// Main code
CSharpCompilation compilation = CSharpCompilation.Create(...);
GetAllSymbolsVisitor visitor = new GetAllSymbolsVisitor();
visitor.Visit(compilation.Assembly.GlobalNamespace);

// Visitor
public class GetAllSymbolsVisitor : SymbolVisitor
{
    public override void VisitNamespace(INamespaceSymbol symbol)
    {
        Parallel.ForEach(symbol.GetMembers(), s => s.Accept(this));
    }

    public override void VisitNamedType(INamedTypeSymbol symbol)
    {
        // Do what you need to here (add to collection, etc.)
    }
}

        private static IEnumerable<INamedTypeSymbol> GetNamedTypeSymbols(Compilation compilation)
        {
            var stack = new Stack<INamespaceSymbol>();
            stack.Push(compilation.GlobalNamespace);

            while (stack.Count > 0)
            {
                var @namespace = stack.Pop();

                foreach (var member in @namespace.GetMembers())
                {
                    if (member is INamespaceSymbol memberAsNamespace)
                    {
                        stack.Push(memberAsNamespace);
                    }
                    else if (member is INamedTypeSymbol memberAsNamedTypeSymbol)
                    {
                        yield return memberAsNamedTypeSymbol;
                    }
                }
            }
        }