How to get CPU frequency in c#

If you want to get the turbo speed, you can make use of the "% Processor Performance" performance counter and multiply it with the WMI "MaxClockSpeed" as follows:

private string GetCPUInfo()
{
  PerformanceCounter cpuCounter = new PerformanceCounter("Processor Information", "% Processor Performance", "_Total");
  double cpuValue = cpuCounter.NextValue();

  Thread loop = new Thread(() => InfiniteLoop());
  loop.Start();

  Thread.Sleep(1000);
  cpuValue = cpuCounter.NextValue();
  loop.Abort();

  foreach (ManagementObject obj in new ManagementObjectSearcher("SELECT *, Name FROM Win32_Processor").Get())
  {
    double maxSpeed = Convert.ToDouble(obj["MaxClockSpeed"]) / 1000;
    double turboSpeed = maxSpeed * cpuValue / 100;
    return string.Format("{0} Running at {1:0.00}Ghz, Turbo Speed: {2:0.00}Ghz",  obj["Name"], maxSpeed, turboSpeed);
  }

  return string.Empty;
}

The InfiniteLoop method is simply an integer that gets 1 added and subtracted:

private void InfiniteLoop()
{
  int i = 0;

  while (true)
    i = i + 1 - 1;
}

The InfiniteLoop method is just added to give the CPU something to do and turbo in the process. The loop is allowed to run for a second before the next value is taken and the loop aborted.