly all the posts on SO about extension methods. I am just trying to play devil's advocate for a minute to consider the alternative to my working opinion.
Recently I was working on a project and a need came up for a method to be a base of an interface. So I suggested we write an extension method and it was shot down. Saying it added complexity and harder to debug.
I of course argued and got on SO to find all the wonderful posts that show the many reasons why to use extension methods. Not to forget that a lot of the .net framework uses them. We eventually did not use it as I was overruled by the team.
But then it got me thinking, are there times when an extension method could be used but shouldn't be?
I really couldn't think of any but thought I would post here and see if anyone could think of any other reasons why they shouldn't be used.
Any time you have a function which is "generally applicable" to an object of a certain type, regardless of its state, an extension method is a good choice.
For example, today I added two new extension methods to our codebase:
public static XElement ToXElement(this XmlElement element) { }
public static XmlElement ToXmlElement(this XElement element) { }
Both of these are, generally speaking, valid on the types they extend regardless of the state of the instance or where we are using it.
If your method does not meet that criteria, it should probably be moved to a helper method closer to the context where the particular case is always true or easily checked.
For example, a developer recently nominated this to be an extension method:
public static bool ParseYesNoBool(this string input) { }
There are two problems here: first, this will appear on all strings in the application, even though the number of strings which might ever be candidates for this case are very small. So we've broken the first rule, in that it is not useful regardless of state. Similarly, but second, the consumer of this functionality is limited to a single parser for one particular connector to an external system. So promoting implementation-specific functionality into the general-use namespace makes no sense. This was downgraded to a helper method in the parser.
As far as readability and debugging, that is just incorrect for a developer of any reasonable skill level.
In general if you control the source-code for the assembly and adding the method does not cause any breaking changes to existing code (which would have been the case if for example LINQ had not been implemented via extension methods) it is better to just add a normal method.
This discussion of the Framework Design Guildelines section on extension methods contains some good advice. I think the relevant portion for your scenario is:
To provide helper functionality relevant to every implementation of an interface, if said functionality can be written in terms of the core interface.
If your proposed usage did not pass that test then it should have been shot down.
I would say you should avoid them when "they do not make the intend of the code more clear". Of course, whether or not some code (or codeing style) is "more clear" varying widely between people, so that's pretty much useless. (I had one boss who said we shoudl avoid using interfaces because they made the code "too complex and hard to understand")
Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
Any time you break the intent and design for the feature I would recommend reconsidering the use of an extension method. I see a few situations when you don't want to use an Extension method:
1) Changing the Object Model to allow for an Extension method: The class you want to create an extension on is an abstract class. This is going to require you either make each inherited class it's own version of the extension or remove abstract from the class. Either way, you are changing the object model in order to use an extension method.
2) Forgetting the Decorator Pattern: The number of extension methods you create for a class exceeds three. I find it is easier to organize/communicate and maintain the domain/object model with decorated objects than with extended objects. However, the opposite is also true: If a decorated object has less than four methods, I find a lot of almost "empty" objects in my project.
3) Private functions: Private functions are meant to modify(create, delete, etc..) the object and extension methods are meant to use the type, much like a struct would. If you find the extension is being assigned to another instance of the type then it probably should not be in an extension.
This question already has answers here:
Are there any reasons to use private properties in C#?
(19 answers)
Closed 9 years ago.
For private class variables, which one is preferred?
If you have a property like int limit, you want it to be:
int Limit {get; set;}
and use it inside the class, like so:
this.Limit
Is there a reason to use it or not use it? Maybe for performance reasons?
I wonder if this is a good practice.
For a private member, I only make it a property when getting and/or setting the value should cause something else to occur, like:
private int Limit
{
get
{
EnsureValue();
return this._limit;
}
}
Otherwise, fields are fine. If you need to increase their accessibility, it's already a big enough change that making it a property at that point isn't a huge deal.
Edit: as Scott reminds us in the comments, side effects in properties can often cause more pain than anything else. Don't violate Single Responsibility and limit property logic to consistent, logical operations on the value only that must be done at the gate - such as lazy loading (as in the example above), transforming an internal structure into a publicly-useful format, etc.
