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Multiple classes in a single .cs file - good or bad? [duplicate]

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Is it a bad practice to have multiple classes in the same file?
Is it advisable to create multiple classes within a .cs file or should each .cs file have an individual class?(Class file name also Animal.cs)
public class Animal
{
}
public class Person
{
}
public class Utility
{
}

Simply say good practice is one class from one file. BUT if your application is quite small you can use several classes in one file.

When designing classes we respect the Single Responsibility Principle. Reading code becomes a lot easier if its shape follows its semantics, hence splitting files by class is sensible.
However if there are inner classes it makes sense to keep them in the same file
Source: Old Post

For the sake of making things simple, you should probably put them in different files. I can't think of any advantages of keeping them in the same file.

It depends about your application. If it is a big application, I think that isn't good. Data must be very organized.
I think that is better to keep them in different files.
But, if you are working for a small application, it is good to keep them in same file.
Hope that I helped you.

Logically its depend on your scope of application.
But Normally best practice to do coding in seperate class (New File , new class )
Splittting files will be more sensible.Because industries follow ths standards...

c# multiple classes in separate files?

I just wondered what other peoples thoughts were regarding related classes in a single or separate .cs file?
If, for example, I have an interface that is implemented by, say an arbitrary 10, other classes, would you place them all in the same file or separate them?
Thanks.

I always go with separate files for each class. It's recommended best practice and it really makes sense.

My approach is that 1 file == 1 class/interface/module/... whatever.
So the filename always reflects what's in there. To me that's the cleanest approach.

I would separate classes into different files. This makes them a lot easier to find in the IDE.

I would place each class in a separate file, and the interface in a separate file as well.
I would give the file the following name .cs
That's a recommended best practice; it allows you to find your classes very fast. I always go with this approach (except when I have inner classes offcourse. :) ).

I must agree with the rest here: 1 class = 1 file.
Also use correct namespacing for full project name as well as folders. Interfaces also go into separate files, but I usually keep enums and structures inside other classes.
Folders can be used to group certain classes together. There is however a small issue when you "run out of names" so to speak.
Example:
Solution: Tedd.CoolApp
Project: Tedd.CoolApp.Engine
Now what do I name the class? I want to name it Engine, but that would give me Tedd.CoolApp.Engine.Engine... :)

The computer could care less about the folder structure you concoct, so this question definitely falls under the category of code readability. As mentioned in this post about standards of code readability, friendly naming, consistency, and logical code separation are fundamental to the creation of readable code.
So, where does that leave us? The creation of files--and the creation of namespaces and file regions--should be consistent. The names should be understandable. And the code in each aggregate category should have something in common, as should be detailed in the category name. Ultimately, with readability, you're considering that your code might be inherited by another poor fellow, and that the naming standards that you've created might help that poor fellow (a "tourist developer", if you will) more easily navigate around in the madness.
That's a lot of talking, so let me get down to brass tacks. These are my rules, but I think they might be helpful to those who are looking to clean up their own code aquariums:
Place one class (or one interface, enum, or struct)
in one file.
The name of the class should be the
name of the file.
Classes that inherit from the same base class should be in the same folder.
If at all possible, a class should be in the same folder as the interface that that class implements.
An interface should have the same name as the class, but should be prefixed with a capitalized "I". It's the only bit of coding advice I still respect from the Hungarians.
The folder name should be a pluralized version of the base class. For example, if we're creating a bunch of Engines, Engine should be the base class name, Engines should be the folder name, and all of the classes that inherit from Engine should be in the Engines folder.
The namespace structure should directly follow the folder structure. So, the namespace for a given set of Engines (example from above) should be placed into a namespace called Engines. If Engines is a subfolder of a subfolder, each subfolder should be its own sub-namespace, e.g. Project1.Subfolder1.Subfolder2.Engines.
When you're dealing with partial classes that need to live in two separate folders (as one piece of the class is autogenerated), place the non-autogenerated class into a folder suffixed with Extensions. In the file, comment out the Extensions namespace like so: namespace FatDish.Engines//.EngineExtensions { ...
When it comes to navigability, the first and second rule are key, as they directly aid in indicating to the "tourist developer" where any given piece of code resides.
That's all I can think of at the moment. It's more important that you're consistent in your conventions than it is that you adopt any particular form of conventionality. That will help other developers understand and consume your code at a quicker rate, and ensure that future developments in the project (written by folks other than yourself) stay within the same conventional, coherent bounds that you've established.
Hope this helps!

