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Can't read connection string from web.config

I have the following in my web.config located at the root of my project:
<configuration>
<connectionStrings>
<clear />
<add name="Default" providerName="System.Data.SqlClient" connectionString="Server=tcp:whoops;Encrypt=True;TrustServerCertificate=False;Connection Timeout=3000;" />
</connectionStrings>
<appSettings>
<add key="ConnectionString" value="test"/>
</appSettings>
....
I read from Startup.cs (this is an asp.net core web app):
string connection = ConfigurationManager.ConnectionStrings["Default"].ConnectionString;
However when I break on this, ConfigurationManager.ConnectionStrings and ConfigurationManager.AppSettings are empty (well, the first has some default connection string that is not the one in web.config).
What's going on here?

You will have to migrate the config to the new file appsettings.json
https://learn.microsoft.com/en-us/aspnet/core/migration/configuration?view=aspnetcore-2.1

Not saying this is how you should do it, but you can do the following...
In ASP.Net Core 2.2, you should be able to add an XML configuration to IConfigurationBuilder using
configBuilder.AddXmlFile("app.config");
Contents is pretty much the same as above...
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<connectionStrings configSource="connectionStrings.config" />
<appSettings>
<add key="Test" value="TestValue" />
</appSettings>
</configuration>
You should then be able to access AppSettings/ConnectionStrings using...
ConfigurationManager.ConnectionStrings
ConfigurationManager.AppSettings.AllKeys
{string[1]}
[0]: "Test"
ConfigurationManager.AppSettings.Get("Test")
"TestValue"

Is it possible to reference the same connection string in multiple places in a config file?

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<connectionStrings>
<add name="DefaultConnection" connectionString="Server=test.com,1234;" providerName="System.Data.SqlClient" />
</connectionStrings>
<appSettings>
<add key="serilog:write-to:MSSqlServer.connectionString" value="here reference the DefaultConnection connection string." />
</appSettings>
</configuration>
Same connection strings needs to be referenced in multiple places. For typo mistakes I would like to be able to reference a single connection string in the same file.
Is this possible?

How to access the default connection string from web.config in c#

I have my config file with below configurations:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="dataConfiguration" type="Microsoft.Practices.EnterpriseLibrary.Data.Configuration.DatabaseSettings, Microsoft.Practices.EnterpriseLibrary.Data" />
</configSections>
<dataConfiguration defaultDatabase="myConnectionString" />
<connectionStrings>
<add name="myConnectionString" connectionString="Data Source=mydatasource;Max Pool Size=100;Pooling=true; Initial Catalog=MyDB;User ID=Myuser;Password=Password;Connection Timeout=60" providerName="System.Data.SqlClient" />
</connectionStrings>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.2" />
</startup>
</configuration>
As you can see, I have set the default dataconfiguration to use myConnectionString as the default connection string, but in code behind (c#) how can I access this connection string without having to provide the name i.e. myConnectionString
So in below code:
string connectionString = ConfigurationManager.ConnectionStrings["myConnectionString"].ConnectionString;
SqlConnection cnn = new SqlConnection(connectionString);
SqlBulkCopy sbc = new SqlBulkCopy(cnn);
I would like to skip hardcoding the name of the connection string.

var dataConfig = (Microsoft.Practices.EnterpriseLibrary.Data.Configuration.DatabaseSettings)System.Configuration.ConfigurationManager.GetSection(
"dataConfiguration");
string connectionString = ConfigurationManager.ConnectionStrings[dataConfig.DefaultDatabase].ConnectionString;
That's what I can tell from typical usage of custom config sections and DatabaseSettings class.
EDIT: I have tested it in LINQPad with your exact app.config, got result:
Data Source=mydatasource;Max Pool Size=100;Pooling=true; Initial Catalog=MyDB;User ID=Myuser;Password=Password;Connection Timeout=60

