Performance optimization for small messages without unknown fields

csharp/src/ProtocolBuffers/UnknownFieldSet.cs

@@ -124,12 +124,16 @@ namespace Google.ProtocolBuffers
         /// Serializes the set and writes it to <paramref name="output"/>.
         /// </summary>
         public void WriteTo(ICodedOutputStream output)
+        {
+            // Avoid creating enumerator for the most common code path.
+            if (fields.Count > 0)
             {
                 foreach (KeyValuePair<int, UnknownField> entry in fields)
                 {
                     entry.Value.WriteTo(entry.Key, output);
                 }
             }
+        }
 
         /// <summary>
         /// Gets the number of bytes required to encode this set.
@@ -138,6 +142,12 @@ namespace Google.ProtocolBuffers
         {
             get
             {
+                // Avoid creating enumerator for the most common code path.
+                if (fields.Count == 0)
+                {
+                    return 0;
+                }
+
                 int result = 0;
                 foreach (KeyValuePair<int, UnknownField> entry in fields)
                 {
@@ -208,12 +218,16 @@ namespace Google.ProtocolBuffers
         /// the MessageSet wire format.
         /// </summary>
         public void WriteAsMessageSetTo(ICodedOutputStream output)
+        {
+            // Avoid creating enumerator for the most common code path.
+            if (fields.Count > 0)
             {
                 foreach (KeyValuePair<int, UnknownField> entry in fields)
                 {
                     entry.Value.WriteAsMessageSetExtensionTo(entry.Key, output);
                 }
             }
+        }
 
         /// <summary>
         /// Gets the number of bytes required to encode this set using the MessageSet
@@ -223,6 +237,12 @@ namespace Google.ProtocolBuffers
         {
             get
             {
+                // Avoid creating enumerator for the most common code path.
+                if (fields.Count == 0)
+                {
+                    return 0;
+                }
+
                 int result = 0;
                 foreach (KeyValuePair<int, UnknownField> entry in fields)
                 {