Override Configuration via Command Line

In my previous blog posts I have talked about creating complex config objects from your app.config file, as well as how to have cascading configuration settings from multiple files. Now I want to build on that concept by taking in configuration from command line in a generic fashion that will override your other cascading settings.

Configuration Object

public class TestConfig

{

    public string Hello { get; set; }

    public string Goodnight { get; set; }

}

App.config

<?xml version="1.0" encoding="utf-8" ?>

<configuration>

  <appSettings>

    <add key="TestConfig.Hello" value="World" />

    <add key="TestConfig.Goodnight" value="Moon" />

  </appSettings>

</configuration>

ConfigurationHelpers.FromCommandLine

Please note that this is a very simple implementation of how to parse the command line arguments and will not be resilient to all inputs.

public static class ConfigurationHelpers

{

    public static NameValueCollection FromCommandLine(

        string args, 

        string argsDelimeter = " ",

        string prefix = "/s:",

        string valueDelimeter = "=")

    {

        var result = new NameValueCollection();

        var splitArgs = args.Split(

            new[] {argsDelimeter},

            StringSplitOptions.RemoveEmptyEntries);

        foreach (var arg in splitArgs)

        {

            if (!arg.StartsWith(prefix))

            {

                continue;

            }

            var i = arg.IndexOf('=');

            if (i == -1)

            {

                continue;

            }

            var key = arg.Substring(prefix.Length, i - prefix.Length);

            var value = arg.Substring(i + 1);

            result.Add(key, value);

        }

        return result;

    }

}

Unit Tests

public class ConfigurationHelpersTests

{

    [Theory]

    [InlineData("/s:TestConfig.Hello=Hello", 1, "Hello")]

    [InlineData("/s:TestConfig.Goodnight=Dawg", 1, "World")]

    [InlineData("/s:TestConfig.Hello=There /s:TestConfig.Goodnight=Dawg", 2, "There")]

    public void FromCommandLine(string args, int count, string helloValue)

    {

        var map = ConfigurationHelpers.FromCommandLine(args);

        Assert.Equal(count, map.Count);

        var combined = map.Combine(ConfigurationManager.AppSettings);

        var config = combined.CreateObject<TestConfig>(string.Empty);

        Assert.Equal(helloValue, config.Hello);

    }

}

Enjoy,
Tom