Updating a config file adds additional config sections

My goal is to programatically update a .config file belonging to another application.
I start off with a simple config file that looks like this...
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="Test1" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true" providerName="System.Data.SqlClient"/>
</connectionStrings>
</configuration>
I then run my code to update the connection string named Test1 and rename it to Test2...
var configMap = new ExeConfigurationFileMap() { ExeConfigFilename = #"test\app.config" };
var externalConfiguration = ConfigurationManager.OpenMappedExeConfiguration(configMap, ConfigurationUserLevel.None);
externalConfiguration.ConnectionStrings.ConnectionStrings[1].Name = "Test2";
externalConfiguration.Save(ConfigurationSaveMode.Minimal, true);
This saves the configuration file, but now looking at the file you will see some other data has been added to it...
ConfigurationSaveMode.Minimal
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="Test2" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true"
providerName="System.Data.SqlClient" />
</connectionStrings>
<system.web>
<trust level="Full" />
<webControls clientScriptsLocation="/aspnet_client/{0}/{1}/" />
</system.web>
</configuration>
ConfigurationSaveMode.Modified
Using the Modified setting I get even more "stuff"...
<configuration>
<appSettings />
<configProtectedData />
<system.diagnostics />
<system.windows.forms />
<uri />
<connectionStrings>
<add name="Test2" connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|aspnetdb.mdf;User Instance=true"
providerName="System.Data.SqlClient" />
</connectionStrings>
<system.net>
<authenticationModules />
<connectionManagement />
<defaultProxy />
...snip...
What is this data and where has it come from? More importantly how can I stop it from being written into my file?

Try changing the ConfigurationSaveMode from
ConfigurationSaveMode.Minimal
to
ConfigurationSaveMode.Modified
Modified only saves the properties that you have changed.
From MSDN:
Full
Causes all properties to be written to the configuration file. This is useful mostly for creating information configuration files or moving configuration values from one machine to another.
Minimal
Causes only properties that differ from inherited values to be written to the configuration file.
Modified
Causes only modified properties to be written to the configuration file, even when the value is the same as the inherited value.

The way I have solved this was to set the Machine Configuration File to be the same as my EXE Configuration file, this way the .net Configuration object does not see that there is any difference between Machine and EXE when it is saving.
For example:
var fm = new ExeConfigurationFileMap();
fm.MachineConfigFilename = fm.ExeConfigFilename = #"C:\dude.config";
var c = ConfigurationManager.OpenMappedExeConfiguration(fm, ConfigurationUserLevel.None);
// Do Stuff
c.Save(ConfigurationSaveMode.Minimal, true);
This way you should not get the section in your config file upon save.

Getting configuration values rom XML file located in the same project

I am making a little program to copy pictures form one location to another. The information for the pictures are stored in a database so I need connections string and also I create a txt file with the final output from the operation and I want to store these two values in a App.Config.xml file.
The structure of my project is very simple :
And the XML files itself is :
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="MyDB" connectionString="Data Source=.\\DVSQLEXPRESS08;Initial Catalog=**;Persist Security Info=True;User ID=**;Password=**;MultipleActiveResultSets=True" />
</connectionStrings>
<createResultFile>
<add key="ResultFile" value="C:\Users\dv\Desktop\Leron\PictureStatus.txt"/>
</createResultFile>
</configuration>
I want to use the connectionString and <createResultFile> value in my PictureTransferTool.cs. This is my first time working with XML file and C# (.NET in general) so I want what is the way to retrieve those config values?

You config file must be like below...
Config File :
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="ResultFile" value="C:\Users\dv\Desktop\Leron\PictureStatus.txt"/>
</appSettings>
<connectionStrings>
<add name="MyDB" connectionString="Data Source=.\\DVSQLEXPRESS08;Initial Catalog=**;Persist Security Info=True;User ID=**;Password=**;MultipleActiveResultSets=True" />
</connectionStrings>
</configuration>
C# :
You can read Connection String like below
var connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MyDB"].ConnectionString;
Console.WriteLine(connectionString);
You can read User Defined Settings like Below
var Resultfile = System.Configuration.ConfigurationManager.AppSettings["ResultFile"];
Console.WriteLine(Resultfile);

The Way I normally do user-defined parameters in my app.config is i put them in the appSettings tab.
<appSettings>
<add key="myStr" value="String Value" />
and then you can access it with
string myStr = System.Configuration.ConfigurationSettings.AppSettings["myStr"];

It works for me.
Linq;
using System.Xml.XPath;
...
var doc = XDocument.Load("test.xml");// You should put the way to your XML
var name = doc.XPathSelectElements("/configuration/connectionStrings/add").Value;
var name = doc.XPathSelectElements("/configuration/createResultFile/add").Value;