SingleResponsibilityPrinciple_Valid.cs

public class Sensor
{
  public string Name { get; }
  public List<double> MeasuredValues { get; }
  public double AverageValue
    => MeasuredValues.Average();
  public double MinimumValue
    => MeasuredValues.Min();
  public double MaximumValue
    => MeasuredValues.Max();

  public Sensor(string name, IEnumerable<double> measuredValues)
  {
    Name = name;
    MeasuredValues = new List<double>(measuredValues);
  }
}

public class PlainTextSensorReportGenerator : ISensorReportGenerator
{
  private readonly IFilesystem _filesystem;

  public PlainTextSensorReportGenerator(IFilesystem filesystem)
  {
    _filesystem = filesystem;
  }

  public void GenerateReport(Sensor sensor, string reportFile)
    => _filesystem.WriteContentToFile(reportFile, ToFileContent(sensor));

  private string ToFileContent(Sensor sensor)
  {
    var fileContent =
      $"#SensorInfo#Name:{sensor.Name};Average:{sensor.AverageValue};"
      + $"Minimum:{sensor.MinimumValue};Maximum:{sensor.MaximumValue}";

    sensor.MeasuredValues.ForEach(
        value => fileContent += $@"{Environment.NewLine}##{value}"
      );
    return fileContent;
  }
}