>I have a DLL library that uses a configuration file to get a value which can be set at the client.
>The configuration file contains:
>

> <appSettings>
>    <add key="CertificateSerialNumber" value="SerialNumber"/>
>  </appSettings>
>

>In the code of the library I try to get the serial number:
>

>SerialNumber = ConfigurationSettings.AppSettings["CertificateSerialNumber"];
>

>Which works fine in debug, but when executing the code at the client, the subsequent error occurs:
>

>Value cannot be Null
>

>So I assume it cannot find the parameter I am looking for. The config file is in the same folder as the dll, so what am I missing?

The config file is based off of the entry executable, not the dll (see https://msdn.microsoft.com/en-us/library/ms229689.aspx). Do you have the config file in the same folder as the exe and named after the exe?