diff --git a/src/Microsoft.Net.Http.Headers/HeaderUtilities.cs b/src/Microsoft.Net.Http.Headers/HeaderUtilities.cs
index e42240a1dd..630e02b93f 100644
--- a/src/Microsoft.Net.Http.Headers/HeaderUtilities.cs
+++ b/src/Microsoft.Net.Http.Headers/HeaderUtilities.cs
@@ -228,5 +228,13 @@ namespace Microsoft.Net.Http.Headers
             }
             return input;
         }
+
+        internal static void ThrowIfReadOnly(bool isReadOnly)
+        {
+            if (isReadOnly)
+            {
+                throw new InvalidOperationException("The object cannot be modified because it is read-only.");
+            }
+        }
     }
 }
diff --git a/src/Microsoft.Net.Http.Headers/MediaTypeHeaderValue.cs b/src/Microsoft.Net.Http.Headers/MediaTypeHeaderValue.cs
index 173bf7c404..968eb4c6cc 100644
--- a/src/Microsoft.Net.Http.Headers/MediaTypeHeaderValue.cs
+++ b/src/Microsoft.Net.Http.Headers/MediaTypeHeaderValue.cs
@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics.Contracts;
 using System.Globalization;
+using System.Linq;
 using System.Text;
 
 namespace Microsoft.Net.Http.Headers
@@ -19,9 +20,10 @@ namespace Microsoft.Net.Http.Headers
         private static readonly HttpHeaderParser<MediaTypeHeaderValue> MultipleValueParser
             = new GenericHeaderParser<MediaTypeHeaderValue>(true, GetMediaTypeLength);
 
-        // Use list instead of dictionary since we may have multiple parameters with the same name.
-        private ICollection<NameValueHeaderValue> _parameters;
+        // Use a collection instead of a dictionary since we may have multiple parameters with the same name.
+        private ObjectCollection<NameValueHeaderValue> _parameters;
         private string _mediaType;
+        private bool _isReadOnly;
 
         private MediaTypeHeaderValue()
         {
@@ -48,6 +50,7 @@ namespace Microsoft.Net.Http.Headers
             }
             set
             {
+                HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
                 // We don't prevent a user from setting whitespace-only charsets. Like we can't prevent a user from
                 // setting a non-existing charset.
                 var charsetParameter = NameValueHeaderValue.Find(_parameters, CharsetString);
@@ -93,6 +96,7 @@ namespace Microsoft.Net.Http.Headers
             }
             set
             {
+                HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
                 if (value == null)
                 {
                     Charset = null;
@@ -112,6 +116,7 @@ namespace Microsoft.Net.Http.Headers
             }
             set
             {
+                HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
                 var boundaryParameter = NameValueHeaderValue.Find(_parameters, BoundaryString);
                 if (string.IsNullOrEmpty(value))
                 {
@@ -141,7 +146,14 @@ namespace Microsoft.Net.Http.Headers
             {
                 if (_parameters == null)
                 {
-                    _parameters = new ObjectCollection<NameValueHeaderValue>();
+                    if (IsReadOnly)
+                    {
+                        _parameters = ObjectCollection<NameValueHeaderValue>.EmptyReadOnlyCollection;
+                    }
+                    else
+                    {
+                        _parameters = new ObjectCollection<NameValueHeaderValue>();
+                    }
                 }
                 return _parameters;
             }
@@ -158,6 +170,7 @@ namespace Microsoft.Net.Http.Headers
             get { return _mediaType; }
             set
             {
+                HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
                 CheckMediaTypeFormat(value, "value");
                 _mediaType = value;
             }
@@ -201,6 +214,11 @@ namespace Microsoft.Net.Http.Headers
             }
         }
 
+        public bool IsReadOnly
+        {
+            get { return _isReadOnly; }
+        }
+
         public bool IsSubsetOf(MediaTypeHeaderValue otherMediaType)
         {
             if (otherMediaType == null)
@@ -253,22 +271,47 @@ namespace Microsoft.Net.Http.Headers
         /// while avoiding the cost of revalidating the components.
         /// </summary>
         /// <returns>A deep copy.</returns>
-        public MediaTypeHeaderValue Clone()
+        public MediaTypeHeaderValue Copy()
         {
+            if (IsReadOnly)
+            {
+                return this;
+            }
+
             var other = new MediaTypeHeaderValue();
             other._mediaType = _mediaType;
 
             if (_parameters != null)
             {
-                other._parameters = new ObjectCollection<NameValueHeaderValue>();
-                foreach (var pair in _parameters)
-                {
-                    other._parameters.Add(pair.Clone());
-                }
+                other._parameters = new ObjectCollection<NameValueHeaderValue>(
+                    _parameters.Select(item => item.Copy()));
             }
             return other;
         }
 
