GetEnvironmentVariable() and SetEnvironmentVariable() for PATH Variable

Question

I want to extend the current PATH variable with a C# program. Here I have several problems:

1. Using GetEnvironmentVariable("PATH", EnvironmentVariableTarget.Machine) replaces the placeholders (i.e. '%SystemRoot%\system32' is replaced by the current path 'C:\Windows\system32'). Updating the PATH variable, I dont want to replace the placeholder with the path.

2. After SetEnvironmentVariable no program can't be opened from the command box anymore (i.e. calc.exe in the command box doesn't work). Im using following code:

String oldPath = Environment.GetEnvironmentVariable("PATH", EnvironmentVariableTarget.Machine);
Environment.SetEnvironmentVariable("PATH", oldPath + ";%MYDIR%", EnvironmentVariableTarget.Machine);

After editing and changing the PATH variable with Windows everything works again. (I thing changes are required, otherwise it is not overwritten)

Answer 1

You can use the registry to read and update:

string keyName = @"SYSTEM\CurrentControlSet\Control\Session Manager\Environment";
//get non-expanded PATH environment variable            
string oldPath = (string)Registry.LocalMachine.CreateSubKey(keyName).GetValue("Path", "", RegistryValueOptions.DoNotExpandEnvironmentNames);

//set the path as an an expandable string
Registry.LocalMachine.CreateSubKey(keyName).SetValue("Path", oldPath + ";%MYDIR%",    RegistryValueKind.ExpandString);

Answer 2

You can use WMI to retrieve the raw values (not sure about updating them though):

ManagementObjectSearcher searcher = new ManagementObjectSearcher("select * from Win32_Environment WHERE Name = 'PATH'");
foreach (ManagementBaseObject managementBaseObject in searcher.Get())
     Console.WriteLine(managementBaseObject["VariableValue"]);

Check WMI Reference on MSDN