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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?

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()
{
}

Config file structure

I got mixed with app.config file.
I had config that failed to get connectionStrings:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<connectionStrings>
<add name="MovieDBContext" connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\Movies.mdf;Integrated Security=True" providerName="System.Data.SqlClient" />
<add name="ProgrammingInCSharpConnection" providerName="System.Data.SqlClient" connectionString="Data Source=(localdb)\v11.0;Initial Catalog=ProgrammingInCSharp;" />
</connectionStrings>
</configSections>
</configuration>
Problem was solved after placing <connectionStrings> outside of <connectionStrings> :
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections />
<connectionStrings>
<add name="MovieDBContext" connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\Movies.mdf;Integrated Security=True" providerName="System.Data.SqlClient" />
<add name="ProgrammingInCSharpConnection" providerName="System.Data.SqlClient" connectionString="Data Source=(localdb)\v11.0;Initial Catalog=ProgrammingInCSharp;" />
</connectionStrings>
</configuration>
But I found many samples with pattern below
<configSections>
<connectionStrings>
...
</connectionStrings>
</configSections>
It might work in some cases?
What is logic of configuration file in general and what kind of information might be located in each section?

No. It is not possible to have a valid app.config file and yet having a connectionStrings in your configSections node.
According to MSDN, this is the list of allowed elements:
section
sectionGroup
add
clear
connectionStrings isn't in the list and I don't expect an application to find the connection strings in that place.

The configSections node contains configuration section per namespace declarations, but is not usually used for connection strings, unless they are namespace - specific.
But I found many samples with pattern below
The patterns you found could never work since <connectionStrings> is not allowed to be nested directly under <configSections>.
This could be achieved, by stating a connection string variable under the <section> itself, and if the server code targets them via a specific namespace.
An example:
<configuration>
<configSections>
<section name="section1" type="System.MyNameSpace" />
</configSections>
<section1 connString="...conn.." />
</configuration>
In the example above the namespace System.MyNameSpace will now have a variable (field) called connString, so it can target it in code behind.
For conclusion, if your connection strings are not per namespace, the <connectionStrings> element should be nested directly under the <configuration> node and no where else.

Using ConnectionStrings and custom ConfigSections in the same App.Config

I have a custom configSection that works as expected. However, when I add a 'connectionStrings' section, I receive error:
Configuration system failed to initialize
on line:
StencilObjects so = ConfigurationManager.GetSection( "stencilObjects" ) as StencilObjects;
Here is the config:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="connection" connectionString="foo"/>
</connectionStrings>
<configSections>
<section name="stencilObjects" type="Stencil.Configuration.StencilObjects, Stencil.Configuration"/>
</configSections>
<stencilObjects>
<tableData>
<table schema="Auth" name="SecurityQuestion" />
</tableData>
</stencilObjects>
</configuration>
Is there any limitation when using a custom config section? Does this not allow the use of connectionstrings?
Again, when I remove the connectionStrings, the app runs as expected.
Any idea on what is going on?

I haven't found a link to back this up yet with an explicit statement, but I've always used configSections at the top of the file without any problems. Try like this:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="stencilObjects" type="Stencil.Configuration.StencilObjects, Stencil.Configuration"/>
</configSections>
<connectionStrings>
<add name="connection" connectionString="foo"/>
</connectionStrings>
<stencilObjects>
<tableData>
<table schema="Auth" name="SecurityQuestion" />
</tableData>
</stencilObjects>
</configuration>
configSections definitely doesn't need to be just before the section(s) it describes. connectionStrings can be in between.

Change app.config connection string depending on PC

I'm doing a c# course at university, and now I'm beginning linQ to xml, linQ to Sql-Server, etc.
I work with the example projects in two PCs (university PC and office PC)
Is there an easy way to change connection string (from app.config) at runtime, or designtime (using a constant in main.cs) so I can use a connection string at university, and a connection string at the office easily?
Thanks a lot in advance,

You could try something like:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="Environment" value="Home"/>
</appSettings>
<connectionStrings>
<add name="Work" connectionString="..."/>
<add name="Home" connectionString="..."/>
</connectionStrings>
</configuration>
and, later:
string environment = ConfigurationManager.AppSettings["Environment"];
ConfigurationManager.ConnectionStrings[environment].ConnectionString;

Another way of doing it:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="HomeEnvironment"
connectionString="Data Source=**HOME-COMPUTER**\SQLEXPRESS;Initial Catalog=**HomeDatabase**;Integrated Security=True;"
providerName="System.Data.SqlClient" />
<add name="WorkEnvironment"
connectionString="DataSource=**WORK-COMPUTER**\SQLEXPRESS;Initial Catalog=**WorkDatabase**;Integrated Security=True;"
providerName="System.Data.SqlClient"/>
</connectionStrings>
</configuration>
and use it like this:
var environment = Environment.MachineName == "HOME-COMPUTER" ? "HomeEnvironment" : "WorkEnvironment";
var connectionString = ConfigurationManager.ConnectionStrings[environment].ConnectionString;
var dbContext = new **Databasename**ContextDataContext(connectionString);
Bold strings should be customised as required