The only real benefit an auto-property has over a field when the accessibility is private is that you can set a breakpoint on accesses and updates of the variable. If that is important to your scenario then definitely use an auto-property. Otherwise, given there is no substantial advantage, I choose to go with the simplest construct which is a field.
I would say its good practice to use a property. If ever you had to expose the limit value and used a local member it will require more coding while if its a property it would only require a change of its modifier.
I think it's cleaner also.
Granted, since it's a private API, its an implementation detail - you can do whatever you want here. However, there is very little reason to not use a property, even for private classes. The properties get inlined away by the JIT, unless there is extra code in place, so there isn't really a performance impact.
The biggest reasons to prefer properties, IMO, are:
Consistency in your API - You'll want properties in publicly exposed APIs, so making them in the private API will make your programming exprience more consistent, which leads to less bugs due to better maintainability
Easier to convert private class to public
From my perspective, using properties in lieu of variables boils down to:
Pros
Can set a break point for debugging, as Jared mentioned,
Can cause side-effects, like Rex's EnsureValue(),
The get and set can have different access restrictions (public get, protected set),
Can be utilized in Property Editors,
Cons
Slower access, uses method calls.
Code bulk, harder to read (IMO).
More difficult to initialize, like requiring EnsureValue();
Not all of these apply to int Limit {get; set;} style properties.
The point of automatic properties is they are very quick at creating a public access to some field in your class. Now, they offer no benefit over exposing straight up fields to the outside world, other than one big one.
Your class' interface is how it communicates with the outside world. Using automatic properties over fields allows you to change the internals of your class down the road in case you need to make setting the value of that property do something or check authorization rules or something similar on the read.
The fact that you already have a property means you can change your implementation without breaking your public interface.
Therefore, if this is just a private field, an automatic property isn't really that useful, not only that, but you can't initialize public properties at declaration like you can with fields.
I generally follow the following principle: If it's for strictly private use, use a field as it is faster.
If you decide that it should become public, protected or internal some day, it's not difficult to refactor to a property anyway, and with tools like ReSharper, it takes about 3 seconds to do so... :)
There's nothing wrong with having private or protected properties; this is mostly useful when there is some rule or side effect associated with the underlying variable.
The reason why properties seem more natural for public variables is that in the public case, it is a way to hedge one's bet against future implementation changes, whereby the property will remain intact but the implementation details somehow move around (and/or some additional business rule will be needed).
On performance, this is typically insignificant, or indeed identical for straight-assignment properties.
I personally dislike (but often use) plain assignment properties because they just clutter the code. I wish C# would allow for "after the fact refactoring".
Properties provide some very good automatic features (like Json and Xml Serialization)
Fields do not.
Properties can also be a part of an Interface. If you decide to refactor later on... this might be something to consider too.
Properties are just syntactic sugar, C# will compile them into get_PropertyName and set_PropertyName, so performance differences are not a consideration.
If your data member need only set and get logic then properties are very good and fast solution in C#
Can I rely on the fact that the underlying field to a property named Foo is called "k__BackingField" ?
No, in short.
And for this very reason, auto-properties are pain when used with field-based serializers like BinaryFormatter (I'm sure I might have mentioned that here before ;-p).
See: Obfuscation, serialization and automatically implemented properties for some thoughts on this (and a solution).
That the spec does not specify the backing field's name (with good reason - you shouldn't take a dependency on it).
You should investigate to see whether the Mono compiler does the same thing for example.
Sanity checking that the field had the CompilerGeneratedAttribute applied to it would be a good idea.
Note that explicit interface implementations would come out (currently) as:
<Full.Namespace.To.Interface.IBlah.Foo>k__BackingField
So code which is trying to serialize a specific aspect of a class in this fashion could be mislead.
If you wanted to be really belt and braces about your code you could use a library like mono's Cecil to inspect the get function of the property and determine the field used.
Of course not. That would be a private implementation detail.
And why on Earth would you want to know?
I don't think so. If you need the backing field, use a 'regular' property.