Personally I adhere to Single Responsibility Principle where each of my classes has a single behaviour
think of a ecommerce site that has
User Registration
User Login
billing
Supplier Ordering
I would separate these out to a User class, Billing Class and Orders class - the same would then adhere for an interface driven approach - 1 interface for each Responsibility
check out SOLID design principles - each class would then be in owns own file and have a suitable naming convention to help

class definition and implementation in C# vs C++

With C++, I can have one class definition in a header file, and have a multiple implementation files by including the header file.
With C#, it seems that there is no such header file, as one class should contain both definition/implementation.
I wonder if the number of lines can be very big, because one can't separate the class into multiple files. Am I correct? I mean, in some cases, one can't change the class design to have smaller classes. In this case, is there a way to solve this problem?

You can separate a class into multiple files using the partial keyword
public partial class ClassNameHere
{
}

It is possible to split the definition of a class or a struct, or an interface over two or more source files using the Partial keyword modifier Link to msdn with the partial class

Partial classes only give you so much. There is still no way, that i know of, to split your class definition from implementation, such that each exists in a separate file. So if you like to develop based on a need-to-know paradigm then you are sort of stuck. Basically there are three levels a developer can work at...
1) Owns all the code and has access to, and maintains all of it.
2) Wishes to use some useful base class(s) which may form part of a framework, or may just be a useful class with some virtual methods, etc, and wishes to extend, or re-implement some virtual base class methods of interest. Now the developer should not need to go and look at the code in the base class(s) in order to understand things at a functional level. If you understand the job of a function, it's input and output parameters, there is no need to go and scratch inside source code. If you think there's a bug, or an optimization is needed, then refer to the developer from 1) who owns and maintains the base code. Of course there's nothing saying that 1) and 2) cannot be associated with the same developer, in which case we have no problem. In fact, this is more than often the case i suspect. Nevertheless, it is still good practice to keep things well separated according to the level at which you are working.
3) A developer needs to use an already packaged / sealed object / component dll, which exposes the relevant interfaces.
Within the context of c#, 1) and 3) have no problems. With 2) i believe there is no way to get round this (unless you change from exposing virtual base methods to exposing interface methods which can be reimplemented in a component owning the would-be base class). If i want to have a look at a class definition to browse over the methods, scaffolding functions, etc, i have to look at a whole lot of source code as well, which just gets in the way of what i am trying to focus on.
Of course if there is class definition documentation external to how we normally do it ( in headers and source files), then i must admit, that within the context of 2), there is not reason to ever look into a class definition file to gain functional knowledge.
So maybe clever Tom's came up with c#, decided to mix class definition with implementation in an attempt to encourage developers to have external documents for their class definitions, and interfaces, which in most IT companies is severely lacking.

Use a partial class as #sparks suggests, or, split into several classes. It's a good rule of thumb that, if you can't fit a class onto a couple of pages, it's complicated enough to need breaking apart.

Creating a Catch-All AppToolbox Class - Is this a Bad Practice?

Never sure where to place functions like:
String PrettyPhone( String phoneNumber ) // return formatted (999) 999-9999
String EscapeInput( String inputString ) // gets rid of SQL-escapes like '
I create a Toolbox class for each application that serves as a repository for functions that don't neatly fit into another class. I've read that such classes are bad programming practice, specifically bad Object Oriented Design. However, said references seem more the opinion of individual designers and developers more than an over-arching consensus. So my question is, Is a catch-all Toolbox a poor design pattern? If so, why, and what alternative is there?

