Readonly members symbol display

src/Compilers/CSharp/Portable/SymbolDisplay/SymbolDisplayVisitor.Members.cs

@@ -82,6 +82,11 @@ public override void VisitProperty(IPropertySymbol symbol)
             AddAccessibilityIfRequired(symbol);
             AddMemberModifiersIfRequired(symbol);
 
+            if (symbol.ShouldDisplayReadOnly())
+            {
+                AddReadOnlyIfRequired();
+            }
+
             if (format.MemberOptions.IncludesOption(SymbolDisplayMemberOptions.IncludeType))
             {
                 if (symbol.ReturnsByRef)
@@ -168,6 +173,12 @@ public override void VisitEvent(IEventSymbol symbol)
             AddAccessibilityIfRequired(symbol);
             AddMemberModifiersIfRequired(symbol);
 
+            var accessor = (symbol.AddMethod ?? symbol.RemoveMethod) as MethodSymbol;
+            if (accessor?.IsDeclaredReadOnly == true)
+            {
+                AddReadOnlyIfRequired();
+            }
+
             if (format.KindOptions.IncludesOption(SymbolDisplayKindOptions.IncludeMemberKeyword))
             {
                 AddKeyword(SyntaxKind.EventKeyword);
@@ -257,6 +268,11 @@ public override void VisitMethod(IMethodSymbol symbol)
                 AddAccessibilityIfRequired(symbol);
                 AddMemberModifiersIfRequired(symbol);
 
+                if ((symbol as MethodSymbol).IsDeclaredReadOnly)
+                {
+                    AddReadOnlyIfRequired();
+                }
+
                 if (format.MemberOptions.IncludesOption(SymbolDisplayMemberOptions.IncludeType))
                 {
                     switch (symbol.MethodKind)
@@ -753,7 +769,7 @@ private void AddParametersIfRequired(bool hasThisParameter, bool isVarargs, Immu
             }
         }
 
-        private void AddAccessor(ISymbol property, IMethodSymbol method, SyntaxKind keyword)
+        private void AddAccessor(IPropertySymbol property, IMethodSymbol method, SyntaxKind keyword)
         {
             if (method != null)
             {
@@ -763,6 +779,11 @@ private void AddAccessor(ISymbol property, IMethodSymbol method, SyntaxKind keyw
                     AddAccessibility(method);
                 }
 
+                if (!property.ShouldDisplayReadOnly() && (method as MethodSymbol)?.IsDeclaredReadOnly == true)
+                {
+                    AddReadOnlyIfRequired();
+                }
+
                 AddKeyword(keyword);
                 AddPunctuation(SyntaxKind.SemicolonToken);
             }
@@ -829,6 +850,17 @@ private void AddRefReadonlyIfRequired()
             }
         }
 
+        private void AddReadOnlyIfRequired()
+        {
+            // 'readonly' in this context is effectively a 'ref' modifier
+            // because it affects whether the 'this' parameter is 'ref' or 'in'.
+            if (format.MemberOptions.IncludesOption(SymbolDisplayMemberOptions.IncludeRef))
+            {
+                AddKeyword(SyntaxKind.ReadOnlyKeyword);
+                AddSpace();
+            }
+        }
+
         private void AddParameterRefKindIfRequired(RefKind refKind)
         {
             if (format.ParameterOptions.IncludesOption(SymbolDisplayParameterOptions.IncludeParamsRefOut))

src/Compilers/CSharp/Portable/Symbols/PropertySymbolExtensions.cs

@@ -70,5 +70,16 @@ public static bool HasRefOrOutParameter(this PropertySymbol property)
             }
             return false;
         }
+
+        public static bool ShouldDisplayReadOnly(this IPropertySymbol property)
+        {
+            var getMethod = property.GetMethod as MethodSymbol;
+            var setMethod = property.SetMethod as MethodSymbol;
+
+            // If at least one accessor is present and all present accessors are readonly, the property should be marked readonly.
+            return (getMethod != null || setMethod != null) &&
+                getMethod?.IsDeclaredReadOnly != false &&
+                setMethod?.IsDeclaredReadOnly != false;
+        }
     }
 }