Introduce VSReuseInstance parameter for test settings

DeveloperGuide.md

@@ -86,6 +86,7 @@ follows:
 | VSHive        | The hive name, like "Exp" or "Default" |
 | VSLaunchTimeoutInSeconds [opt] | The number of seconds to wait for launch |
 | VSDebugMixedMode | True to use mixed-mode debugging for tests |
+| VSReuseInstance [opt] | False to disable the reuse of the same VS instance for distinct tests |
 | ScreenCapture [opt] | Relative path to capture screenshots to |
 | ScreenCaptureInterval [opt] | Milliseconds between screenshots |
 

VSTestHost/TesterTestAdapter.cs

@@ -40,6 +40,7 @@ namespace Microsoft.VisualStudioTools.VSTestHost {
         private IRunContext _runContext;
         private TesteeTestAdapter _remote;
         private bool _mockVs;
+        private bool _reuseInstance;
 
         public TesterTestAdapter() { }
 
@@ -54,7 +55,8 @@ namespace Microsoft.VisualStudioTools.VSTestHost {
         ) {
             var hiveOption = string.IsNullOrEmpty(hive) ? "" : (" /rootSuffix " + hive);
 
-            if (_ide != null &&
+            if (_reuseInstance &&
+                _ide != null &&
                 _remote != null &&
                 application == _currentApplication &&
                 executable == _currentExecutable &&
@@ -169,6 +171,7 @@ namespace Microsoft.VisualStudioTools.VSTestHost {
             Version version;
             string launchTimeoutInSecondsString;
             int launchTimeoutInSeconds;
+            string reuseInstanceString;
 
             // VSApplication is the registry key name like 'VisualStudio'
             application = vars[VSTestProperties.VSApplication.Key] ?? VSTestProperties.VSApplication.VisualStudio;
@@ -216,6 +219,12 @@ namespace Microsoft.VisualStudioTools.VSTestHost {
                 return;
             }
 
+            if (!vars.TryGetValue(VSTestProperties.VSReuseInstance.Key, out reuseInstanceString) ||
+                !bool.TryParse(reuseInstanceString, out _reuseInstance))
+            {
+                // the default behavior is to reuse the current instance when possible
+                _reuseInstance = true;
+            }
 
             // TODO: Detect and perform first run of VS if necessary.
             // The first time a VS hive is run, the user sees a dialog allowing

VSTestHost/VSTestProperties.cs

@@ -50,5 +50,9 @@ namespace Microsoft.VisualStudioTools.VSTestHost {
         public static class VSDebugMixedMode {
             public const string Key = "VSDebugMixedMode";
         }
+
+        public static class VSReuseInstance {
+            public const string Key = "VSReuseInstance";
+        }
     }
 }