fix null property of generic collection

Author: mythz

src/ServiceStack.Text/CollectionExtensions.cs

@@ -7,17 +7,21 @@ public static class CollectionExtensions
     {
         public static ICollection<T> CreateAndPopulate<T>(Type ofCollectionType, T[] withItems)
         {
-            if (ofCollectionType == null) return new List<T>(withItems);
+            if (withItems == null)
+                return null;
+
+            if (ofCollectionType == null)
+                return new List<T>(withItems);
 
             var genericType = ofCollectionType.FirstGenericType();
             var genericTypeDefinition = genericType != null
                 ? genericType.GetGenericTypeDefinition()
                 : null;
 #if !XBOX
-            if (genericTypeDefinition == typeof(HashSet<T>))
+            if (genericTypeDefinition == typeof(HashSet<>))
                 return new HashSet<T>(withItems);
 #endif
-            if (genericTypeDefinition == typeof(LinkedList<T>))
+            if (genericTypeDefinition == typeof(LinkedList<>))
                 return new LinkedList<T>(withItems);
 
             var collection = (ICollection<T>)ofCollectionType.CreateInstance();

tests/ServiceStack.Text.Tests/JsonTests/BasicPropertiesTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Runtime.Serialization;
 using System.Text;
 using NUnit.Framework;
 
@@ -172,5 +173,21 @@ static string DictStr(IDictionary d)
             foreach (var key in d.Keys) { sb.AppendLine(key + " = " + d[key]); }
             return sb.ToString();
         }
+
+        public class ModelWithHashSet
+        {
+            public HashSet<string> Set { get; set; }
+        }
+
+        [Test]
+        public void Can_deserialize_null_Nested_HashSet()
+        {
+            JsConfig.ThrowOnDeserializationError = true;
+            string json = @"{""set"":null}";
+            var o = json.FromJson<ModelWithHashSet>();
+            Assert.That(o.Set, Is.Null);
+
+            JsConfig.Reset();
+        }
     }
 }