How to detect if my application is running in a virtual machine?

This is what I use:

using (var searcher = new System.Management.ManagementObjectSearcher("Select * from Win32_ComputerSystem"))
{
  using (var items = searcher.Get())
  {
    foreach (var item in items)
    {
      string manufacturer = item["Manufacturer"].ToString().ToLower();
      if ((manufacturer == "microsoft corporation" && item["Model"].ToString().ToUpperInvariant().Contains("VIRTUAL"))
          || manufacturer.Contains("vmware")
          || item["Model"].ToString() == "VirtualBox")
      {
        return true;
      }
    }
  }
}
return false;

Edit 2014-12-02: Updated code so that it no longer detects a Microsoft Surface Pro as a VM. Thanks to Erik Funkenbusch for pointing this out.

Edit 2017-06-29: Updated code so that it also checks the value of the HypervisorPresent property.

Edit 2018-02-05: removed check for the HypervisorPresent property since it's incorrect. This property could return true if running on the host O/S on a hyper-V server.

According to Virtual PC Guy's blog post "Detecting Microsoft virtual machines", you can use WMI to check the manufacturer of the motherboard. In PowerShell:

 (gwmi Win32_BaseBoard).Manufacturer -eq "Microsoft Corporation"