+        /// <summary>
+        /// Performs a deep copy of this object and all of it's NameValueHeaderValue sub components,
+        /// while avoiding the cost of revalidating the components. This copy is read-only.
+        /// </summary>
+        /// <returns>A deep, read-only, copy.</returns>
+        public MediaTypeHeaderValue CopyAsReadOnly()
+        {
+            if (IsReadOnly)
+            {
+                return this;
+            }
+
+            var other = new MediaTypeHeaderValue();
+            other._mediaType = _mediaType;
+            if (_parameters != null)
+            {
+                other._parameters = new ObjectCollection<NameValueHeaderValue>(
+                    _parameters.Select(item => item.CopyAsReadOnly()), isReadOnly: true);
+            }
+            other._isReadOnly = true;
+            return other;
+        }
+
         public override string ToString()
         {
             return _mediaType + NameValueHeaderValue.ToString(_parameters, ';', true);
diff --git a/src/Microsoft.Net.Http.Headers/NameValueHeaderValue.cs b/src/Microsoft.Net.Http.Headers/NameValueHeaderValue.cs
index 08a08f96ef..a91b98e358 100644
--- a/src/Microsoft.Net.Http.Headers/NameValueHeaderValue.cs
+++ b/src/Microsoft.Net.Http.Headers/NameValueHeaderValue.cs
@@ -20,6 +20,7 @@ namespace Microsoft.Net.Http.Headers
 
         private string _name;
         private string _value;
+        private bool _isReadOnly;
 
         private NameValueHeaderValue()
         {
@@ -49,17 +50,25 @@ namespace Microsoft.Net.Http.Headers
             get { return _value; }
             set
             {
+                HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
                 CheckValueFormat(value);
                 _value = value;
             }
         }
 
+        public bool IsReadOnly { get { return _isReadOnly; } }
+
         /// <summary>
         /// Provides a copy of this object without the cost of re-validating the values.
         /// </summary>
         /// <returns>A copy.</returns>
-        public NameValueHeaderValue Clone()
+        public NameValueHeaderValue Copy()
         {
+            if (IsReadOnly)
+            {
+                return this;
+            }
+
             return new NameValueHeaderValue()
             {
                 _name = _name,
@@ -67,6 +76,21 @@ namespace Microsoft.Net.Http.Headers
             };
         }
 
+        public NameValueHeaderValue CopyAsReadOnly()
+        {
+            if (IsReadOnly)
+            {
+                return this;
+            }
+
+            return new NameValueHeaderValue()
+            {
+                _name = _name,
+                _value = _value,
+                _isReadOnly = true
+            };
+        }
+
         public override int GetHashCode()
         {
             Contract.Assert(_name != null);
diff --git a/src/Microsoft.Net.Http.Headers/ObjectCollection.cs b/src/Microsoft.Net.Http.Headers/ObjectCollection.cs
index 90e7718c4c..24237b6ff1 100644
--- a/src/Microsoft.Net.Http.Headers/ObjectCollection.cs
+++ b/src/Microsoft.Net.Http.Headers/ObjectCollection.cs
@@ -2,6 +2,8 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 
 namespace Microsoft.Net.Http.Headers
@@ -10,40 +12,74 @@ namespace Microsoft.Net.Http.Headers
     // type to throw if 'null' gets added. Collection<T> internally uses List<T> which comes at some cost. In addition
     // Collection<T>.Add() calls List<T>.InsertItem() which is an O(n) operation (compared to O(1) for List<T>.Add()).
     // This type is only used for very small collections (1-2 items) to keep the impact of using Collection<T> small.
-    internal class ObjectCollection<T> : Collection<T> where T : class
+    internal class ObjectCollection<T> : ICollection<T> where T : class
     {
-        private static readonly Action<T> DefaultValidator = CheckNotNull;
+        internal static readonly Action<T> DefaultValidator = CheckNotNull;
+        internal static readonly ObjectCollection<T> EmptyReadOnlyCollection
+            = new ObjectCollection<T>(DefaultValidator, isReadOnly: true);
 
-        private Action<T> _validator;
+        private readonly Collection<T> _collection = new Collection<T>();
+        private readonly Action<T> _validator;
+        private readonly bool _isReadOnly;
 
         public ObjectCollection()
             : this(DefaultValidator)
         {
         }
 
