pem.cs

static void Main(string[] args)
{
    var cert = new X509Certificate2("foo.cer"); //Or however you load the certificate.
    var octets = cert.Export(X509ContentType.Cert);
    var formatted = Convert.ToBase64String(octets);
    var builder = new StringBuilder();
    builder.AppendLine("-----BEGIN CERTIFICATE-----");
    var i = 0;
    while(i < formatted.Length)
    {
        const int MAX_LINE_SIZE = 64;
        var size = Math.Min(MAX_LINE_SIZE, formatted.Length - i);
        builder.AppendLine(formatted.Substring(i, size));
        i += size;
    }
    builder.AppendLine("-----END CERTIFICATE-----");
    Console.WriteLine(builder);
}