C#: Fix some alerts, and fix a potential NullReferenceException.

Author: calumgrant

csharp/extractor/Semmle.Extraction.CIL/Entities/Method.cs

@@ -402,6 +402,9 @@ public MemberReferenceMethod(GenericContext gc, MemberReferenceHandle handle) :
 
             declType = parentMethod == null ? parent as Type : parentMethod.DeclaringType;
 
+            if (declType is null)
+                throw new InternalError("Parent context of method is not a type");
+
             ShortId = MakeMethodId(declType, nameLabel);
 
             var typeSourceDeclaration = declType.SourceDeclaration;

csharp/extractor/Semmle.Extraction.CIL/Entities/Type.cs

@@ -1121,7 +1121,7 @@ class GenericMethodParameter : ITypeSignature
             static readonly Id excl = Id.Create("M!");
             public Id MakeId(GenericContext outerGc)
             {
-                if (innerGc != outerGc && innerGc is Method method)
+                if (!ReferenceEquals(innerGc, outerGc) && innerGc is Method method)
                     return open + method.Label.Value + close + excl + index;
                 return excl + index;
             }

csharp/extractor/Semmle.Extraction.CSharp/Entities/Modifier.cs

@@ -134,6 +134,9 @@ public static void ExtractModifiers(Context cx, IEntity key, ISymbol symbol)
             if (symbol.Kind == SymbolKind.NamedType)
             {
                 INamedTypeSymbol nt = symbol as INamedTypeSymbol;
+                if (nt is null)
+                    throw new InternalError(symbol, "Symbol kind is inconsistent with its type");
+
                 if (nt.TypeKind == TypeKind.Struct)
                 {
                     // Sadly, these properties are internal so cannot be accessed directly.

csharp/extractor/Semmle.Extraction/Context.cs

@@ -93,7 +93,7 @@ private void CheckEntityHasUniqueLabel(IId id, ICachedEntity entity)
         {
             if (idLabelCache.TryGetValue(id, out var originalEntity))
             {
-                Extractor.Message(new Message { message = "Label collision for " + id.ToString(), severity = Severity.Warning });
+                Extractor.Message(new Message { message = "Label collision for " + id, severity = Severity.Warning });
             }
             else
             {

csharp/extractor/Semmle.Extraction/Symbol.cs

@@ -57,7 +57,7 @@ protected void WithDuplicationGuard(System.Action a, IEntity location)
         public override bool Equals(object obj)
         {
             var other = obj as CachedEntity<Initializer>;
-            return obj != null && obj.GetType() == GetType() && Equals(other.symbol, symbol);
+            return other != null && other.GetType() == GetType() && Equals(other.symbol, symbol);
         }
 
         public abstract TrapStackBehaviour TrapStackBehaviour { get; }