Great question. I always find that any sufficiently complex project require "utility" classes. I think this is simply because the nature of object-oriented programming forces us to place things in a neatly structured hierarchical taxonomy, when this isn't always feasible or appropriate (e.g. try creating an object model for mammals, and then squeeze the platypus in). This is the problem which motivates work into aspect oriented programming (c.f. cross cutting concern). Often what goes into a utility class are things that are cross-cutting concerns.
One alternative to using toolbox or utility classes, are to use extension methods to provide additional needed functionality to primitive types. However, the jury is still out on whether or not that constitutes good software design.
My final word on the subject is: go with it if you need, just make sure that you aren't short-cutting better designs. Of course, you can always refactor later on if you need to.

I think a static helper class is the first thing that comes to mind. It is so common that some even refer to it as part of the object-oriented design. However, the biggest problem with helper classes is that they tend to become a large dump. I think i saw this happen on a few of the larger projects i was involved in. You're working on a class and don't know where to stick this and that function so you put it in your helper class. At which point your helpers don't communicate well what they do. The name 'helper' or 'util' itself in the class name doesn't mean anything. I think nearly all OO gurus advocate against helpers since you can very easily replace them with more descriptive classes if you give it enough thought. I tend to agree with this approach as I believe that helpers violate the single responsibility principle. Honestly, take this with a grain of salt. I'm a little opinionated on OOP :)

In these examples I would be more inclined to extend String:
class PhoneNumber extends String
{
public override string ToString()
{
// return (999) 999-9999
}
}
If you write down all the places you need these functions you can figure out what actually uses it and then add it to the appropriate class. That can sometimes be difficult but still something you should aim for.
EDIT:
As pointed out below, you cannot override String in C#. The point I was trying to make is that this operation is made on a phone number so that is where the function belongs:
interface PhoneNumber
{
string Formatted();
}
If you have different formats you can interchange implementations of PhoneNumber without littering your code with if statements, e.g.,
Instead of:
if(country == Countries.UK) output = Toolbox.PhoneNumberUK(phoneNumber);
else ph = Toolbox.PhoneNumberUS(phoneNumber);
You can just use:
output = phoneNumber.Formatted();

There is nothing wrong with this. One thing is try to break it up into logical parts. By doing this you can keep your intellisense clean.
MyCore.Extensions.Formatting.People
MyCore.Extensions.Formatting.Xml
MyCore.Extensions.Formatting.Html

My experience has been that utility functions seldom occur in isolation. If you need a method for formatting telephone numbers, then you will also need one for validating phone numbers, and parsing phone numbers. Following the YAGNI principle, you certainly wouldn't want to write such things until they're actually needed, but I think it's helpful to just go ahead and separate such functionality into individual classes. The growth of those classes from single methods into minor subsystems will then happen naturally over time. I have found this to be the easiest way to keep the code organized, understandable, and maintainable over the long term.

