diff --git a/objectivec/GPBUnknownField.m b/objectivec/GPBUnknownField.m
new file mode 100644
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--- /dev/null
+++ b/objectivec/GPBUnknownField.m
@@ -0,0 +1,326 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBUnknownField_PackagePrivate.h"
+
+#import "GPBArray.h"
+#import "GPBCodedOutputStream.h"
+
+@implementation GPBUnknownField {
+ @protected
+  int32_t number_;
+  GPBUInt64Array *mutableVarintList_;
+  GPBUInt32Array *mutableFixed32List_;
+  GPBUInt64Array *mutableFixed64List_;
+  NSMutableArray *mutableLengthDelimitedList_;
+  NSMutableArray *mutableGroupList_;
+}
+
+@synthesize number = number_;
+@synthesize varintList = mutableVarintList_;
+@synthesize fixed32List = mutableFixed32List_;
+@synthesize fixed64List = mutableFixed64List_;
+@synthesize lengthDelimitedList = mutableLengthDelimitedList_;
+@synthesize groupList = mutableGroupList_;
+
+- (instancetype)initWithNumber:(int32_t)number {
+  if ((self = [super init])) {
+    number_ = number;
+  }
+  return self;
+}
+
+- (void)dealloc {
+  [mutableVarintList_ release];
+  [mutableFixed32List_ release];
+  [mutableFixed64List_ release];
+  [mutableLengthDelimitedList_ release];
+  [mutableGroupList_ release];
+
+  [super dealloc];
+}
+
+- (id)copyWithZone:(NSZone *)zone {
+  GPBUnknownField *result =
+      [[GPBUnknownField allocWithZone:zone] initWithNumber:number_];
+  result->mutableFixed32List_ = [mutableFixed32List_ copyWithZone:zone];
+  result->mutableFixed64List_ = [mutableFixed64List_ copyWithZone:zone];
+  result->mutableLengthDelimitedList_ =
+      [mutableLengthDelimitedList_ copyWithZone:zone];
+  result->mutableVarintList_ = [mutableVarintList_ copyWithZone:zone];
+  if (mutableGroupList_.count) {
+    result->mutableGroupList_ = [[NSMutableArray allocWithZone:zone]
+        initWithCapacity:mutableGroupList_.count];
+    for (GPBUnknownFieldSet *group in mutableGroupList_) {
+      GPBUnknownFieldSet *copied = [group copyWithZone:zone];
+      [result->mutableGroupList_ addObject:copied];
+      [copied release];
+    }
+  }
+  return result;
+}
+
+- (BOOL)isEqual:(id)object {
+  if (self == object) return YES;
+  if (![object isKindOfClass:[GPBUnknownField class]]) return NO;
+  GPBUnknownField *field = (GPBUnknownField *)object;
+  BOOL equalVarint =
+      (mutableVarintList_.count == 0 && field->mutableVarintList_.count == 0) ||
+      [mutableVarintList_ isEqual:field->mutableVarintList_];
+  if (!equalVarint) return NO;
+  BOOL equalFixed32 = (mutableFixed32List_.count == 0 &&
+                       field->mutableFixed32List_.count == 0) ||
+                      [mutableFixed32List_ isEqual:field->mutableFixed32List_];
+  if (!equalFixed32) return NO;
+  BOOL equalFixed64 = (mutableFixed64List_.count == 0 &&
+                       field->mutableFixed64List_.count == 0) ||
+                      [mutableFixed64List_ isEqual:field->mutableFixed64List_];
+  if (!equalFixed64) return NO;
+  BOOL equalLDList =
+      (mutableLengthDelimitedList_.count == 0 &&
+       field->mutableLengthDelimitedList_.count == 0) ||
+      [mutableLengthDelimitedList_ isEqual:field->mutableLengthDelimitedList_];
+  if (!equalLDList) return NO;
+  BOOL equalGroupList =
+      (mutableGroupList_.count == 0 && field->mutableGroupList_.count == 0) ||
+      [mutableGroupList_ isEqual:field->mutableGroupList_];
+  if (!equalGroupList) return NO;
+  return YES;
+}
+
+- (NSUInteger)hash {
+  // Just mix the hashes of the possible sub arrays.
