我想我找到了...这个用的是弹跳城堡巧克力包。

using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.OpenSsl;
using Org.BouncyCastle.Pkcs;
using Org.BouncyCastle.Security;
using System.Security.Cryptography.X509Certificates;
using System;
using System.IO;

string pemKey = @"-----BEGIN EC PRIVATE KEY-----
MHcCAQEEIKpAuZ/Wwp7FTSCNJ56fFM4Y/rf8ltXp3xnrooPxNc1UoAoGCCqGSM49
AwEHoUQDQgAEqiRaEw3ItPsRAqdDjJCyqxhfm8y3tVrxLBAGhPM0pVhHuqmPoQFA
zR5FA3IJZaWcopieEX5uZ4KMtDhLFu/FHw==
-----END EC PRIVATE KEY-----";

string pemCert = @"-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----";

var keyPair = (AsymmetricCipherKeyPair)new PemReader(new StringReader(pemKey)).ReadObject();
var cert = (Org.BouncyCastle.X509.X509Certificate)new PemReader(new StringReader(pemCert)).ReadObject();

var builder = new Pkcs12StoreBuilder();
builder.SetUseDerEncoding(true);
var store = builder.Build();

var certEntry = new X509CertificateEntry(cert);
store.SetCertificateEntry("", certEntry);
store.SetKeyEntry("", new AsymmetricKeyEntry(keyPair.Private), new[] { certEntry });

byte[] data;
using (var ms = new MemoryStream())
{
    store.Save(ms, Array.Empty<char>(), new SecureRandom());
    data = ms.ToArray();
}

var x509Cert = new X509Certificate2(data);

诀窍似乎是将cert和key组合到一个pkcs12容器中，然后一次性将其输入X509Certificate2。

将证书和密钥与openssl结合起来，并将其输入x509certificate类的工作原理是：

openssl pkcs12 -export -in certificate.pem -inkey privatekey.pem -out cert-and-key.pfx

然后使用此函数获取一个具有指定私钥的类：

X509Certificate2 certificate = new X509Certificate2("cert-and-key.pfx", "password");

然后我问题中的代码就起作用了。

您可以简单地使用方法X509Certificate2.CreateFromPemFile。

var httpClientHandler = new HttpClientHandler();

var cert = X509Certificate2.CreateFromPemFile(certificateFile, privateKeyFile);
httpClientHandler.ClientCertificates.Add(cert);

var httpClient = new HttpClient(httpClientHandler);
HttpResponseMessage result = httpClient.GetAsync("https://server.cryptomix.com/secure/").Result;

string str = result.Content.ReadAsStringAsync().Result;

其中certificateFile和privateKeyFile是PEM文件的文件路径，