Develop Reference

Should I be using "internal" or "private" instead of "public" for methods that call other methods?

In my C# project I have methods that call other methods like this:
options = ReferenceUtilities.GetMenuStatuses();
In my ReferenceUtilities I have coded:
internal static SelectList GetMenuStatuses()
{
throw new NotImplementedException();
}
But should I be using internal or private? I am not sure of the difference here.

As people have already answered, internal means that the member can be accesed by other code in the same assembly. private means that it can be accessed from other code in the same class.
However, one important point to add:
In Properties/Assemblyinfo.cs, you can add the [assembly: InternalsVisibleTo("something")] statement, that lets you access the internal methods from a different assembly.
This can be extremely useful for unit testing purposes, and is a good reason to sometimes use internal over private.
(There is a huge debate over unit testing internals or not, but it is good to know about the possibility.)

internal means that the member can be accesed by other code in the same assembly. private means that it can be accessed from other code in the same class.
This has nothing to do with whether the method calls other methods.

Internal means that other classes in the same assembly can see the method. Private means only the class where the method is defined can see it. If the method will only ever be called by the class that defines it, use private. Otherwise, use internal. Public should only be used when classes outside the assembly need to call the method directly.
As always, there are exceptions, but this is a good general rule to live by.
Going a bit further, service classes (i.e. methods that exist solely to provide a service or feature) should implement interfaces that define the contract for that service or feature. Other classes should pass around an instance of that interface so that only the interface methods are available.

internal is between assemblies while private is between classes
internal: not visible to code from other assemblies, only visible in this assembly
private: not visible to other classes. only visible in this class
public: visible to other classes or assemblies [for class]

Why use a public method in an internal class?

There is a lot of code in one of our projects that looks like this:
internal static class Extensions
{
public static string AddFoo(this string s)
{
if (s == null)
{
return "Foo";
}
return $({s}Foo);
}
}
Is there any explicit reason to do this other than "it is easier to make the type public later?"
I suspect it only matters in very strange edge cases (reflection in Silverlight) or not at all.

UPDATE: This question was the subject of my blog in September 2014. Thanks for the great question!
There is considerable debate on this question even within the compiler team itself.
First off, it's wise to understand the rules. A public member of a class or struct is a member that is accessible to anything that can access the containing type. So a public member of an internal class is effectively internal.
So now, given an internal class, should its members that you wish to access in the assembly be marked as public or internal?
My opinion is: mark such members as public.
I use "public" to mean "this member is not an implementation detail". A protected member is an implementation detail; there is something about it that is going to be needed to make a derived class work. An internal member is an implementation detail; something else internal to this assembly needs the member in order to work correctly. A public member says "this member represents the key, documented functionality provided by this object."
Basically, my attitude is: suppose I decided to make this internal class into a public class. In order to do that, I want to change exactly one thing: the accessibility of the class. If turning an internal class into a public class means that I have to also turn an internal member into a public member, then that member was part of the public surface area of the class, and it should have been public in the first place.
Other people disagree. There is a contingent that says that they want to be able to glance at the declaration of a member and immediately know whether it is going to be called only from internal code.
Unfortunately, that doesn't always work out nicely; for example, an internal class that implements an internal interface still has to have the implementing members marked as public, because they are part of the public surface of the class.

If the class is internal, it doesn't matter from an accessibility standpoint whether you mark a method internal or public. However it is still good to use the type you would use if the class were public.
While some have said that this eases transitions from internal to public. It also serves as part of the description of the method. Internal methods typically are considered unsafe for unfettered access, while public methods are considered to be (mostly) free game.
By using internal or public as you would in a public class, you ensure that you are communicating what style of access is expected, while also easing the work required to make the class public in the future.

I suspect that "it is easier to make the type public later?" is it.
The scoping rules mean that the method will only be visible as internal - so it really doesn't matter whether the methods are marked public or internal.
One possibility that comes to mind is that the class was public and was later changed to internal and the developer didn't bother to change all the method accessibility modifiers.

I often mark my methods in internal classes public instead of internal as a) it doesn't really matter and b) I use internal to indicate that the method is internal on purpose (there is some reason why I don't want to expose this method in a public class. Therefore, if I have an internal method I really have to understand the reason why it's internal before changing it to public whereas if I am dealing with a public method in an internal class I really have to think about why the class is internal as opposed to why each method is internal.

In some cases, it may also be that the internal type implements a public interface which would mean that any methods defined on that interface would still need to be declared as public.

It's the same, the public method will be really marked as internal since it's inside a internal class, but it has an advantaje(as you guested), if you want to mark the class as public, you have to change fewer code.

