I wanted a way to get the latest version of SolidWorks installed (if SolidWorks wasn't already open) on a machine to automate with a Standalone. I thought this might be useful for anyone working on machines where the installer neglected to uninstall the previous versions of SolidWorks, but the latest version is preferred.

I do this by:

1. getting the current year (System.DateTime.Now.Year) at the time of execution
2. adding two years (for good measure, maybe only one is necessary)
3. subtracting the first SW year (1992) to get that RevisionNumber
4. iterating backwards until SolidWorks app is found, or REV# < 0 to end processing.

using System;
using SolidWorks.Interop.sldworks;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;//For MessageBox

/// 

/// Gets SolidWorks Session
/// 
/// True gets only the current session, False opens new session if session not already active.
/// "YYYY" for year, or "VV" for version. 
/// /// True to end if null, False to return null if app not found
/// app if found, null if not.
public static SldWorks GetSolidWorks(bool CurrentSessionOnly = false, string version = "", bool EndIfNull = false)
{
int i = DateTime.Now.Year + 2 - 1992;            
	SldWorks app = null;
if (version != "")
{
int j = Convert.ToInt32(version);
if (version.Length == 4)
			j = j - 1992;
try
{
			app = (SldWorks)Marshal.GetActiveObject(\$"SldWorks.Application.{j.ToString()}");
}
catch (Exception) { }
if (app == null && CurrentSessionOnly == false)
{
try
{
				app = (SldWorks)Activator.CreateInstance(Type.GetTypeFromProgID(\$"SldWorks.Application.{j.ToString()}"));
}
catch (Exception) { }
}
if (app == null)
			Debug.Print("Could not get that version of Solidworks. Latest installed version will be fetched.");
}

if (app == null)
{
try
{
			app = (SldWorks)Marshal.GetActiveObject("SldWorks.Application");
}
catch (Exception)
{
if (CurrentSessionOnly == false)
{
do
{
try
{
						app = (SldWorks)Activator.CreateInstance(Type.GetTypeFromProgID(\$"SldWorks.Application.{i.ToString()}"));
}
catch (Exception) { }
--i;
if (i < 0)
break;
} while (app == null);
}
}
}
if (app == null)
{
		MessageBox.Show("SolidWorks not found or\n" +
"system date may be incorrect.\n" +
"Program will terminate.",
"SolidWorks Not Found");
if (EndIfNull == true)
			System.Environment.Exit(0);
}
else
		app.Visible = true;            
return app;‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

Feel free to post if you have a better solution or suggestions on this code.