+  const int prime = 31;
+  NSUInteger result = prime + [mutableVarintList_ hash];
+  result = prime * result + [mutableFixed32List_ hash];
+  result = prime * result + [mutableFixed64List_ hash];
+  result = prime * result + [mutableLengthDelimitedList_ hash];
+  result = prime * result + [mutableGroupList_ hash];
+  return result;
+}
+
+- (void)writeToOutput:(GPBCodedOutputStream *)output {
+  NSUInteger count = mutableVarintList_.count;
+  if (count > 0) {
+    [output writeUInt64Array:number_ values:mutableVarintList_ tag:0];
+  }
+  count = mutableFixed32List_.count;
+  if (count > 0) {
+    [output writeFixed32Array:number_ values:mutableFixed32List_ tag:0];
+  }
+  count = mutableFixed64List_.count;
+  if (count > 0) {
+    [output writeFixed64Array:number_ values:mutableFixed64List_ tag:0];
+  }
+  count = mutableLengthDelimitedList_.count;
+  if (count > 0) {
+    [output writeBytesArray:number_ values:mutableLengthDelimitedList_];
+  }
+  count = mutableGroupList_.count;
+  if (count > 0) {
+    [output writeUnknownGroupArray:number_ values:mutableGroupList_];
+  }
+}
+
+- (size_t)serializedSize {
+  __block size_t result = 0;
+  int32_t number = number_;
+  [mutableVarintList_
+      enumerateValuesWithBlock:^(uint64_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        result += GPBComputeUInt64Size(number, value);
+      }];
+
+  [mutableFixed32List_
+      enumerateValuesWithBlock:^(uint32_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        result += GPBComputeFixed32Size(number, value);
+      }];
+
+  [mutableFixed64List_
+      enumerateValuesWithBlock:^(uint64_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        result += GPBComputeFixed64Size(number, value);
+      }];
+
+  for (NSData *data in mutableLengthDelimitedList_) {
+    result += GPBComputeBytesSize(number, data);
+  }
+
+  for (GPBUnknownFieldSet *set in mutableGroupList_) {
+    result += GPBComputeUnknownGroupSize(number, set);
+  }
+
+  return result;
+}
+
+- (void)writeAsMessageSetExtensionToOutput:(GPBCodedOutputStream *)output {
+  for (NSData *data in mutableLengthDelimitedList_) {
+    [output writeRawMessageSetExtension:number_ value:data];
+  }
+}
+
+- (size_t)serializedSizeAsMessageSetExtension {
+  size_t result = 0;
+  for (NSData *data in mutableLengthDelimitedList_) {
+    result += GPBComputeRawMessageSetExtensionSize(number_, data);
+  }
+  return result;
+}
+
+- (NSString *)description {
+  NSMutableString *description = [NSMutableString
+      stringWithFormat:@"<%@ %p>: Field: %d {\n", [self class], self, number_];
+  [mutableVarintList_
+      enumerateValuesWithBlock:^(uint64_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        [description appendFormat:@"\t%llu\n", value];
+      }];
+
+  [mutableFixed32List_
+      enumerateValuesWithBlock:^(uint32_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        [description appendFormat:@"\t%u\n", value];
+      }];
+
+  [mutableFixed64List_
+      enumerateValuesWithBlock:^(uint64_t value, NSUInteger idx, BOOL *stop) {
+#pragma unused(idx, stop)
+        [description appendFormat:@"\t%llu\n", value];
+      }];
+
+  for (NSData *data in mutableLengthDelimitedList_) {
+    [description appendFormat:@"\t%@\n", data];
+  }
+
+  for (GPBUnknownFieldSet *set in mutableGroupList_) {
+    [description appendFormat:@"\t%@\n", set];
+  }
+  [description appendString:@"}"];
+  return description;
+}
+
+- (void)mergeFromField:(GPBUnknownField *)other {
+  GPBUInt64Array *otherVarintList = other.varintList;
+  if (otherVarintList.count > 0) {
+    if (mutableVarintList_ == nil) {
+      mutableVarintList_ = [otherVarintList copy];
+    } else {
+      [mutableVarintList_ addValuesFromArray:otherVarintList];
+    }
+  }
+
+  GPBUInt32Array *otherFixed32List = other.fixed32List;
+  if (otherFixed32List.count > 0) {
+    if (mutableFixed32List_ == nil) {
+      mutableFixed32List_ = [otherFixed32List copy];
+    } else {
+      [mutableFixed32List_ addValuesFromArray:otherFixed32List];
+    }
+  }
+
+  GPBUInt64Array *otherFixed64List = other.fixed64List;
+  if (otherFixed64List.count > 0) {
+    if (mutableFixed64List_ == nil) {
+      mutableFixed64List_ = [otherFixed64List copy];
+    } else {
+      [mutableFixed64List_ addValuesFromArray:otherFixed64List];
+    }
+  }
+
+  NSArray *otherLengthDelimitedList = other.lengthDelimitedList;
+  if (otherLengthDelimitedList.count > 0) {
+    if (mutableLengthDelimitedList_ == nil) {
+      mutableLengthDelimitedList_ = [otherLengthDelimitedList mutableCopy];
+    } else {
+      [mutableLengthDelimitedList_
+          addObjectsFromArray:otherLengthDelimitedList];
+    }
+  }
+
+  NSArray *otherGroupList = other.groupList;
+  if (otherGroupList.count > 0) {
+    if (mutableGroupList_ == nil) {
+      mutableGroupList_ =
+          [[NSMutableArray alloc] initWithCapacity:otherGroupList.count];
+    }
+    // Make our own mutable copies.
+    for (GPBUnknownFieldSet *group in otherGroupList) {
+      GPBUnknownFieldSet *copied = [group copy];
+      [mutableGroupList_ addObject:copied];
+      [copied release];
+    }
+  }
+}
+
+- (void)addVarint:(uint64_t)value {
+  if (mutableVarintList_ == nil) {
+    mutableVarintList_ = [[GPBUInt64Array alloc] initWithValues:&value count:1];
+  } else {
+    [mutableVarintList_ addValue:value];
+  }
+}
+
+- (void)addFixed32:(uint32_t)value {
+  if (mutableFixed32List_ == nil) {
+    mutableFixed32List_ =
+        [[GPBUInt32Array alloc] initWithValues:&value count:1];
+  } else {
+    [mutableFixed32List_ addValue:value];
+  }
+}
+
+- (void)addFixed64:(uint64_t)value {
+  if (mutableFixed64List_ == nil) {
+    mutableFixed64List_ =
+        [[GPBUInt64Array alloc] initWithValues:&value count:1];
+  } else {
+    [mutableFixed64List_ addValue:value];
+  }
+}
+
+- (void)addLengthDelimited:(NSData *)value {
+  if (mutableLengthDelimitedList_ == nil) {
+    mutableLengthDelimitedList_ =
+        [[NSMutableArray alloc] initWithObjects:&value count:1];
+  } else {
+    [mutableLengthDelimitedList_ addObject:value];
+  }
+}
+
+- (void)addGroup:(GPBUnknownFieldSet *)value {
+  if (mutableGroupList_ == nil) {
+    mutableGroupList_ = [[NSMutableArray alloc] initWithObjects:&value count:1];
+  } else {
+    [mutableGroupList_ addObject:value];
+  }
+}
+
+@end