Using System.Diagnostic.DiagnosticSource to intercept database requests– Improvements

Yesterday I blogged about the usage of DiagnosticSource to capture database specific events. The code we ended up with looked like this:

	public sealed class SqlClientObserver : IObserver<DiagnosticListener>, IObserver<KeyValuePair<string, object>>
	{
	private readonly IList<IDisposable> _listeners = new List<IDisposable>();
	void IObserver<DiagnosticListener>.OnNext(DiagnosticListener diagnosticListener)
	{
	lock (_listeners)
	{
	if (diagnosticListener.Name == "SqlClientDiagnosticListener")
	{
	var subscription = diagnosticListener.Subscribe(this);
	_listeners.Add(subscription);
	}
	}
	}
	void IObserver<DiagnosticListener>.OnError(Exception error)
	{ }
	void IObserver<DiagnosticListener>.OnCompleted()
	{
	lock(_listeners)
	{
	_listeners.ForEach(x => x.Dispose());
	_listeners.Clear();
	}
	}
	void IObserver<KeyValuePair<string, object>>.OnNext(KeyValuePair<string, object> pair)
	{
	Console.WriteLine(pair.Key);
	Console.WriteLine(pair.Value)
	}
	void IObserver<KeyValuePair<string, object>>.OnError(Exception error)
	{ }
	void IObserver<KeyValuePair<string, object>>.OnCompleted()
	{ }
	}

view raw SqlClientObserver.cs hosted with ❤ by GitHub

What is rather unfortunate is that you are working with generic events and that you need to use magic strings and reflection to capture the object data:

	void IObserver<KeyValuePair<string, object>>.OnNext(KeyValuePair<string, object> pair)
	{
	switch (pair.Key)
	{
	case"System.Data.SqlClient.WriteCommandAfter":
	{
	var command = GetProperty<SqlCommand>(pair.Value, "Command");
	Console.WriteLine($"CommandText: {command.CommandText}");
	Console.WriteLine($"Elapsed: {stopwatch.Elapsed}");
	Console.WriteLine();
	break;
	}
	}
	}
	private static T GetProperty<T>(objectvalue, string name)
	{
	return (T) value.GetType()
	.GetProperty(name)
	.GetValue(value);
	}

view raw OnNext.cs hosted with ❤ by GitHub

A solution exists through the usage of the Microsoft.Extensions.DiagnosticAdapter package. This Nuget package will use code generation to avoid the cost of reflection.

• Add a reference to the Microsoft.Extensions.DiagnosticAdapter package:

dotnet add package Microsoft.Extensions.DiagnosticAdapter

• Update the implementation to use the SubscribeWithAdapter extension method:

	void IObserver<DiagnosticListener>.OnNext(DiagnosticListener diagnosticListener)
	{
	lock (_listeners)
	{
	if (diagnosticListener.Name == "SqlClientDiagnosticListener")
	{
	var subscription = diagnosticListener.SubscribeWithAdapter(this);
	_listeners.Add(subscription);
	}
	}
	}

view raw SubscribeWithAdapter.cs hosted with ❤ by GitHub

• Now we no longer need to implement the IObserver<KeyValuePair<string, object>> interface. Instead, for each event that we want to handle, we need to declare a separate method, marking it with an attribute DiagnosticNameAttribute. The parameters of these methods are the parameters of the event being processed:

	[DiagnosticName("System.Data.SqlClient.WriteCommandAfter")]
	public void OnCommandAfter(DbCommand command)
	{
	Console.WriteLine($"CommandText: {command.CommandText}");
	}

view raw OnCommandAfter.cs hosted with ❤ by GitHub