No longer crash when PrivateFieldNamingRule encounters WithEvents

Fixes: #216

Author: davkean

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/PrivateFieldNamingRuleTests.cs

@@ -14,7 +14,7 @@ internal override IGlobalSemanticFormattingRule Rule
             get { return new Rules.PrivateFieldNamingRule(); }
         }
 
-        private sealed class CSharpFields : PrivateFieldNamingRuleTests
+        public sealed class CSharpFields : PrivateFieldNamingRuleTests
         {
             [Fact]
             public void TestUnderScoreInPrivateFields()
@@ -215,7 +215,7 @@ int M(C p)
             }
         }
 
-        private sealed class VisualBasicFields : PrivateFieldNamingRuleTests
+        public sealed class VisualBasicFields : PrivateFieldNamingRuleTests
         {
             [Fact]
             public void Simple()
@@ -314,6 +314,23 @@ End Function
 
                 Verify(text, expected, languageName: LanguageNames.VisualBasic);
             }
+
+            [Fact]
+            public void FieldMarkedWithEvents()
+            {   // See:  https://github.com/dotnet/codeformatter/issues/216
+
+                var text = @"
+Class C1
+    Private Field WithEvents As Integer
+End Class";
+
+                var expected = @"
+Class C1
+    Private _field WithEvents As Integer
+End Class";
+
+                Verify(text, expected, languageName: LanguageNames.VisualBasic);
+            }
         }
     }
 }

src/Microsoft.DotNet.CodeFormatting/Rules/PrivateFieldNamingRule.cs

@@ -63,7 +63,10 @@ private async Task<Solution> RenameFields(Solution solution, DocumentId document
                     var semanticModel = await solution.GetDocument(documentId).GetSemanticModelAsync(cancellationToken);
                     var root = await semanticModel.SyntaxTree.GetRootAsync(cancellationToken);
                     var declaration = root.GetAnnotatedNodes(s_markerAnnotation).ElementAt(i);
-                    var fieldSymbol = (IFieldSymbol)semanticModel.GetDeclaredSymbol(declaration, cancellationToken);
+
+                    // Make note, VB represents "fields" marked as "WithEvents" as properties, so don't be
+                    // tempted to treat this as a IFieldSymbol. We only need the name, so ISymbol is enough.
+                    var fieldSymbol = semanticModel.GetDeclaredSymbol(declaration, cancellationToken);
                     var newName = GetNewFieldName(fieldSymbol);
 
                     // Can happen with pathologically bad field names like _
@@ -79,7 +82,7 @@ private async Task<Solution> RenameFields(Solution solution, DocumentId document
                 return solution;
             }
 
-            private static string GetNewFieldName(IFieldSymbol fieldSymbol)
+            private static string GetNewFieldName(ISymbol fieldSymbol)
             {
                 var name = fieldSymbol.Name.Trim('_');
                 if (name.Length > 2 && char.IsLetter(name[0]) && name[1] == '_')