Mock Configuration.GetSection

mock.cs

Mock<IConfiguration> configuration = new Mock<IConfiguration>();  
configuration.Setup(c => c.GetSection(It.IsAny<String>())).Returns(new Mock<IConfigurationSection>().Object);

Sorry the title of this is misleading, I should have mentioned it is "Mock Configuration.GetSection"

Because GetValue is an extension method, and as far as I know extension methods can not be mocked (correct me if I am wrong). You will have to use GetSection as its not an extension method. And as per line 2 its mocking c.GetSection.

It's okay man.
I solved my problem as follows:

var _configuration = new Mock<IConfiguration>();
var _configurationSection = new Mock<IConfigurationSection>();
_configurationSection.Setup(a => a.Value).Returns("1, 2, 3, 5, 7, 9");
_configuration.Setup(c => c.GetSection(It.IsAny<String>())).Returns(new Mock<IConfigurationSection>().Object);
_configuration.Setup(a => a.GetSection("MyKey")).Returns(_configurationSection.Object);

`
// setup
var inMemorySettings = new Dictionary<string, string> {
{"SectionName:SomeKey", "SectionValue"},
//...populate as needed for the test
};

        _configuration = new ConfigurationBuilder()
            .AddInMemoryCollection(inMemorySettings)
            .Build();

`
[()](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/?view=aspnetcore-6.0&viewFallbackFrom=aspnetcore-3.0#memory-configuration-provider)