Entity Framework Core: Log queries for a single db context instance

Call DbContextOptionsBuilder.UseLoggerFactory(loggerFactory) method to log all SQL output of a particular context instance. You could inject a logger factory in the context’s constructor.

Here is a usage example:

//this context writes SQL to any logs and to ReSharper test output window
using (var context = new TestContext(_loggerFactory))
{
    var customers = context.Customer.ToList();
}

//this context doesn't
using (var context = new TestContext())
{
    var products = context.Product.ToList();
}

Generally, I use this feature for manual testing. To keep the original context class clean, a derived testable context is declared with overridden OnConfiguring method:

public class TestContext : FooContext
{
    private readonly ILoggerFactory _loggerFactory;

    public TestContext() { }

    public TestContext(ILoggerFactory loggerFactory)
    {
        _loggerFactory = loggerFactory;
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        base.OnConfiguring(optionsBuilder);

        optionsBuilder.UseLoggerFactory(_loggerFactory);
    }
}

It’s enough to log SQL queries. Don’t forget to attach a suitable logger (like Console) to loggerFactory before you pass it to context.

Part II: Pass logs to xUnit output and ReSharper test output window

We can create a loggerFactory in a test class constructor:

public class TestContext_SmokeTests : BaseTest
{
    public TestContext_SmokeTests(ITestOutputHelper output)
        : base(output)
    {
        var serviceProvider = new ServiceCollection().AddLogging().BuildServiceProvider();

        _loggerFactory = serviceProvider.GetService<ILoggerFactory>();

        _loggerFactory.AddProvider(new XUnitLoggerProvider(this));
    }

    private readonly ILoggerFactory _loggerFactory;
}

The test class is derived from BaseTest which supports the writing to xUnit output:

public interface IWriter
{
    void WriteLine(string str);
}

public class BaseTest : IWriter
{
    public ITestOutputHelper Output { get; }

    public BaseTest(ITestOutputHelper output)
    {
        Output = output;
    }

    public void WriteLine(string str)
    {
        Output.WriteLine(str ?? Environment.NewLine);
    }
}

The most tricky part is to implement a logging provider accepting IWriter as a parameter:

public class XUnitLoggerProvider : ILoggerProvider
{
    public IWriter Writer { get; private set; }

    public XUnitLoggerProvider(IWriter writer)
    {
        Writer = writer;
    }
    public void Dispose()
    {
    }

    public ILogger CreateLogger(string categoryName)
    {
        return new XUnitLogger(Writer);
    }

    public class XUnitLogger : ILogger
    {
        public IWriter Writer { get; }

        public XUnitLogger(IWriter writer)
        {
            Writer = writer;
            Name = nameof(XUnitLogger);
        }

        public string Name { get; set; }

        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception,
            Func<TState, Exception, string> formatter)
        {
            if (!this.IsEnabled(logLevel))
                return;

            if (formatter == null)
                throw new ArgumentNullException(nameof(formatter));

            string message = formatter(state, exception);
            if (string.IsNullOrEmpty(message) && exception == null)
                return;

            string line = $"{logLevel}: {this.Name}: {message}";

            Writer.WriteLine(line);

            if (exception != null)
                Writer.WriteLine(exception.ToString());
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            return true;
        }

        public IDisposable BeginScope<TState>(TState state)
        {
            return new XUnitScope();
        }
    }

    public class XUnitScope : IDisposable
    {
        public void Dispose()
        {
        }
    }
}

We’ve done here! All the SQL logs will be shown in Rider/Resharper test output window.