System.NullReferenceException in a class library

Question

I'm trying to configure some connection strings in app.config, but keep getting this error message,

System.NullReferenceException: Object reference not set to an instance of an object.

I've looked at numerous solutions on the web, but nothing directly applies,

I'm copying the app.config file to the target directory
I've referenced System.Configuration and am using ConfigurationManager
The XML / source is almost a verbatim copy of the Microsoft guidelines

The only difference is that the project is a class library, which I'm exposing to Excel via ExcelDNA.

Is there something special I need to do to get this to work with Excel?

Here's a snippet of the XML,

<configuration>
    <connectionStrings>
        <add name="ConnectionCSV" providerName="System.Data.OleDb" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;" />
    </connectionStrings>  
</configuration>

This is the source code (have got a using System.Configuration along with necessary reference),

string constr = ConfigurationManager.ConnectionStrings["ConnectionCSV"].ConnectionString;

The above line throws the Object reference not set to an instance of an object error.

C. Augusto Proiete · Accepted Answer · 2016-12-14 18:09:40Z

2

The name of the configuration file needs to match the name of the .XLL file that is created by ExcelDNA.

e.g.

MyApp-AddIn.xll -> MyApp-AddIn.xll.config

MyApp-AddIn64.xll -> MyApp-AddIn64.xll.config

answered Dec 14, 2016 at 18:09

C. Augusto Proiete

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1 Comment

insomniac Over a year ago

Thanks a lot, Caio. You hit the nail on the head!

Dai · Accepted Answer · 2016-12-14 17:51:18Z

1

Libraries don't have their own app.config files, they belong to the parent executable.
Excel.exe does not have its own app.config file.
The connection string ConnectionCSV does not exist in whatever configuration file, if any, is being used at runtime. You can verify this by checking if ConfigurationManager.ConnectionStrings["ConnectionCSV"] != null first and warning the user if the key doesn't exist.

In your case it might be better to store the connection string in the user's registry instead. To whit:

Update:

I just found this page on MSDN which describes your exact problem and explains how you can use app.config files with Office add-ins: https://msdn.microsoft.com/en-us/library/16e74h9c.aspx

Specifically:

In the [app.config filename] box, type the same name as the assembly plus the extension .config. For example, a configuration file for an Excel project assembly called ExcelWorkbook1.dll would be named ExcelWorkbook1.dll.config.

answered Dec 14, 2016 at 17:51

Dai

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1 Comment

insomniac Over a year ago

Thanks a lot, Dai. That further clarification helped. File this question under 'bleeding obvious'. ;-)

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