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Application Settings in MVC5 Web App - Using C# class vs Web.Config?

I'm building out my application and I'm at a point where I've hardcoded a lot of settings at the top of my class files - stuff like ApiSid and ApiKey, SmtpServiceUsername, MyEmailPassword etc. I'm now trying to consolidate these and I see two options:
1) Push them all into web.config. I don't like the thought of muddling up my web.config with tens (almost 100) settings though... I also feel uncomfortable with security here.
2) Build a static class that just contains these settings (Settings.cs) - basically housing a bunch of constants that are referenced throughout the app.
I feel more comfortable with the second approach because I can keep my settings totally isolated and not worry about exposing them via web.config - is there anything inherently wrong with this approach?

is there anything inherently wrong with this approach?
What makes you think putting constants in the code is any more secure than in the config? The compiled DLLs are right there next to the Web.Config, if somebody can examine one of them they can examine the other one. Hard-coded values can be de-compiled pretty easily.
Config files exist for a reason. Specifically, if any value is going to change per environment then it belongs in the config file. That way the same identical codebase can be used in any environment (development, test, production, etc.) and you'd just edit the config values for that environment. Having to re-compile the code just to deploy the same version to a new environment is less than ideal, since it's no longer the same version.
I don't like the thought of muddling up my web.config with tens (almost 100) settings though
Why not? If they're all flat static values, a list of appSettings keys would be fine. If there's more structure to them, create custom config sections.

This is not necessarily the best approach but I'd store these kind of settings in the database. This gives you database security for the settings plus it's easy to update the settings without having to stop / restart the application so you avoid kicking out users.
Once you have your settings in the database, you can load them periodically (like every 15-20 minutes) to detect changes. In the meantime, create a dictionary of the data and either wrap it in a class that provides type-safe access through properties or just use the dictionary directly. Since this is web application, you'll have to use a thread-safe class (like ConcurrentDictionary) to make sure multiple threads can safely access your settings.
If you have so many settings, web.config would be cluttered and every change would force an app pool restart. As #David mentions in his answer, the config file gives you an easy way to have different settings for different environments but this is also easy to do with a database approach where settings may be present once per environment.

Best way to store configuration settings outside of web.config

I'm starting to consider creating a class library that I want to make generic so others can use it. While planning it out, I came to thinking about the various configuration settings that I would need. Since the idea is to make it open/shared, I wanted to make things as easy on the end user as possible. What's the best way to setup configuration settings without making use of web.config/app.config?

Why are you rejecting the use of web.config/app.config? That's what they're for, and that's totally the way I'd do it.
Put an example block of code for the web.config/app.config in your documantation that could be copy-and-pasted into the real config file. If documented well, it shouldn't really be a problem for your users.
Just be sure you don't crash because the settings don't exist. Or, if you do crash, give a detailed error message telling the user to edit the proper config file and where in the documentation they can find examples. That would make it easier on your user.

I think it's not wrong to use the web.config for storing you configuration items. But you could be more flexible offering 3 configuration options as many popular libraries do (nhibernate, log4net, etc)
Use Web.config
Use a external xml file
Configure library programatically
Doing it like this you leave the decision on your library users instead of supposing they won't be happy using the web.config.

If you are worried about all the other tags in web.config, why don't you "include" your configuration file that contains only those settings that your users would customise?
The syntax is
<include uri="your file.xml" />
By having your "file.xml" outside of web.config, you prevent common mistakes by your users to modify your web.config inadvertably.

Using the database to hold application settings

I am looking at ways to make our application more extensible and easier to manipulate without having to alter the web.config (or, in our case, application.config files, which contain the appsettings node).
One way I have thought about is keeping the app settings in the database table that has a sqlcachedependancy. This means that:
Any time a setting is changed in the database, the cache is invalidated, and the settings are retrieved again, thus updating the application in realtime without having to alter files and restart the entire app.
We can create a custom tool which allows us to alter the settings.
The cons as I see it are that this may cause serious logic problems in that, if you have something that checks an appsetting at the start of a process, and it then changes halfway through, you could end up unintentionally altering the process flow, as the requirement for a complete application restart is negated.
Is there a way round this?
Is there a better way to manage appsettings, so that you can alter them on the fly remotely for one, several, or all servers in one go?

I think you've nailed the two major players:
either you have access to the file system and you put all your settings in a plethora of *.config files there
OR:
you don't have access (or only very limited access) to the server's file system and thus you're probably better off putting config settings and user preferences in a database, basically leaving nothing but the connection string to the config file on disk
Both approaches have their pros and cons. I've been trying for a long time to find a way to "materialize" a config section from a database field, so that I could basically just use the config XML, but stored in a database field. Unfortunately, the entire .NET 2.0 config system is very much "locked down" and just only assumes data will come from files - there's no way to plug in e.g. a database provider to allow the config system to read its contents from a database field :-( Really too bad!
The only other approach I've seen is a "ConfigurationService" in the StockTrader 2.0 sample app provided by Microsoft, but for my needs, it felt like overkill and like a really complex, really heavy-weight subsystem.

You could use SQLite, which will be a self-contained DB in a single file. Two birds with one stone?

