Avoid DataTip crash in .cshtml...

There are cases were a DataBuffer span may map to two zero-length spans in the _subjectBuffer.  In these cases, rather than throwing an Exception, we can simply try returning a DataTip from each one.

src/VisualStudio/Core/Def/Implementation/Venus/VenusCommandFilter.cs

@@ -61,49 +61,52 @@ public override int GetDataTipText(TextSpan[] pSpan, out string pbstrText)
                 return VSConstants.E_FAIL;
             }
 
-            // We need to map the TextSpan from the DataBuffer to our subject buffer. We'll
-            // only consider spans whose length matches our input span (which we expect to
-            // always be zero). This is to address the case where the position is on a seam
-            // and maps to multiple source spans.
-            // If there is not exactly one matching span, just return.
+            // We need to map the TextSpan from the DataBuffer to our subject buffer.
             var span = textViewModel.DataBuffer.CurrentSnapshot.GetSpan(pSpan[0]);
-            var spanLength = span.Length;
-            Debug.Assert(spanLength == 0, $"Expected zero length span (got '{spanLength}'.");
-            var subjectSpan = WpfTextView.BufferGraph.MapDownToBuffer(span, SpanTrackingMode.EdgeInclusive, _subjectBuffer)
-                                .SingleOrDefault(x => x.Length == spanLength);
+            var subjectSpans = WpfTextView.BufferGraph.MapDownToBuffer(span, SpanTrackingMode.EdgeInclusive, _subjectBuffer);
 
-            if (subjectSpan == default(SnapshotSpan))
+            // The following loop addresses the case where the position is on a seam and maps to multiple source spans.
+            // In these cases, we assume it's okay to return the first span that successfully returns a DataTip.
+            // It's most likely that either only one will succeed or both with fail.
+            var expectedSpanLength = span.Length;
+            foreach (var candidateSpan in subjectSpans)
             {
-                pbstrText = null;
-                return VSConstants.E_FAIL;
-            }
+                // First, we'll only consider spans whose length matches our input span. 
+                if (candidateSpan.Length != expectedSpanLength)
+                {
+                    continue;
+                }
 
-            pSpan[0] = subjectSpan.ToVsTextSpan();
+                // Next, we'll check to see if there is actually a DataTip for this candidate.
+                // If there is, we'll map this span back to the DataBuffer and return it.
+                pSpan[0] = candidateSpan.ToVsTextSpan();
+                int hr = base.GetDataTipText(pSpan, out pbstrText);
+                if (ErrorHandler.Succeeded(hr))
+                {
+                    var subjectSpan = _subjectBuffer.CurrentSnapshot.GetSpan(pSpan[0]);
 
-            int hr = base.GetDataTipText(pSpan, out pbstrText);
+                    // When mapping back up to the surface buffer, if we get more than one span,
+                    // take the span that intersects with the input span, since that's probably
+                    // the one we care about.
+                    // If there are no such spans, just return.
+                    var surfaceSpan = WpfTextView.BufferGraph.MapUpToBuffer(subjectSpan, SpanTrackingMode.EdgeInclusive, textViewModel.DataBuffer)
+                                        .SingleOrDefault(x => x.IntersectsWith(span));
 
-            // pSpan is an in/out parameter, so map it back to the Databuffer.
-            if (ErrorHandler.Succeeded(hr))
-            {
-                subjectSpan = _subjectBuffer.CurrentSnapshot.GetSpan(pSpan[0]);
+                    if (surfaceSpan == default(SnapshotSpan))
+                    {
+                        pbstrText = null;
+                        return VSConstants.E_FAIL;
+                    }
 
-                // When mapping back up to the surface buffer, if we get more than one span,
-                // take the span that intersects with the input span, since that's probably
-                // the one we care about.
-                // If there are no such spans, just return.
-                var surfaceSpan = WpfTextView.BufferGraph.MapUpToBuffer(subjectSpan, SpanTrackingMode.EdgeInclusive, textViewModel.DataBuffer)
-                                    .SingleOrDefault(x => x.IntersectsWith(span));
+                    // pSpan is an in/out parameter
+                    pSpan[0] = surfaceSpan.ToVsTextSpan();
 
-                if (surfaceSpan == default(SnapshotSpan))
-                {
-                    pbstrText = null;
-                    return VSConstants.E_FAIL;
+                    return hr;
                 }
-
-                pSpan[0] = surfaceSpan.ToVsTextSpan();
             }
 
-            return hr;
+            pbstrText = null;
+            return VSConstants.E_FAIL;
         }
     }
 }