Diagnostics.Process.Start: UseShellExecute=true and EnvironmentVariables gotcha #58069

I was working on a program that needs to shell out to an OS handler to do some printing, and was continually getting this error while I was debugging.

The Process object must have the UseShellExecute property set to false in order to use environment variables.

Short summary; if you (or anything including a debugger, serializer, etc) accesses ProcessStartInfo.EnvironmentVariables it gets set to not null and causes an exception later on when you try to use UseShellExecute = true and start the process.

What is occurring is the check here Process.Win32.cs#L47 has a conflict with the lazy initialization logic here ProcessStartInfo.cs#L78.

• Suggested Improvement 1; In StartWithShellExecuteEx during the argument validation phase, rather than just check _environmentVariables = null, add a check for an empty collection.

if (startInfo._environmentVariables != null && _environmentVariables.Any())
                throw new InvalidOperationException(SR.CantUseEnvVars);

This would be the least impactful to the API and allows a user struggling with this problem to simply issue myProcessStartInfo.EnvironmentVariables.Clear(); to remove the environment variables. However, if a user does clear the EnvironmentVariables collection, they still had to silently load them at some point, due to the lazy loading logic.

• Suggested Improvement 2; In ProcessStartInfo, add a check for UseShellExecute to gate access or creation of the EnvironmentVariables collection. Something like this:

        public IDictionary<string, string?> Environment
        {
            get
            {
                if (_environmentVariables == null && UseShellExecute == false)
                {
                 [..do the needful..]

This isn't a full solution since there's partial classes at work here, but this would in principle prevent the _environmentVariables variable from getting initialized if UseShellExecute was set to true. This creates an order of operations issue which would also have to be considered. If you accessed EnvironmentVariables and then later set UseShellExecute=true, what should happen?