PR: Suggested fix for 'Possible Recursion Detected' exception on specific scenario.

src/Examples/Recursion.cs

@@ -1,6 +1,8 @@
 using System.IO;
 using Xunit;
 using ProtoBuf;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace Examples
 {
@@ -10,6 +12,45 @@ public class RecursiveObject
         [ProtoMember(1)]
         public RecursiveObject Yeuch { get; set; }
     }
+
+    [ProtoContract]
+    public class MySurrogate
+    {
+        [ProtoMember(1)]
+        public TreeNode treeNode { get; set; }
+        [ProtoConverter]
+        public static INode From(MySurrogate surrogate)
+        {
+            return surrogate.treeNode;
+        }
+        [ProtoConverter]
+        public static MySurrogate To(INode value)
+        {
+            var surrogate = new MySurrogate();
+            if (value is TreeNode treeNode)
+                surrogate.treeNode = treeNode;
+            return surrogate;
+        }
+    }
+    [ProtoContract(Surrogate = typeof(MySurrogate))]
+    public interface INode
+    {
+        public string Name { get; }
+    }
+    [ProtoContract]
+    public class TreeNode : INode
+    {
+        [ProtoMember(1)]
+        public string Name { get; }
+        [ProtoMember(2)]
+        public List<INode> Children { get; }
+        public TreeNode(string Name) : this()
+        {
+            this.Name = Name;
+        }
+        public TreeNode() { Children = new List<INode>(); }
+        public string AsString() => Children != null ? $"{Name} => {{{string.Join(",", Children)}}}" : Name;
+    }
 
     public class Recursion
     {
@@ -23,5 +64,26 @@ public void BlowUp()
                 Serializer.Serialize(Stream.Null, obj);
             });
         }
+
+        [Fact]
+        public void Tree_NoRecursionDetected()
+        {
+            var tree = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToArray().Reverse().Select(letter => new TreeNode(letter.ToString()))
+            .Aggregate((prev, next) => { next.Children.Add(prev); return next; });
+            //A => {B => {C => {D => {E => {F => {G => {H => {I => {J => {K => {L => {M => {N => {...}}}}}}}}}}}}}}
+            var cloned = Serializer.DeepClone(tree);
+            Assert.Equal(tree.AsString(), cloned.AsString());
+        }
+
+        [Fact]
+        public void Graph_RecursionDetected()
+        {
+            Program.ExpectFailure<ProtoException>(() =>
+            {
+                var graph = new TreeNode("A");
+                graph.Children.Add(graph);
+                Serializer.DeepClone(graph);
+            });
+        }
     }
 }

src/protobuf-net.Core/ProtoWriter.cs

@@ -254,7 +254,7 @@ protected private virtual void ClearKnownObjects()
         protected internal virtual void WriteMessage<[DynamicallyAccessedMembers(DynamicAccess.ContractType)] T>(ref State state, T value, ISerializer<T> serializer, PrefixStyle style, bool recursionCheck)
         {
 #pragma warning disable CS0618 // StartSubItem/EndSubItem
-            var tok = state.StartSubItem(TypeHelper<T>.IsReferenceType & recursionCheck ? (object)value : null, style);
+            var tok = state.StartSubItem(TypeHelper<T>.IsReferenceType & !typeof(T).IsInterface & recursionCheck ? (object)value : null, style);
             (serializer ?? TypeModel.GetSerializer<T>(model)).Write(ref state, value);
             state.EndSubItem(tok, style);
 #pragma warning restore CS0618