NpgsqlConnectionExtensions.cs

    public static class NpgsqlConnectionExtensions
    {
        public static void ExecuteFromResource(this NpgsqlConnection connection, string resourceName, object parameters = null)
        {
            var sql = ResourceUtil.Get<NpgsqlContentTreeProvider>($"Sql/{resourceName}.sql");

            connection.Execute(sql, parameters);
        }

        public static IEnumerable<T> QueryFromResource<T>(this NpgsqlConnection connection, string resourceName, object parameters = null)
        {
            var sql = ResourceUtil.Get<NpgsqlContentTreeProvider>($"Sql/{resourceName}.sql");

            return connection.Query<T>(sql, parameters);
        }

        public static T QuerySingleFromResource<T>(this NpgsqlConnection connection, string resourceName, object parameters = null)
        {
            return QueryFromResource<T>(connection, resourceName, parameters).SingleOrDefault();
        }
    }

ResourceUtil.cs

    public static class ResourceUtil
    {
        #region Public Methods

        public static string Get<TAssembly>(string name)
        {
            if (name == null) { throw new ArgumentNullException(nameof(name)); }

            var assembly = typeof(TAssembly).GetTypeInfo().Assembly;

            name = name.Replace('/', '.');
            name = assembly.GetName().Name + '.' + name;

            using (var stream = assembly.GetManifestResourceStream(name))
            {
                if (stream == null)
                {
                    throw new ResourceNotFoundException(name);
                }

                using (var reader = new StreamReader(stream))
                {
                    return reader.ReadToEnd();
                }
            }
        }

        #endregion
    }

    public class ResourceNotFoundException : Exception
    {
        #region Public Constructors

        public ResourceNotFoundException(string resourceName)
            : base($"Resource not found {resourceName}")
        {
        }

        #endregion
    }