Harden BindingGroup against re-entrant binding-expression changes

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Data/BindingGroup.cs

@@ -382,9 +382,14 @@ public void CancelEdit()
             }
 
             // update targets
-            for (int i=_bindingExpressions.Count - 1; i>=0; --i)
+            BindingExpressionBase[] bindingExpressions = CopyBindingExpressions();
+            for (int i=bindingExpressions.Length - 1; i>=0; --i)
             {
-                _bindingExpressions[i].UpdateTarget();
+                BindingExpressionBase bindingExpression = bindingExpressions[i];
+                if (bindingExpression.BindingGroup != this)
+                    continue;
+
+                bindingExpression.UpdateTarget();
             }
 
             // also update dependent targets.  These are one-way bindings that
@@ -1127,13 +1132,26 @@ private void UpdateNotifyDataErrorValidationErrors(WeakReference itemWR, List<ob
             }
         }
 
+        // return a snapshot of the binding expressions - see https://github.com/dotnet/wpf/issues/1690
+        private BindingExpressionBase[] CopyBindingExpressions()
+        {
+            BindingExpressionBase[] copy = new BindingExpressionBase[_bindingExpressions.Count];
+            _bindingExpressions.CopyTo(copy, 0);
+            return copy;
+        }
+
         // apply conversions to each binding in the group
         private bool ObtainConvertedProposedValues()
         {
             bool result = true;
-            for (int i=_bindingExpressions.Count-1; i>=0; --i)
+            BindingExpressionBase[] bindingExpressions = CopyBindingExpressions();
+            for (int i=bindingExpressions.Length-1; i>=0; --i)
             {
-                result = _bindingExpressions[i].ObtainConvertedProposedValue(this) && result;
+                BindingExpressionBase bindingExpression = bindingExpressions[i];
+                if (bindingExpression.BindingGroup != this)
+                    continue;
+
+                result = bindingExpression.ObtainConvertedProposedValue(this) && result;
             }
 
             return result;
@@ -1144,9 +1162,14 @@ private bool UpdateValues()
         {
             bool result = true;
 
-            for (int i=_bindingExpressions.Count-1; i>=0; --i)
+            BindingExpressionBase[] bindingExpressions = CopyBindingExpressions();
+            for (int i=bindingExpressions.Length-1; i>=0; --i)
             {
-                result = _bindingExpressions[i].UpdateSource(this) && result;
+                BindingExpressionBase bindingExpression = bindingExpressions[i];
+                if (bindingExpression.BindingGroup != this)
+                    continue;
+
+                result = bindingExpression.UpdateSource(this) && result;
             }
 
             if (_proposedValueBindingExpressions != null)
@@ -1172,9 +1195,16 @@ private bool CheckValidationRules()
             ClearValidationErrors(_validationStep);
 
             // check rules attached to the bindings
-            for (int i=_bindingExpressions.Count-1; i>=0; --i)
+            BindingExpressionBase[] bindingExpressions = CopyBindingExpressions();
+            for (int i=bindingExpressions.Length-1; i>=0; --i)
             {
-                if (!_bindingExpressions[i].CheckValidationRules(this, _validationStep))
+                BindingExpressionBase bindingExpression = bindingExpressions[i];
+                if (bindingExpression.BindingGroup != this)
+                {
+                    continue;
+                }
+
+                if (!bindingExpression.CheckValidationRules(this, _validationStep))
                 {
                     result = false;
                 }