Process Environment Variable Testing

ExtractedFromLINQPad.cs

void Main()
{
	var psi = new ProcessStartInfo("cmd")
	{
		UseShellExecute = false
	};
	
	psi.Environment.OrderBy(kvp => kvp.Key).Dump("Current Process");
	
	var original = new Dictionary<string, string>(psi.Environment);
	
	psi.Environment.Clear();
	
	var systemVars = Environment.GetEnvironmentVariables(EnvironmentVariableTarget.Machine).Cast<DictionaryEntry>().ToDictionary(kvp => (string)kvp.Key, kvp => (string)kvp.Value);
	var userVars = Environment.GetEnvironmentVariables(EnvironmentVariableTarget.User).Cast<DictionaryEntry>().ToDictionary(kvp => (string)kvp.Key, kvp => (string)kvp.Value);

	foreach (var kvp in systemVars)
	{
		psi.Environment.Add(kvp.Key, kvp.Value);
	}

	foreach (var kvp in userVars)
	{
		if (psi.Environment.TryGetValue(kvp.Key, out var existingValue))
		{
			if (kvp.Key.ToUpperInvariant() == "PATH")
			{
				psi.Environment[kvp.Key] = $"{existingValue};{kvp.Value}";
			}
			else
			{
				psi.Environment[kvp.Key] = kvp.Value;
			}
		}
		else
		{
			psi.Environment.Add(kvp.Key, kvp.Value);
		}
	}
		
	systemVars.OrderBy(kvp => kvp.Key).Dump("System");
	userVars.OrderBy(kvp => kvp.Key).Dump("User");
	psi.Environment.OrderBy(kvp => kvp.Key).Dump("Flattened");
	
	var missingKeys = original.Keys.ToHashSet();
	missingKeys.ExceptWith(psi.Environment.Keys);
	
	missingKeys.Dump("Current Process Only?");
	
	foreach (var key in missingKeys)
	{
		psi.Environment.Add(key, original[key]);
	}
	
	Process.Start(psi);
}

// Define other methods and classes here

Note for anyone who stumbles on this, the Windows API I was looking for was CreateEnvironmentBlock. There is a sample here, though it would be nice if the parsing code were a little more elegant and safer with handle destruction, so perhaps I will update this later when I've written my own solution.