For the same reason as using public methods in any other class - so that they're public to the outside of the containing type.
Type's access modifier has exactly zero to do with its members' access modifiers. The two decisions are made completely independently.
Just because certain combinations of type and members' modifiers produce seemingly (or as others call it "effectively") the same result doesn't mean they're semantically the same.
Local access modifier of a an entity (as declared in code) and its global effective access level (as evaluated through the chain of containment) are completely different things, too. An open office inside of a locked building is still open, even though you can't really enter it from the street.
Don't think of the end effect. Think of what you need locally, first.
Public's Public: classic situation.
Public's Internal: type is public but you want some semi-legal access in the assembly to do some hacky-wacky stuff.
Internal's Public: you hide the whole type but within the assembly it has a classic public surface
Internal's Internal: I can't think of any real world example. Perhaps something soon to become public's internal?
Internal's Public vs Internal's Internal is a false dilemma. The two have completely different meaning and should be used each in their own set of situations, non-overlapping.

internal says the member can only be accessed from within the same assembly. Other classes in that assembly can access the internal public member, but would not be able to access a private or protected member, internal or not.

I actually struggled with this today. Until now I would have said that methods should all be marked with internal if the class was internal and would have considered anything else simply bad coding or laziness, specially in enterprise development; however, I had to sub class a public class and override one of it's methods:
internal class SslStreamEx : System.Net.Security.SslStream
{
public override void Close()
{
try
{
// Send close_notify manually
}
finally
{
base.Close();
}
}
}
The method MUST be public and it got me thinking that there's really no logical point to setting methods as internal unless they really must be, as Eric Lippert said.
Until now I've never really stopped to think about it, I just accepted it, but after reading Eric's post it really got me thinking and after a lot of deliberating it makes a lot of sense.

There does be a difference.
In our project we have made a lot of classes internal, but we do unit test in another assembly and in our assembly info we used InternalsVisibleTo to allow the UnitTest assembly to call the internal classes.
I've noticed if internal class has an internal constructor we are not able to create instance using Activator.CreateInstance in the unit test assembly for some reason. But if we change the constructor to public but class is still internal, it works fine.
But I guess this is a very rare case (Like Eric said in the original post: Reflection).

I think I have an additional opinion on this. At first, I was wondering about how it makes sense to declare something to public in an internal class. Then I have ended up here, reading that it could be good if you later decide to change the class to public. True. So, a pattern formed in my mind: If it does not change the current behavior, then be permissive, and allow things that does not makes sense (and does not hurt) in the current state of code, but later it would, if you change the declaration of the class.
Like this:
public sealed class MyCurrentlySealedClass
{
protected void MyCurretlyPrivateMethod()
{
}
}
According to the "pattern" I have mentioned above, this should be perfectly fine. It follows the same idea. It behaves as a private method, since you can not inherit the class. But if you delete the sealed constraint, it is still valid: the inherited classes can see this method, which is absolutely what I wanted to achieve. But you get a warning: CS0628, or CA1047. Both of them is about do not declare protected members in a sealed class. Moreover, I have found full agreement, about that it is senseless: 'Protected member in sealed class' warning (a singleton class)
So after this warning and the discussion linked, I have decided to make everything internal or less, in an internal class, because it conforms more that kind of thinking, and we don't mix different "patterns".

When defining a class as internal, do you define what would usually be public fields as internal?

When defining a class as internal, do you define what would usually be public fields as internal? Or do you leave them as public? I have a set of classes with public/private methods that I have decided to set as internal. Now, should I change the class' modifier to internal and let the rest of the methods/properties as they are (public/private) or switch them to (internal/private)?
I don't see a big point in changing it to internal, and if by some reason later I want to set them back to public it's going to give a lot of work to have to put them back to public again.
Any other thoughts on this?

I can't see any reason not to leave them as public, as your class won't be visible to outside assemblies anyway. The only case where I think this might matter is when using reflection over that class.

If I have a class that is internal, I leave the class members as public (or protected/private of course if that's what they were). I find that often I have classes that I hope I can keep internal that I end up having to expose eventually and switching all the appropriate members back to public is annoying.

You defnitely shouldn't change private members to internal as that would make them more accessible. There is no need to change public members to internal since nothing outside of the defining assembly will ever be able to get a reference to an internal class anyway.

I think you should give generally members the same visibility as you would if the Type were itself public.
That is, members that are part of the public API should be public, and members that are special-purpose helpers that should only be visible to "friend" classes should be internal.
This means there will be no changes to member visibility if you ever decide to make the Type public.
More importantly, it also documents your intention - anyone reading your code will be able to identify which (if any) members are intended to be internal.

We use internal keyword for members in internal classes, so that the intention is clear. However it fails if one implicitly implement internal interfaces, where the members have to be defined as public. We dont know why and see this as an accidental mistake in the language specification that we have to live with.