When I create an application, I typically create a static class that contains static methods and properties that I can't figure out where to put anywhere else.
It's not an especially good design, but that's sort of the point: it gives me a place to localize a whole class of design decisions that I haven't thought out yet. Generally as the application grows and is refined through refactoring, it becomes clearer where these methods and properties actually ought to reside. Mercifully, the state of refactoring tools is such that those changes are usually not exceptionally painful to make.
I've tried doing it the other way, but the other way is basically implementing an object model before I know enough about my application to design the object model properly. If I do that, I spend a fair amount of time and energy coming up with a mediocre solution that I have to revisit and rebuild from the ground up at some point in the future. Well, okay, if I know I'm going to be refactoring this code, how about I skip the step of designing and building the unnecessarily complicated classes that don't really work?
For instance, I've built an application that is being used by multiple customers. I figured out pretty early on that I needed to have a way of separating out methods that need to work differently for different customers. I built a static utility method that I could call at any point in the program where I needed to call a customized method, and stuck it in my static class.
This worked fine for months. But there came a point at which it was just beginning to look ugly. And so I decided to refactor it out into its own class. And as I went through my code looking at all the places where this method was being called, it became extremely clear that all of the customized methods really needed to be members of an abstract class, the customers' assemblies needed to contain a single derived class that implements all of the abstract methods, and then the program just needed to get the name of the assembly and the namespace out of its configuration and create an instance of the custom features class at startup. It was really simple for me to find all of the methods that had to be customized, since all I needed to do was find every place that my load-a-custom-feature method was being called. It took me the better part of an afternoon to go through the entire codebase and rationalize this design, and the end result is really flexible and robust and solves the right problem.
The thing is, when I first implemented that method (actually it was three or four interrelated methods), I recognized that it wasn't the right answer. But I didn't know enough to decide what the right answer was. So I went with the simplest wrong answer until the right answer became clear.

I think the reason it's frowned upon is because the "toolbox" can grow and you will be loading a ton of resources every time you want to call a single function.
It's also more elegant to have the methods that apply to the objects in the actual class - just makes more sense.
That being said, I personally don't think it's a problem, but would avoid it simply for the reasons above.

I posted a comment, but thought I'd elaborate a bit more.
What I do is create a Common library with namespaces: [Organisation].[Product].Common as the root and a sub namespace Helpers.
A few people on here mention things like creating a class and shoving some stuff they don't know where else to put in there. Wrong. I'd say, even if you need one helper method, it is related to something, so create a properly named (IoHelper, StringHelper, etc.) static helper class and put it in the Helpers namespace. That way, you get some structure and you get some sort of separation of concerns.
In the root namespace, you can use instance utility classes that do require state (they exist!). And needless to say also use an appropriate class name, but don't suffix with Helper.

C# classes in separate files? [closed]

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Should each class in my C# project get its own file (in your opinion)?

While the one class per file policy is strictly enforced in Java, it's not required by C#. However, it's generally a good idea.
I typically break this rule if I have a very small helper class that is only used by the main class, but I prefer to do that as a nested inner class for clarity's sake.
You can however, split a single class into multiple files using the partial keyword. This is useful for separating your code from wizard-generated code.

Files are cheap, you aren't doing anyone a favor by consolidating many classes into single files.
In Visual Studio, renaming the file in Solution Explorer will rename the class and all references to that class in your project. Even if you rarely use that feature, the cheapness of files and the ease of managing them mean the benefit is infinitely valuable, when divided by its cost.

As others have said, one file per type in general - although where others have made the public/private distinction, I'd just say "one top-level file per type" (so even top-level internal types get their own files).
I have one exception to this, which is less relevant with the advent of the Func and Action delegate types in .NET 3.5: if I'm defining several delegate types in a project, I often bunch them together in a file called Delegates.cs.
There are other very occasional exceptions too - I recently used partial classes to make several autogenerated classes implement the same interface. They already defined the appropriate methods, so it was just a case of writing:
public partial class MessageDescriptor : IDescriptor<MessageDescriptorProto> {}
public partial class FileDescriptor : IDescriptor<FileDescriptorProto> {}
etc. Putting all of those into their own files would have been slightly silly.
One thing to bear in mind with all of this: using ReSharper makes it easier to get to your classes whether they're in sensibly named files or not. That's not to say that organising them properly isn't a good thing anyway; it's more to reinforce the notion that ReSharper rocks :)

I personally believe that every class should be in its own file, this includes nested types as well. About the only exceptions to this rule for me are custom delegates.
Most answers have excluded private classes from this rule but I think those should be in their own file as well. Here is a pattern that I currently use for nested types:
Foo.cs: // Contains only Foo implementation
public partial class Foo
{
// Foo implementation
}
Foo.Bar.cs: // Contains only Foo.Bar implementation
public partial class Foo
{
private class Bar
{
// Bar implementation
}
}

It depends. Most of the time I would say yes, put them in separate files. But if I had a private helper class that would only be used by one other class (like a Linked List's Node or Element) I wouldn't recommend separating them.

