Fix Extract Method operations within an event accessor

Fixes #17474

src/EditorFeatures/CSharpTest/ExtractMethod/ExtractMethodTests.cs

@@ -10079,6 +10079,47 @@ private async Task<int> NewMethod(S s)
             await TestExtractMethodAsync(code, expected, dontPutOutOrRefOnStruct: true);
         }
 
+        [Theory]
+        [InlineData("add", "remove")]
+        [InlineData("remove", "add")]
+        [WorkItem(17474, "https://github.com/dotnet/roslyn/issues/17474")]
+        [Trait(Traits.Feature, Traits.Features.ExtractMethod)]
+        public async Task TestExtractMethodEventAccessorUnresolvedName(string testedAccessor, string untestedAccessor)
+        {
+            // This code intentionally omits a 'using System;'
+            var code =
+$@"namespace ClassLibrary9
+{{
+    public class Class
+    {{
+        public event EventHandler Event
+        {{
+            {testedAccessor} {{ [|throw new NotImplementedException();|] }}
+            {untestedAccessor} {{ throw new NotImplementedException(); }}
+        }}
+    }}
+}}";
+            var expected =
+$@"namespace ClassLibrary9
+{{
+    public class Class
+    {{
+        public event EventHandler Event
+        {{
+            {testedAccessor} {{ NewMethod(); }}
+            {untestedAccessor} {{ throw new NotImplementedException(); }}
+        }}
+
+        private static void NewMethod()
+        {{
+            throw new NotImplementedException();
+        }}
+    }}
+}}";
+
+            await TestExtractMethodAsync(code, expected);
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.ExtractMethod)]
         public async Task ExtractMethod_Argument1()
         {

src/Features/CSharp/Portable/ExtractMethod/CSharpSelectionResult.StatementResult.cs

@@ -71,13 +71,23 @@ public override ITypeSymbol GetContainingScopeType()
                 switch (node)
                 {
                     case AccessorDeclarationSyntax access:
-                        // property case
+                        // property or event case
                         if (access.Parent == null || access.Parent.Parent == null)
                         {
                             return null;
                         }
 
-                        return ((IPropertySymbol)semanticModel.GetDeclaredSymbol(access.Parent.Parent)).Type;
+                        switch (semanticModel.GetDeclaredSymbol(access.Parent.Parent))
+                        {
+                            case IPropertySymbol propertySymbol:
+                                return propertySymbol.Type;
+
+                            case IEventSymbol eventSymbol:
+                                return eventSymbol.Type;
+
+                            default:
+                                return null;
+                        }
 
                     case MethodDeclarationSyntax method:
                         return semanticModel.GetDeclaredSymbol(method).ReturnType;