X509SubjectAlternativeNameExtension 找不到

[FreeVB]

private static IEnumerable<string> ReadDnsNames(X509Certificate? cert) { if (cert is X509Certificate2 x509) { var extension = x509.Extensions.OfType<X509SubjectAlternativeNameExtension>().FirstOrDefault(); if (extension != null) { return extension.EnumerateDnsNames(); } } return Array.Empty<string>(); }

请问这里为什么提示X509SubjectAlternativeNameExtension 找不到，没有引用呢？我下载后没有修改东西呀。