ConnectionString from app.config of a DLL is null

I have a class library that contains a valid connectionString inside the app.config. Inside that class library I want to use it with

ConfigurationManager.ConnectionStrings["NAME"].ConnectionString

My ASP.net 4.0 framework application references that DDL and retrieves data from it. I want create a Entity Framework 4 DataContext within my DDL with the ConnectionString from the App.config. (I do not want to pass the connectionString from my ASP.net application in every single method. (I’m using ObjectDataSources))

However, this line inside my DLL throws a NullReferenceException.

ConfigurationManager.ConnectionStrings["NAME"].ConnectionString

How can I fix this issue?

Answers:

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Method 1

I have a class library that contains a
valid connectionString inside the
app.config

A class library doesn’t have an app.config file associated. It’s the application consuming this assembly that does. So you need to put the connection string inside this config file (if this is an ASP.NET application this would be web.config). Thus adding an App.config file in a project of type class library in Visual Studio makes no sense.

Method 2

In this case you put the same <connectionStrings> entry (the <add> in question) in your web app’s web.config, ConfigurationManager.ConnectionStrings always looks at the current config, that’s a web.config in your case.

Method 3

In case if you don’t waana use ConfigurationManager

If i assume the config file is mydll.dll.config i can load it as XElement and parse it using Linq as

var xe = XElement.Load("mydll.dll.config");
var connectionString = xe.Descendants("connectionStrings")
     .Elements("add")
     .FirstOrDefault(a => a.Attribute("name").Value == Name)
     .Attribute("connectionString").Value;

where Name is the connectionString name in the XML. Without using the ConfigurationManager import and other stuffs. The only requirement for this to make sure that the config file sit next to the dll.

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