InterfaceGenerator/InterfaceGenerator/SyntaxReceiver.cs

using System.Collections.Generic;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace InterfaceGenerator
{
    internal class SyntaxReceiver : ISyntaxReceiver
    {
        public IList<TypeDeclarationSyntax> CandidateTypes { get; } = new List<TypeDeclarationSyntax>();
        
        public void OnVisitSyntaxNode(SyntaxNode syntaxNode)
        {
            if (syntaxNode is TypeDeclarationSyntax typeDeclarationSyntax &&
                IsClassOrRecord(typeDeclarationSyntax) &&
                typeDeclarationSyntax.AttributeLists.Count > 0)
            {
                CandidateTypes.Add(typeDeclarationSyntax);
            }
        }

        private static bool IsClassOrRecord(TypeDeclarationSyntax typeDeclarationSyntax)
        {
            return typeDeclarationSyntax is ClassDeclarationSyntax || typeDeclarationSyntax is RecordDeclarationSyntax;
        }
    }
}