xUnit.net does not capture console output

This can help if your Console.Write is embedded deep down some class hierarchy that you don't want to refactor:

    public MyTestClass(ITestOutputHelper output)
    {
        var converter = new Converter(output);
        Console.SetOut(converter);
    }

    private class Converter : TextWriter
    {
        ITestOutputHelper _output;
        public Converter(ITestOutputHelper output)
        {
            _output = output;
        }
        public override Encoding Encoding
        {
            get { return Encoding.Whatever; }
        }
        public override void WriteLine(string message)
        {
            _output.WriteLine(message);
        }
        public override void WriteLine(string format, params object[] args)
        {
            _output.WriteLine(format, args);
        }

        public override void Write(char value)
        {
            throw new NotSupportedException("This text writer only supports WriteLine(string) and WriteLine(string, params object[]).");
        }
    }

The situation has changed a little with xUnit.net 2. I know the question is about an earlier version, but as people will land here having performed the upgrade, I thought it was worth pointing this out.

In order to see some kind of output in the test output in version 2 you will need to take a dependency in your test class (via a constructor argument) on an instance of Xunit.Abstractions.ITestOutputHelper, then use the WriteLine method on this interface. E.g.:

public class MyTestSpec
{
  private readonly ITestOutputHelper _testOutputHelper;

  public MyTestSpec(ITestOutputHelper testOutputHelper)
  {
    _testOutputHelper = testOutputHelper;
  }

  [Fact]
  public void MyFact()
  {
    _testOutputHelper.WriteLine("Hello world");
  }
}

You could choose to hook up your logging framework to this interface, perhaps by injecting an ILog implementation that forwarded all calls to ITestOutpuHelper.

I acknowledge that you won't want to do this by default, but for diagnostic purposes from time to time it can be quite useful. This is especially true where your tests only fail on some cloud based build & test server!