Merge pull request #18246 from michaelnebel/csharp/fixwarnings

C#: Fix some new compiler warnings

Author: michaelnebel

csharp/.config/dotnet-tools.json

@@ -3,10 +3,10 @@
   "isRoot": true,
   "tools": {
     "paket": {
-      "version": "9.0.1",
+      "version": "9.0.2",
       "commands": [
         "paket"
       ]
     }
   }
-}
+}

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/NugetPackageRestorer.cs

@@ -604,6 +604,16 @@ private bool IsFeedReachable(string feed, int timeoutMilliSeconds, int tryCount,
                 {
                     httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, _) =>
                     {
+                        if (chain is null || cert is null)
+                        {
+                            var msg = cert is null && chain is null
+                                ? "certificate and chain"
+                                : chain is null
+                                    ? "chain"
+                                    : "certificate";
+                            logger.LogWarning($"Dependabot proxy certificate validation failed due to missing {msg}");
+                            return false;
+                        }
                         chain.ChainPolicy.TrustMode = X509ChainTrustMode.CustomRootTrust;
                         chain.ChainPolicy.CustomTrustStore.Add(this.dependabotProxy.Certificate);
                         return chain.Build(cert);