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Can you split up a connection string in app.config?

I am very new to app.config files:
In an app.config file which look like this
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="connectionstring" connectionString="Server=server;User ID=bar;Password=foobar" providerName="System.Data.SqlClient" />
</connectionStrings>
</configuration>
I want to split up the connection string into various sections like Server, UserId, etc in a different file and then concat it together in my app.config:
Pseucode
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="connectionstring" connectionString=Server+UserId+Password providerName="System.Data.SqlClient />
</connectionStrings>
</configuration>
Is this possible?

Custom section in App.Config, ConfigurationManager stops working

I have created a console app in c# that reads information from App.config. if i add things in appSettings section, i can acces them and it works, but as soon as i add some custom sections i cant read anything from it. I am using ConfigurationManager, and i have the reference for it included.
My app config looks like this:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
</startup>
<appSettings>
<add key="overwriteBackupFiles" value="False"/>
<add key="path" value="c:\temp"/>
</appSettings>
<ImageFormatsINeed>
<add key="type1" value="width=180&height=180"></add>
<add key="type2" value="width=220&height=220"></add>
<add key="type3" value="width=500&height=500"></add>
</ImageFormatsINeed>
</configuration>
and i am trying to acces those information like this:
string path = ConfigurationManager.AppSettings["path"];
var settings = ConfigurationManager.GetSection("ImageFormatsINeed");
When i didnt have the ImageFormatsINeed section i could get the path from AppSettings and it was working. But as soon as i added my ImageFormatsINeed section, everything stops working.
Now my question is how can i add custom sections in app.config that it will work, or should i just read my ImageInformation from some custom xml file or config file?

You have to use the tag <configSections> at the top in your app.config, for this case you should use the type AppSettingsSection
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="ImageFormatsINeed" type="System.Configuration.AppSettingsSection" />
</configSections>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
</startup>
<appSettings>
<add key="overwriteBackupFiles" value="False"/>
<add key="path" value="c:\temp"/>
</appSettings>
<ImageFormatsINeed>
<add key="type1" value="width=180&height=180"></add>
<add key="type2" value="width=220&height=220"></add>
<add key="type3" value="width=500&height=500"></add>
</ImageFormatsINeed>
</configuration>
Then in your C# code:
NameValueCollection settings_section = ConfigurationManager.GetSection("ImageFormatsINeed") as NameValueCollection;
Console.WriteLine(settings_section["type1"]);

MS Unit test Data Source cannot be found in test configuration settings

I've a data source details defined in app.config file in a test project, but when I used the data source name in a test function it's not executing it rather throwing error like "Data source 'MyDataSourceName' cannot be found in test configuration settings".
Exception:
My Data Source in app.config file:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="UnitTestConfiguration" type="Microsoft.VisualStudio.TestTools.UnitTesting.TestConfigurationSection, Microsoft.VisualStudio.QualityTools.UnitTestFramework, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
</configSections>
<connectionStrings>
<add name="DataSourceConnection" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|\DataSource;Extended Properties='text;HDR=Yes;FMT=Delimited'"
providerName="System.Data.OleDb" />
</connectionStrings>
<UnitTestConfiguration>
<dataSources>
<add name="TargetDataSource" connectionString="DataSourceConnection" dataTableName="UserData#CSV" dataAccessMethod="Sequential"/>
</dataSources>
</UnitTestConfiguration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
</startup>
</configuration>
My test function:
[DataSource("TargetDataSource")]
[TestMethod]
public void UserValidationTest()
{
}

How to use custom configuration file or app.config in .NET application

I have MVC5 .NET 4.6.1 C# web application
I want to create a custom config file separate from web.config to store some settings my application uses.
I tried to follow this article https://support.microsoft.com/en-us/kb/815786
however the items I set in app.config:
<?xml version="1.0"?>
<configuration>
<system.web>
<compilation debug="true" targetFramework="4.6.1" />
<httpRuntime targetFramework="4.6.1" />
</system.web>
<appSettings>
<add key="Key0" value="0" />
<add key="Key1" value="1" />
<add key="Key2" value="2" />
</appSettings>
</configuration>
are not seen in my application see , eg. they come as null:
string attr = ConfigurationManager.AppSettings["Key0"];
Why isn't it working? Am I missing something?
Alternatively I would like to create a custom config file eg. mycustom.config to define my global app settings.
EDIT
Solution I used
Follwing this post https://social.msdn.microsoft.com/Forums/vstudio/en-US/11e6d326-c32c-46b1-a9a2-1fbef96f33ee/howto-custom-configuration-files?forum=netfxbcl
In web.config:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="newAppSettings" type="System.Configuration.AppSettingsSection, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</configSections>
<newAppSettings file="C:\mycustom.config"/>
</configuration>
Then mycustom.config
<?xml version="1.0" encoding="utf-8" ?>
<newAppSettings>
<add key="OurKey" value="OurValue"/>
</newAppSettings>
And reading the value:
System.Collections.Specialized.NameValueCollection newAppSettings = (System.Collections.Specialized.NameValueCollection)System.Configuration.ConfigurationManager.GetSection("newAppSettings");
string key = Convert.ToDateTime(newAppSettings["OurKey"]);

You can use separate config file for connection strings and app settings:
<appSettings configSource="appSettings.config" />
<connectionStrings configSource="connectionStrings.config"/>
appSettings.config file
<?xml version="1.0" encoding="utf-8" ?>
<appSettings>
<add key="Setting1" value="App setting 1" />
</appSettings>
connectionStrings.config file
<?xml version="1.0" encoding="utf-8"?>
<connectionStrings>
<add name="MyConnStr1" connectionString="My connection string" />
</connectionStrings>
Usage is same as it was before:
var setting1 = ConfigurationManager.AppSettings["Setting1"];
var connString1 = ConfigurationManager.ConnectionStrings["MyConnStr1"].ConnectionString;

//Helps to open the Root level web.config file.
Configuration webConfigApp = WebConfigurationManager.OpenWebConfiguration("~");
//Modifying the AppKey from AppValue to AppValue1
webConfigApp.AppSettings.Settings["AppKey"].Value = "AppValue1";
<appSettings>
<add key="AppKey" value="AppValue"/>
</appSettings>

Error : The key 'SecurityKey' does not exist in the appSettings configuration section

When I execute my app (Winform)from the computer where it was developed there is no error, but When I execute this in another computer I get the error. My App.config is like this:
<?xml version="1.0"?>
<configuration>
<appSettings>
<add key="SecurityKey"/>
</appSettings>
<startup>
<supportedRuntime version="v4.0"sku="...."/>
</startup>
</configuration>
and this is the line that I use:
string key = (string)settingsReader.GetValue("SecurityKey", typeof(String));
I already tried to follow this The key 'UserID' does not exist in the appSettings configuration section, but it is still the same.
Any suggestions?

the appSettings in the .config file is different from .settings file.
Take a look at ConfigurationManager.AppSettings Property.
I'd also mention that I have no idea how either the settingsReader nor the ConfigurationManager work with a key with no value:
<add key="SecurityKey"/> <!-- no value? -->
<add key="SecurityKeyWithValue" value="myvalue"/>

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="SecurityKey" value="Syed Moshiur Murshed"/>
</appSettings>
</configuration>