Add IsObsolete to ISymbolExtensions and use it.

src/Features/Core/Portable/Completion/CommonCompletionUtilities.cs

@@ -171,16 +171,13 @@ public static bool IsStartingNewWord(SourceText text, int characterPosition, Fun
             Workspace workspace, SemanticModel semanticModel, int position, IReadOnlyList<ISymbol> symbols, SupportedPlatformData supportedPlatforms, CancellationToken cancellationToken)
         {
             var symbol = symbols[0];
-            if (IsObsolete(symbol))
+            if (symbol.IsObsolete())
             {
-                // The symbol is obsolete/deprecated. Lets try to find another one that is not and take that instead.
-                symbol = symbols.FirstOrDefault(s => !IsObsolete(s)) ?? symbol;
+                // The first symbol is obsolete/deprecated. Lets try to find another one that is not and take that instead.
+                symbol = symbols.Skip(1).FirstOrDefault(s => !s.IsObsolete()) ?? symbol;
             }
 
             return CreateDescriptionAsync(workspace, semanticModel, position, symbol, overloadCount: symbols.Count - 1, supportedPlatforms, cancellationToken);
-
-            static bool IsObsolete(ISymbol symbol)
-                => symbol.GetAttributes().Any(x => x.AttributeClass.MetadataName == "ObsoleteAttribute");
         }
 
         private static void AddOverloadPart(List<TaggedText> textContentBuilder, int overloadCount, bool isGeneric)

src/Features/Core/Portable/LanguageServices/SymbolDisplayService/AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.cs

@@ -247,7 +247,7 @@ protected void AddCaptures(SyntaxNode syntax)
 
             private async Task AddDescriptionPartAsync(ISymbol symbol)
             {
-                if (symbol.GetAttributes().Any(x => x.AttributeClass.MetadataName == "ObsoleteAttribute"))
+                if (symbol.IsObsolete())
                 {
                     AddDeprecatedPrefix();
                 }

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/Extensions/ISymbolExtensions.cs

@@ -749,5 +749,15 @@ private static bool VerifyGetAsyncEnumerator(IMethodSymbol getAsyncEnumerator)
         public static bool IsSymbolWithSpecialDiscardName(this ISymbol symbol)
             => symbol.Name.StartsWith("_") &&
                (symbol.Name.Length == 1 || uint.TryParse(symbol.Name.Substring(1), out _));
+
+        /// <summary>
+        /// Returns <see langword="true"/>, if the symbol is marked with the <see cref="System.ObsoleteAttribute"/>.
+        /// </summary>
+        /// <param name="symbol"></param>
+        /// <returns><see langword="true"/> if the symbol is marked with the <see cref="System.ObsoleteAttribute"/>.</returns>
+        public static bool IsObsolete(this ISymbol symbol)
+            => symbol.GetAttributes().Any(x =>
+                x.AttributeClass?.MetadataName == nameof(ObsoleteAttribute) &&
+                x.AttributeClass?.ContainingNamespace.Name == nameof(System));
     }
 }