static string CacheFileName(GravatarImageSource gis, float scale) => $"{GetMd5Hash(gis.Email)}-{gis.Size}@{scale}x.png";
static string CacheFileName(GravatarImageSource gis, float scale) { _ = gis.Email ?? throw new InvalidOperationException($"{nameof(gis.Email)} cannot be null"); return($"{GetMd5Hash(gis.Email)}-{gis.Size}@{scale}x.png"); }