CSHARP-4436: .NET/C# driver fails to create new mongo client in xamarin. (#1036)

Author: DmitryLukyanov

src/MongoDB.Driver.Core/Core/Misc/DnsClientWrapper.cs

@@ -31,42 +31,42 @@ internal class DnsClientWrapper : IDnsResolver
         #endregion
 
         // private fields
-        private readonly LookupClient _lookupClient;
+        private readonly Lazy<LookupClient> _lookupClient;
 
         // constructors
         private DnsClientWrapper()
         {
-            _lookupClient = new LookupClient();
+            _lookupClient = new Lazy<LookupClient>(() => new LookupClient());
         }
 
         // public methods
         public List<SrvRecord> ResolveSrvRecords(string service, CancellationToken cancellationToken)
         {
             Ensure.IsNotNull(service, nameof(service));
             cancellationToken.ThrowIfCancellationRequested();
-            var response = _lookupClient.Query(service, QueryType.SRV, QueryClass.IN);
+            var response = _lookupClient.Value.Query(service, QueryType.SRV, QueryClass.IN);
             return GetSrvRecords(response);
         }
 
         public async Task<List<SrvRecord>> ResolveSrvRecordsAsync(string service, CancellationToken cancellationToken)
         {
             Ensure.IsNotNull(service, nameof(service));
-            var response = await _lookupClient.QueryAsync(service, QueryType.SRV, QueryClass.IN, cancellationToken).ConfigureAwait(false);
+            var response = await _lookupClient.Value.QueryAsync(service, QueryType.SRV, QueryClass.IN, cancellationToken).ConfigureAwait(false);
             return GetSrvRecords(response);
         }
 
         public List<TxtRecord> ResolveTxtRecords(string domainName, CancellationToken cancellationToken)
         {
             Ensure.IsNotNull(domainName, nameof(domainName));
             cancellationToken.ThrowIfCancellationRequested();
-            var response = _lookupClient.Query(domainName, QueryType.TXT, QueryClass.IN);
+            var response = _lookupClient.Value.Query(domainName, QueryType.TXT, QueryClass.IN);
             return GetTxtRecords(response);
         }
 
         public async Task<List<TxtRecord>> ResolveTxtRecordsAsync(string domainName, CancellationToken cancellationToken)
         {
             Ensure.IsNotNull(domainName, nameof(domainName));
-            var response = await _lookupClient.QueryAsync(domainName, QueryType.TXT, QueryClass.IN, cancellationToken).ConfigureAwait(false);
+            var response = await _lookupClient.Value.QueryAsync(domainName, QueryType.TXT, QueryClass.IN, cancellationToken).ConfigureAwait(false);
             return GetTxtRecords(response);
         }
 

tests/MongoDB.Driver.Core.Tests/Core/Clusters/DnsClientWrapperTests.cs

@@ -29,11 +29,11 @@ namespace MongoDB.Driver.Core.Clusters
     public class DnsClientWrapperTests
     {
         [Fact]
-        public void constructor_should_initialize_instance()
+        public void constructor_should_initialize_lazy_instance()
         {
             var subject = CreateSubject();
 
-            subject._lookupClient().Should().NotBeNull();
+            subject._lookupClient().IsValueCreated.Should().BeFalse();
         }
 
         [Fact]
@@ -186,6 +186,6 @@ private DnsClientWrapper CreateSubject()
 
     internal static class DnsClientWrapperReflector
     {
-        public static LookupClient _lookupClient(this IDnsResolver dnsResolver) => (LookupClient)Reflector.GetFieldValue(dnsResolver, nameof(_lookupClient));
+        public static Lazy<LookupClient> _lookupClient(this IDnsResolver dnsResolver) => (Lazy<LookupClient>)Reflector.GetFieldValue(dnsResolver, nameof(_lookupClient));
     }
 }