-Improve ignore case get property performance

Src/Newtonsoft.Json/Serialization/JsonPropertyCollection.cs

@@ -38,6 +38,7 @@ namespace Newtonsoft.Json.Serialization
     public class JsonPropertyCollection : KeyedCollection<string, JsonProperty>
     {
         private readonly Type _type;
+        private readonly List<JsonProperty> _list;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="JsonPropertyCollection"/> class.
@@ -48,6 +49,9 @@ namespace Newtonsoft.Json.Serialization
         {
             ValidationUtils.ArgumentNotNull(type, "type");
             _type = type;
+
+            // foreach over List<T> to avoid boxing the Enumerator
+            _list = (List<JsonProperty>)Items;
         }
 
         /// <summary>
@@ -70,7 +74,9 @@ namespace Newtonsoft.Json.Serialization
             {
                 // don't overwrite existing property with ignored property
                 if (property.Ignored)
+                {
                     return;
+                }
 
                 JsonProperty existingProperty = this[property.PropertyName];
                 bool duplicateProperty = true;
@@ -102,7 +108,9 @@ namespace Newtonsoft.Json.Serialization
                 }
 
                 if (duplicateProperty)
+                {
                     throw new JsonSerializationException("A member with the name '{0}' already exists on '{1}'. Use the JsonPropertyAttribute to specify another name.".FormatWith(CultureInfo.InvariantCulture, property.PropertyName, _type));
+                }
             }
 
             Add(property);
@@ -119,7 +127,9 @@ namespace Newtonsoft.Json.Serialization
         {
             JsonProperty property = GetProperty(propertyName, StringComparison.Ordinal);
             if (property == null)
+            {
                 property = GetProperty(propertyName, StringComparison.OrdinalIgnoreCase);
+            }
 
             return property;
         }
@@ -148,13 +158,16 @@ namespace Newtonsoft.Json.Serialization
             {
                 JsonProperty property;
                 if (TryGetValue(propertyName, out property))
+                {
                     return property;
+                }
 
                 return null;
             }
 
-            foreach (JsonProperty property in this)
+            for (int i = 0; i < _list.Count; i++)
             {
+                JsonProperty property = _list[i];
                 if (string.Equals(propertyName, property.PropertyName, comparisonType))
                 {
                     return property;