CSHARP-3066: Improve MongoX509Credential API

Author: MikalaiMazurenka

src/MongoDB.Driver/MongoCredential.cs

@@ -284,7 +284,7 @@ public static MongoCredential CreateMongoCRCredential(string databaseName, strin
         /// </summary>
         /// <param name="username">The username.</param>
         /// <returns>A credential for MONGODB-X509.</returns>
-        public static MongoCredential CreateMongoX509Credential(string username)
+        public static MongoCredential CreateMongoX509Credential(string username = null)
         {
             return FromComponents(
                 mechanism: "MONGODB-X509",

tests/MongoDB.Driver.Tests/AuthenticationTests.cs

@@ -341,10 +341,14 @@ public void Authentication_succeeds_with_MONGODB_X509_mechanism(
 
             var settings = DriverTestConfiguration.GetClientSettings().Clone();
             var serverVersion = CoreTestConfiguration.ServerVersion;
-            var serverExtractsUsernameFromCertificate =
-                serverVersion >= Feature.ServerExtractsUsernameFromX509Certificate.FirstSupportedVersion;
-            settings.Credential = MongoCredential.CreateMongoX509Credential(
-                username: serverExtractsUsernameFromCertificate ? null : userName);
+            if (Feature.ServerExtractsUsernameFromX509Certificate.IsSupported(serverVersion))
+            {
+                settings.Credential = MongoCredential.CreateMongoX509Credential();
+            }
+            else
+            {
+                settings.Credential = MongoCredential.CreateMongoX509Credential(userName);
+            }
             settings.SslSettings = settings.SslSettings.Clone();
             settings.SslSettings.ClientCertificates = new[] { clientCertificate };
 

tests/MongoDB.Driver.Tests/MongoCredentialTests.cs

@@ -44,7 +44,7 @@ public void TestCreateMongoX509Credential()
         [Fact]
         public void TestCreateMongoX509Credential_without_username()
         {
-            var credential = MongoCredential.CreateMongoX509Credential(null);
+            var credential = MongoCredential.CreateMongoX509Credential();
             Assert.Equal("MONGODB-X509", credential.Mechanism);
             Assert.Equal(null, credential.Username);
             Assert.IsType<ExternalEvidence>(credential.Evidence);