var rs = RunspaceFactory.CreateRunspace();
            rs.Open();

// create a pipeline to add the variable into the runspace
            var pipeline = PowerShell.Create();
            // create a command object, add commands and parameters to it...
            var cmd = new PSCommand();
            cmd.AddCommand("New-Variable");
            cmd.AddParameter("Name", "foo");
            cmd.AddParameter("Value", "@()");
            // associate the command with the pipeline and runspace, and then invoke
            pipeline.Commands = cmd;
            pipeline.Runspace = rs;
            pipeline.Invoke();

cmd.AddParameter("Value", new object[0]);

pipeline.Runspace.SessionStateProxy.PSVariable.Set("foo", new object[0]);