Thread safe ID generation

src/DnsClient/DnsRequestHeader.cs

@@ -5,7 +5,9 @@ namespace DnsClient
 {
     internal class DnsRequestHeader
     {
+#if !NET6_0_OR_GREATER
         private static readonly Random s_random = new Random();
+#endif
         public const int HeaderLength = 12;
         private ushort _flags = 0;
 
@@ -102,7 +104,14 @@ public void RefreshId()
 
         private static ushort GetNextUniqueId()
         {
-            return (ushort)s_random.Next(1, ushort.MaxValue);
+#if NET6_0_OR_GREATER
+            return (ushort)Random.Shared.Next(1, ushort.MaxValue);
+#else
+            lock (s_random)
+            {
+                return (ushort)s_random.Next(1, ushort.MaxValue);
+            }
+#endif
         }
     }
 }

test/DnsClient.Tests/DnsRequestHeaderTest.cs

@@ -1,4 +1,5 @@
-using System;
+using System.Collections.Concurrent;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace DnsClient.Tests
@@ -49,5 +50,20 @@ public void DnsRequestHeader_ChangeRecursion()
 
             Assert.Equal(DnsOpCode.Notify, header.OpCode);
         }
+
+        [Fact]
+        public void DnsRequestHeader_IdIsPseudoUnique()
+        {
+            ConcurrentDictionary<int, int> ids = new ConcurrentDictionary<int, int>();
+
+            Parallel.For(0, 1000, i =>
+            {
+                var header = new DnsRequestHeader(DnsOpCode.Query);
+                Assert.NotEqual(0, header.Id);
+                ids.TryAdd(header.Id, 0);
+            });
+
+            Assert.True(ids.Count > 950, $"Only {ids.Count} of 1000 ids are unique!");
+        }
     }
-}
+}