How to pass a POCO class to .NET Core configuration

Question

I am working on integration tests of .NET Core application and want to use some test configuration. My configuration is a POCO class that is configured via appsettings.json and then consumed via IOptions<T>. In my tests I'd like to use an instance of that class.

Here is the code:

 var mySettings = GetTestSettings(); // factory method returning POCO class 
 var configurationBuilder = new ConfigurationBuilder();

 // I am looking for something like 
 configurationBuilder.AddInMemoryObject("Settings", mySettings);

 // does not work, result is just string 
 configurationBuilder.AddInMemoryCollection("Settings",
        JsonConvert.SerializeObject(mySettings)); 

// requires filename
configurationBuilder.AddJsonFile("filename.json");

What is the easiest way to feed POCO to configuration?

Answer 1

AddInMemoryCollection takes a collection of KeyValuePair where a key is setting key and value is its value. This call from the question

configurationBuilder.AddInMemoryCollection("Settings", JsonConvert.SerializeObject(mySettings));

actually passes "Settings" as a key and whole JSON as one setting value, which expectedly does not work.

But the overall approach is correct, you should use AddInMemoryCollection extension call. In collection passed to this call, setting keys are full paths within configuration, delimited by a colon. Say if you have following settings POCO:

public class SomeSettings
{
    public string SomeKey1 { get; set; }
    public int SomeKey2 { get; set; }
}

and it's loaded from following JSON

{
    "SomeSettings": {
        "SomeKey1": "SomeData",
        "SomeKey2": 123
    } 
}

the keys would be SomeSettings:SomeKey1 and SomeSettings:SomeKey2.

You could then add such configuration with following AddInMemoryCollection call:

configurationBuilder.AddInMemoryCollection(new Dictionary<string, string>
{
    { "SomeSettings:SomeKey1", "SomeData" },
    { "SomeSettings:SomeKey2", 123 },
});

Now, if you want to add settings POCO with one call, you could write simple extension method that will enumerate setting class properties using reflection and return collection of key-value pairs for properties names and values.

Here is a sample:

public static class ObjectExtensions
{
    public static IEnumerable<KeyValuePair<string, string>> ToKeyValuePairs(this Object settings, string settingsRoot)
    {
        foreach (var property in settings.GetType().GetProperties())
        {
            yield return new KeyValuePair<string, string>($"{settingsRoot}:{property.Name}", property.GetValue(settings).ToString());
        }
    }
}

public static class ConfigurationBuilderExtensions
{
    public static void AddInMemoryObject(this ConfigurationBuilder configurationBuilder, object settings, string settingsRoot)
    {
        configurationBuilder.AddInMemoryCollection(settings.ToKeyValuePairs(settingsRoot));
    }
}

In the test:

var configurationBuilder = new ConfigurationBuilder();
var mySettings = GetTestSettings();
configurationBuilder.AddInMemoryObject(mySettings, "Settings");

Such simple ObjectExtensions.ToKeyValuePairs extension method will work for plain POCO like SomeSettings above. However it will not work if some of the properties are also objects like here:

public class InnerSettings
{
    public string SomeKey { get; set; }
}

public class OuterSettings
{
    public InnerSettings InnerSettings { get; set; }
}

However I believe you got an idea and could make required adjustments if required.