>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?

When I run into something such as that it ends up being a minor spelling issue that I don't notice by glancing at it. I would wrap that call anyway in your own code somewhere making a method such as GetConfigSetting(). Pass in the key and also a default value for those times it doesn't contain a value.
then you can return

GetConfigSetting(string config, string default)
{
     string result = null;
     result = ConfigurationManager.AppSettings[config];
     return result  != null ? result : default;
}