As someone who has been coding in large files for years (limited to 1,000 lines), in fact, since I started programming as a child, I was surprised at the huge consensus in this "one class per source file" rule.
The "one class per source file" is not without its problems. If you are working on a lot of things at once, you will have many files open. Sure, you could close files once you're finished with them, but what if you needed to re-open them? There is usually a delay every time I open a file.
I am now going to address points others have made and explain what I think are bad reasons for the "one class per source file" rule. A lot of the problems with multiple classes in one source file are resolved with modern source-editing technology.
"I hate having to scroll up and down" - Bad Reason - Modern IDEs now either have built-in functionality for getting quickly to the code you want or you can install extensions/plugins for that task. Visual Studio's Solution Explorer does this with its search function, but if that's not enough, buy VisualAssist. VisualAssist provides an outline of the items in your source file. No need to scroll, but double-click on what you want.
There is also code-folding. Too much code? Just collapse it into one line! Problem solved.
"Things are easier to find because they're identified by file" - Bad Reason - Again, modern IDEs make it easy to find what you're looking for. Just use Solution Explorer or buy VisualAssist!! The technology is out there, use it!!
"Easier to read/too much code" - Bad Reason - I am not blind. I can see. Again, with code-folding I can easily eliminate the clutter and collapse the code I don't need to see. This is not the Stone Age of programming.
"You will forget where the classes are in large projects" - Bad Reason - Easy solution: Solution Explorer in Visual Studio and the VisualAssist extension.
"You know what's in a project without opening anything" - Good Reason - no dispute with that one.
Source Control/Merging - Good Reason - This is actually one good argument in favour of the "one class per source file" rule, especially in team projects. If multiple people are working on the same project. It allows people to see what has changed, at a glance. I can also see how it can complicate merging processes if you use large, multiple-class files.
Source control and merging processes are really the only compelling reason IMO that the "one class per source file" rule should apply. If I'm working on my own individual projects, no, it's not so important.

They should be in different files, even when it seems like overkill. It's a mistake I still frequently make.
There always comes a time when you you've added enough code to a class that it deserves it's own file. If you decide to create a new file for it at that point then you lose your commit history, which always bites you when you lest want it too.

Public classes: yes
Private classes: (needless to say) no

I actually prefer pretty big .cs files, 5000 lines is pretty reasonable IMO, although most of my files at the moment are only about 500-1000 (In C++, however, I've had some scary files), however, . The Object Browser/Class View, Go to Definition, and incremental search (-I; Thanks for that tip, Jeff Atwood!), all make finding any specific class or method pretty easy.
This is probably all because I am terrible about closing unneded tabs.
This is of course highly dependant on how you work, but there are more than enough tools to not need to use horrible old '70s based file source navigation (Joking, if it wasn't obvious).

Of course! Why wouldn't you? Other than private classes it is silly to have multiple classes in a single file.

I think the one-class-per-file approach makes sense. Certainly for different classes, but especially for base and derived classes, whose interactions and dependencies are often non-obvious and error-prone. Separate files makes it straightforward to view/edit base and derived classes side-by-side and scroll independently.
In the days of printed source code listings running to many hundreds of pages (think of a phone book), the "three finger rule" was good a working limit on complexity: if you needed more than three fingers (or paper clips or post-its) as placeholders to understand a module, that module's dependency set was probably too complex. Given that almost no one uses printed source code listings anymore, I'll suggest that this should be updated as the "three window rule" - if you have to open more than three additional windows to understand code displayed in another window, this code probably should be refactored.
A class hierarchy of more than four levels is a code smell, which is in evidence if you need more than four open windows to see the totality of its behavior. Keeping each class in its own file will improve understandability for depth less than four and will give an indirect warning otherwise.