Add SymbolVisitor<TArgument, TResult>

[siegfriedpammer]

Background and Motivation

In order to avoid shared state and allowing parallel invocations of the same visitor, it would be nice to be able to use a SymbolVisitor<TArgument, TResult>. We already have a similar pattern with OperationVisitor.

Proposed API

namespace Microsoft.CodeAnalysis
{
+    public abstract class SymbolVisitor<TArgument, TResult>
+    {
+        public virtual TResult? Visit(ISymbol? symbol, TArgument argument)
+        {
+            if (symbol != null)
+            {
+                return symbol.Accept(this, argument);
+            }
+
+            return default(TResult);
+        }
+
+        public virtual TResult? DefaultVisit(ISymbol symbol, TArgument argument)
+        {
+            return default(TResult);
+        }
+
+        public virtual TResult? VisitAlias(IAliasSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitArrayType(IArrayTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitAssembly(IAssemblySymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitDiscard(IDiscardSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitDynamicType(IDynamicTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitEvent(IEventSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitField(IFieldSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitLabel(ILabelSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitLocal(ILocalSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitMethod(IMethodSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitModule(IModuleSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitNamedType(INamedTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitNamespace(INamespaceSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitParameter(IParameterSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitPointerType(IPointerTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitFunctionPointerType(IFunctionPointerTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitProperty(IPropertySymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitRangeVariable(IRangeVariableSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult? VisitTypeParameter(ITypeParameterSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+    }

	public interface ISymbol : IEquatable<ISymbol?>
    {
		// ...
        void Accept(SymbolVisitor visitor);

        TResult? Accept<TResult>(SymbolVisitor<TResult> visitor);

+        TResult? Accept<TArgument, TResult>(SymbolVisitor<TArgument, TResult> visitor, TArgument argument);
		// ...
    }
}

Usage Examples

      var result = symbol.Accept(new GenerateIdVisitor(), new GeneratorContext());

Alternative Designs

It would be possible to implement a visitor based on dynamic invocation, however, I think this is not the optimal solution.

Risks

none I can think of

[333fred]

Seems reasonable. The only change I think we need to make is remove the ? from TArgument: the user should decide whether the argument is allowed to be nullable or not. @siegfriedpammer let me know when that's been updated and I can tag this for review.

[siegfriedpammer]

Done. Yeah, I am pretty new to NRTs, so I had a small misconception regarding nullability of unconstrained generics, thanks for pointing that out.

Thank you very much for considering this!

[333fred]

We'll review this during the next review session, which would normally be next week, but as I'll be out then it will be in about 3 weeks.

[333fred]

API Review

We slightly changed the shape of the API to allow for nullability to play better with the visitor result generic. This is inconsistent with the rest of our visitors, but we can look at taking a breaking change in a future API version to allow them to better-express the TResult nullability as well. The amended shape is as follows:

namespace Microsoft.CodeAnalysis
{
+    public abstract class SymbolVisitor<TArgument, TResult>
+    {
+        protected abstract TResult DefaultResult { get; }
+
+        public virtual TResult Visit(ISymbol? symbol, TArgument argument)
+        {
+            if (symbol != null)
+            {
+                return symbol.Accept(this, argument);
+            }
+
+            return DefaultResult;
+        }
+
+        public virtual TResult DefaultVisit(ISymbol symbol, TArgument argument)
+        {
+            return DefaultResult;
+        }
+
+        public virtual TResult VisitAlias(IAliasSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitArrayType(IArrayTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitAssembly(IAssemblySymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitDiscard(IDiscardSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitDynamicType(IDynamicTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitEvent(IEventSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitField(IFieldSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitLabel(ILabelSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitLocal(ILocalSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitMethod(IMethodSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitModule(IModuleSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitNamedType(INamedTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitNamespace(INamespaceSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitParameter(IParameterSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitPointerType(IPointerTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitFunctionPointerType(IFunctionPointerTypeSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitProperty(IPropertySymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitRangeVariable(IRangeVariableSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+
+        public virtual TResult VisitTypeParameter(ITypeParameterSymbol symbol, TArgument argument)
+        {
+            return DefaultVisit(symbol, argument);
+        }
+    }

[333fred]

@siegfriedpammer sorry for the review delay, but it's now approved with the above shape. Would you be interested in implementing the API?

[siegfriedpammer]

Yes, of course!