I have an old library that used .NetFramework 4.7.2. It uses System.Configuration and App.config file. I migrated the library to target framework of netstandard2.0. My unit tests, I migrated to .Net 8.0. Now, the test acts as if the App.config does not exist. My App.config file is as follows:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.8" />
  </startup>
  <appSettings>
    <add key="Email:ApiKey" value="SG.11111111111"/>
    <!— keys removed ... -->
    <!-- Audit=0, Error = 1, Warning=2, Info=3, Debug=4 -->
    <add key="LogLevel" value="4"/>
  </appSettings>
  <system.net>
    <mailSettings>
      <smtp deliveryMethod="Network">
        <network host="localhost" port="25" enableSsl="false"/>
      </smtp>
    </mailSettings>
  </system.net>
</configuration>

A simple test is as follows:

[TestMethod]
public void ConfigurationManager_AppSettings_Exists_Test()
{
    string _apiKey = System.Configuration.ConfigurationManager.AppSettings["Email:ApiKey"].ToString();
    Assert.AreEqual("SG.11111111111", _apiKey);
}

It errors on the assignment of the variable _apiKey, with the following message:

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

It used to work 6 years ago. Does App.config work under .Net 8.0?