If you reference an external config file that contains appsettings (leaving everything else in the normal app.config) then I believe editing it only reloads those settings, it doesn't force the whole app to restart.
There's a similar question on the subject here:
Nested app.config (web.config) files
WRT the problem of values changing in the middle of program execution, I guess you could locally cache the values, and raise an event when they change, allowing routines to reach a suitable point before using the updated values.
I think in asp.net we sort of get this for free because each page lifecyle is distinct, so the value is simply applied to new page requests only, not in the middle of an execution.
Edit: A little extra info:
Configuration Changes Cause a Restart of the Application Domain
From MSDN:
Changes to configuration settings in Web.config files indirectly cause the application domain to restart. This behavior occurs by design. You can optionally use the configSource attribute to reference external configuration files that do not cause a restart when a change is made. For more information, see configSource in General Attributes Inherited by Section Elements.
More information on the ConfigurationManager class in the System.Configuration namespace which could be used to modify the config files programatically (ie in a custom tool, if relevant disk read permissions can be provided). If you stick to using the built in configuration classes, I think changing the external configs, would not cause application restart, but would raise events (such as property changed) which you could handle, to ensure your code is not caught out by changing settings.

Is switching app.config at runtime possible?

Is there a way at runtime to switch out an applications app.config (current.config to new.config, file for file). I have a backup/restore process which needs to replace its own application.exe.config file. I have seen this post but it does not answer how to do this at runtime.

Turns out I can swap the .config file for the new one and do a ConfigurationManager.RefreshSection(...) for each section. It will update from the new .config file.

Microsoft .NET's app.config is not designed for your scenario, as well as many others. I often encounter a similar need, so I have spent a lot of effort designing a solution.
Redesign to use app.config only as a configuration bootstrap: specify where to find the rest of the real configuration data. This information should almost never change, so there is no need to handle file watching or application restarts.
Pick an alternate location for the real configuration data: a file, a database, perhaps even a web service. I prefer a database most of the time, so I create a configuration table with a simple structure that allows me to store my data.
Implement a simple library to wrap your configuration access so that you have a simple API for the rest of your application (via dependency injection). Hide the usage of app.config as well as your real configuration storage location(s). Since .NET is strongly-typed, make the configuration settings so--convert each string retrieved into the most-specific type available (URL, Int32, FileInfo, etc.).
Determine which configuration settings can be safely changed at runtime versus those that can't. Typically, some settings need to change along with others, or it simply makes no sense to allow them to change at all. If all your configuration data can safely change at runtime, then that makes things easy, but I HIGHLY doubt such a scenario. Hide the changeability and interdependencies of the configuration settings to the extent possible.
Design the response to the unavailability of your real configuration data. I prefer to treat the absence of any configuration setting as a fatal error that aborts the application, unless I can identify a usable default. Likewise, I abort in the absence of the configuration storage container (file, database table, etc.).
Enjoy, and best wishes.

Are you able to restart the application when you detect that you need to switch files? If so, it's just a matter of switching the files and restarting. Now, the tricky bit is if .NET keeps the app.config file open while the program is running. I suspect it doesn't, but if the most obviously approach fails, I suggest you have a second application (cfgswitcher.exe) which waits for the process with a PID specified on the command line to terminate, then switches config files and relaunches the original process. Then your app would just need to launch cfgswitcher.exe (passing in its own PID as a command line argument) and terminate.
As I say though, it's worth trying the more obvious approach first.
EDIT: If you can't restart the application (or even part of it in a new AppDomain) then various aspects of app.config (assembly bindings etc) can't be changed. If you're only interested in your own configuration sections changing, then I suggest you store them in a separate config file and reload them whenever you want to.

Look at the events available to you on the ApplicationSettingsBase class. There are PropertyChanged & SettingChanging that may give you what you need.
You could also watch the file and if it has changed call the reload method to get the new settings.

I don't think it is possible at all to switch the configuration at runtime without restarting, so if you can't apply Jon's approach, you should try to come up with an other approach.
Anyway, maybe it's just me not having enough information about your scenario, but this kind of feels fishy.
Are you sure that swapping the configuration file is the best way to achieve whatever requirement you need to meet? I mean, this is quite an uncommon thing. If I were you, I would try to come up with some other approach.

Letting several assemblies access the same text file

I've got many assemblies/projects in the same c#/.net solution. A setting needs to be saved by people using the web application gui, and then a console app and some test projects need to access the same file. Where should I put the file and how to access it?
I've tried using "AppDomain.CurrentDomain.BaseDirectory" but that ends up being different for my assemblies. Also the "System.Reflection.Assembly.Get*Assembly.Location" fail to give me what I need.
Maybe this isn't something I should but in a file, but rather the database? But it feels so complicated doing that for a few lines of configuration.

Put the file in
Path.Combine(
Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData),
"[Company Name]\[Application Suite]");

Personally, I would be leveraging the database because the alternative is either a configuration headache or is more trouble than it's worth.
You could configure each application to point to the same file, but that becomes problematic if you want to move the file. Or you could write a service to manage the file and expose that to clients, but at this point you may as well just use the DB.

Thought about storing it in the registry or in Isolated Storage? Not sure if multiple applications can share Isolated Storage or not, though.

projects can have build events -- why not add a post-build event to copy the file to all required locations?