codereview feedback.

Use Threading.Interlocked instead of direct assigned.

src/System.Net.Http.Formatting/Formatting/FormDataCollection.cs

@@ -6,6 +6,7 @@ using System.Net.Http.Formatting.Parsers;
 using System.Net.Http.Internal;
 using System.Runtime.Serialization;
 using System.Text;
+using System.Threading;
 
 namespace System.Net.Http.Formatting
 {    
@@ -101,7 +102,8 @@ namespace System.Net.Http.Formatting
             {
                 // Initialize in a private collection to be thread-safe, and swap the finished object.
                 // Ok to double initialize this. 
-                _nameValueCollection = HttpValueCollection.Build(this);
+                NameValueCollection newCollection = HttpValueCollection.Build(this);
+                Interlocked.Exchange(ref _nameValueCollection, newCollection);
             }
             return _nameValueCollection;
         }

test/System.Net.Http.Formatting.Test.Unit/Formatting/FormDataCollectionTests.cs

@@ -30,7 +30,7 @@ namespace System.Net.Http.Formatting
         {
             FormDataCollection form = new FormDataCollection(new Uri("http://foo.com"));
 
-            Assert.Equal(0, form.Count());            
+            Assert.Empty(form);            
         }
 
         [Fact]
@@ -45,7 +45,7 @@ namespace System.Net.Http.Formatting
         {
             FormDataCollection form = new FormDataCollection("");
 
-            Assert.Equal(0, form.Count());
+            Assert.Empty(form);       
         }
 
         [Fact]
@@ -53,7 +53,7 @@ namespace System.Net.Http.Formatting
         {
             FormDataCollection form = new FormDataCollection((string) null);
 
-            Assert.Equal(0, form.Count());
+            Assert.Empty(form);       
         }
 
         [Fact]