Dig around in Reflector for a bit and you'll see that the BCL itself is wildly inconsistent over this. You'll see many internal classes with public members and many others with internal members. Several classes even mix and match the two with no particular rhyme or reason that I'm able to discern.
There is no "right" answer here, but there are a few things you should consider whenever you need to make a decision on this:
internal members cannot implicitly implement an interface, and explicit implementations are always private. So if you want interface members to be accessible through the class instance (the Dispose method of IDisposable is a common one), they need to be public.
Type visibilities can change. You might decide down the road that an internal class has some valuable functionality that you want to make available to the outside. But if you do, then all public members become accessible by everyone. You should decide in advance if this is what you want.
On the other hand, another reason you might make an internal class public is if you decide that you need to subclass it and that the derived classes should be in a different assembly. In this case, some of your internal members should probably be protected internal instead, otherwise derived classes won't have access to members they might need.
In the end, what it all comes down to is writing code to be read and maintained by other people. The modifier internal can mean two very different things to a maintenance programmer:
That it doesn't seem useful to the outside world, but wouldn't actually be harmful either. A typical example would be a utility class that was whipped up in 5 minutes and doesn't do much validation or error checking. In this case, it's OK for someone to make it public as long as they tighten up the code a little and/or document how to use it properly. Make this assumption explicit by making the members public.
That it's actually not safe for outside consumption; it might manipulate some protected state, leave handles or transactions open, etc. In this case, you really want to make the individual methods internal to make it absolutely clear that nobody else should be using this class, ever.
Choose whichever one is appropriate for your scenario.

Is there any constraint to specify the access specifier of members of a class when we have specified access specifier of the class?

Suppose that we have a class named class1.
The class1 has several properties and methods and we have decided to specify the access specifier of class1 as internal.
Now, can we set the access specifier of class1 methods as public?

For your specific question, Class 1 which is declared as internal can have a public method.
Why?
Look at Jon Skeets explanation:
You can certainly mark a class as
internal, but that's different from
making its public members internal.
For instance, suppose you have a class
which implements a public interface.
Even though the class may be internal,
an instance may still "get out of the
assembly" by being returned from a
member in another (public) class. That
instance would have to be referenced
by the interface it implements rather
than the class name itself (as the
class isn't known to the outside
assembly) but the public methods can
still be called.
If the public methods aren't
implementing any interfaces, I suspect
it would only make a difference in a
very few reflection cases which you
may not care about.
community wiki - as credit should go to Jon Skeet

Yes, you can set public on members of internal/private/etc types.
As other replies have noted, external code won't be able to see the properties unless it can see the type - but there are lots of nuances:
if the member is on an interface it will be (essentially) part of the public API
the member might be a public override of a virtual/abstract member - in which case it will truly be publicly visible, but via the base-class (again, similar to interfaces)
But there is a lot of other code in the framework that uses reflection and demands public accessibility:
data binding usually works on the public properties
security checks for partial-trust can be fussy about public members
serialization (for example XmlSerializer) may want public members
etc
So there are still lots of reasons to think about public rather than just internal, even if your code is only referenced by the local assembly.

By rule access specifiers on methods and properties can not be more more accessible than that of the class containing it.
But I've tried this:
internal class Test
{
public string Testing{get;set;}
}
and it compiles without any exception! I think it is okay as the class Test will not be accessible outside the namespace assembly we have declared so public property will not make any difference.
This does not works:
private class Test
{
public string Testing{get;set;}
internal string TestingAgain{get;set;}
}

How do you "override" an Internal Class in C#?

There's something I want to customize in the System.Web.Script.Services.ScriptHandlerFactory and other .NET stuff inside an internal class. Unfortunately, it's an internal class. What options do I have when trying to customize a method in this class?

You might find this recent article enlightening. Basically, it says that you can't override anything marked internal, and the source is about as authoritative as it gets. Best you can hope for is an extension method.

The internal keyword signifies that a unit of code (class, method, etc.) is "public" to the assembly it is in, but private to any other assembly.
Because you are not in the same assembly, you cannot do anything. If it wasn't internal you could use the new keyword on the method you're overriding (to hide the original implementation) when extending the class.
In short: you are to be SOL.
The only thing i can think of you could do is write a proxy class, where one of your private fields is the class you'd want to extend and you implement all it's methods and proxy their calls. that way you can still customize output, but you'd have to get your class used, and considering it's marked internal, i'm not sure that's possible without some serious hacking.
using System;
...
using System.Web.Script.Services
namespace MyGreatCompany.ScriptServices
{
public class MyScriptHandlerFactory /* implement all the interfaces */
{
private ScriptHandlerFactory internalFactory;
public MyScriptHandlerFactory()
{
internalFactory = new ScriptHandlerFactory();
}
...
}
}
This could make what you want to accomplish possible, but it won't be pretty.

I believe you can use Reflection to get around the access modifiers on a class, so perhaps you can use Reflection.Emit to generate a type that inherits from an internal type (but NOT the sealed modifier), though I can't find an example of this online.
This certainly works for accessing private members of classes, and probably for inheritance of non-sealed classes. But it doesn't help much if the target methods are not already marked virtual.

It depends on the assembly. This could possibly violate some licensing (although its similar to some sort of static linking), and maybe even make deployment a nightmare, but you could consider:
Decompile and copy the code over to your own project; modify as needed
Recompile/patch the assembly and add an "InternalsVisibleToAttribute"