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Tobias Punke

React to configuration changes in .NET

  • C#
  • .NET
  • Thursday, May 16, 2024

OptionsMonitor
is a powerful tool that allows you to dynamically monitor configuration options and react to changes.

In this example, we consider the Starfleet's warp drive, which will be commissioned a few years after 2063. Undoubtedly, the system must be informed if the chief engineer has made some changes to the operating parameters.
x
–
+
warpcore.json
{
	"WarpCore":
	{
		"DilithiumFractionRatio": 0.456,
		"MaximumSafeWarpFactor": 9.99656,
		"CanTransWarp": false
	}
}
x
–
+
WarpCoreOptions.cs
public record WarpCoreOptions
{
	public Double DilithiumFractionRatio { get; init; }
	public Double MaximumSafeWarpFactor { get; init; }
	public Boolean CanTransWarp { get; init; }
}

Request 
IOptionsMonitor
from the DI container.

Instead of using 
IOptions
rom the DI container, one should alternatively request 
IOptionsMonitor
. This interface provides the capability to register a delegate as soon as the configuration source (often a JSON file) changes.
x
–
+
MarkSixWarpCore.cs
public class MarkSixWarpCore : IWarpCore, IDisposable
{
	private readonly IDisposable? _OptionsChangedListener;

	public MarkSixWarpCore(IOptionsMonitor<WarpCoreOptions> options)
	{
	    _OptionsChangedListener = options.OnChange(OnOptionsChanged);
	    ConfigureWarpCore(options.CurrentValue);
	}

	private void OnOptionsChanged(WarpCoreOptions options)
	{
		ConfigureWarpCore(options);
	}

	private void ConfigureWarpCore(WarpCoreOptions options)
	{
	    SetTransWarpCapability(options.CanTransWarp);
	    ...
	}

	/// <inheritdoc />
	public void Dispose()
	{
		_OptionsChangedListener?.Dispose();
	}
}

Add the JSON file to the 
IConfigurationBuilder
.

The JSON file is registered, specifying 
reloadOnChange
to indicate that the file should be reloaded when changes are detected in the file system.
x
–
+
Program.cs
private static IHostBuilder CreateHostBuilder(String[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureHostConfiguration(configHost =>
        {
            configHost.AddJsonFile("warpcore.json", reloadOnChange: true);
        });

A glue to bind them all

In addition to setting up the options class using the 
ConfigureOptions
class, which I examine in more detail in this article , an 
IOptionsChangeTokenSource
must also be registered. The 
ConfigurationChangeTokenSource
implementation is suitable for this purpose, as it handles the notifications.
x
–
+
Startup.cs
public void ConfigureServices(IServiceCollection services)
{
	services.TryAddEnumerable(
		ServiceDescriptor.Singleton<IConfigureOptions<WarpOptions>, WarpOptionsSetup>());

	services.AddSingleton<IOptionsChangeTokenSource<TOptions>>(serviceProvider =>
	{
		var configuration = serviceProvider.GetRequiredService<IConfiguration>();
		return new ConfigurationChangeTokenSource<TOptions>(name, configuration);
	});
}

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Topics

Read configuration data in .NET

Read configuration data in .NET

Automatic parsing of configuration data from JSON into a DTO using the 

ConfigureOptions
class.