public class MyFixture
{
    private readonly ICloseLogFileCommand _closeCommand = Mock.Of<ICloseLogFileCommand>();

    private readonly ILog _log = Mocks.Of<ILog>().
        First(x => x.Name == @"C:\Path\To\File.log" &&
                   x.Count == 42 &&
                   x.Offset == TimeSpan.FromSeconds(69));
    ...
}