-        public ObjectCollection(Action<T> validator)
+        public ObjectCollection(Action<T> validator, bool isReadOnly = false)
         {
             _validator = validator;
+            _isReadOnly = isReadOnly;
         }
 
-        protected override void InsertItem(int index, T item)
+        public ObjectCollection(IEnumerable<T> other, bool isReadOnly = false)
         {
-            if (_validator != null)
+            _validator = DefaultValidator;
+            foreach (T item in other)
             {
-                _validator(item);
+                Add(item);
             }
-            base.InsertItem(index, item);
+            _isReadOnly = isReadOnly;
         }
 
-        protected override void SetItem(int index, T item)
+        public int Count
         {
-            if (_validator != null)
-            {
-                _validator(item);
-            }
-            base.SetItem(index, item);
+            get { return _collection.Count; }
         }
 
+        public bool IsReadOnly
+        {
+            get { return _isReadOnly; }
+        }
+
+        public void Add(T item)
+        {
+            HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
+            _validator(item);
+            _collection.Add(item);
+        }
+
+        public bool Remove(T item)
+        {
+            HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
+            return _collection.Remove(item);
+        }
+
+        public void Clear()
+        {
+            HeaderUtilities.ThrowIfReadOnly(IsReadOnly);
+            _collection.Clear();
+        }
+
+        public bool Contains(T item) => _collection.Contains(item);
+
+        public void CopyTo(T[] array, int arrayIndex) => _collection.CopyTo(array, arrayIndex);
+
+        public IEnumerator<T> GetEnumerator() => _collection.GetEnumerator();
+
+        IEnumerator IEnumerable.GetEnumerator() => _collection.GetEnumerator();
+
         private static void CheckNotNull(T item)
         {
             // null values cannot be added to the collection.
diff --git a/src/Microsoft.Net.Http.Headers/project.json b/src/Microsoft.Net.Http.Headers/project.json
index 15f82024e2..fcbada8643 100644
--- a/src/Microsoft.Net.Http.Headers/project.json
+++ b/src/Microsoft.Net.Http.Headers/project.json
@@ -12,6 +12,7 @@
                 "System.Diagnostics.Contracts": "4.0.0-beta-*",
                 "System.Globalization": "4.0.10-beta-*",
                 "System.Globalization.Extensions": "4.0.0-beta-*",
+                "System.Linq": "4.0.0-beta-*",
                 "System.Text.Encoding": "4.0.10-beta-*",
                 "System.Runtime": "4.0.20-beta-*"
             }
diff --git a/test/Microsoft.Framework.WebEncoders.Tests/project.json b/test/Microsoft.Framework.WebEncoders.Tests/project.json
index b0ba4aa9fe..e44f880a72 100644
--- a/test/Microsoft.Framework.WebEncoders.Tests/project.json
+++ b/test/Microsoft.Framework.WebEncoders.Tests/project.json
@@ -20,5 +20,5 @@
       }
     }
   },
-  "resources": "..\\..\\unicode\\UnicodeData.txt"
+  "resource": "..\\..\\unicode\\UnicodeData.txt"
 }
diff --git a/test/Microsoft.Net.Http.Headers.Tests/MediaTypeHeaderValueTest.cs b/test/Microsoft.Net.Http.Headers.Tests/MediaTypeHeaderValueTest.cs
index 92ec8f9726..cfd389e4d4 100644
--- a/test/Microsoft.Net.Http.Headers.Tests/MediaTypeHeaderValueTest.cs
+++ b/test/Microsoft.Net.Http.Headers.Tests/MediaTypeHeaderValueTest.cs
@@ -65,10 +65,10 @@ namespace Microsoft.Net.Http.Headers
         }
 
         [Fact]
-        public void Clone_SimpleMediaType_Copied()
+        public void Copy_SimpleMediaType_Copied()
         {
             var mediaType0 = new MediaTypeHeaderValue("text/plain");
-            var mediaType1 = mediaType0.Clone();
+            var mediaType1 = mediaType0.Copy();
             Assert.NotSame(mediaType0, mediaType1);
             Assert.Same(mediaType0.MediaType, mediaType1.MediaType);
             Assert.NotSame(mediaType0.Parameters, mediaType1.Parameters);
@@ -76,11 +76,26 @@ namespace Microsoft.Net.Http.Headers
         }
 
         [Fact]
-        public void Clone_WithParameters_Copied()
+        public void CopyAsReadOnly_SimpleMediaType_CopiedAndReadOnly()
+        {
+            var mediaType0 = new MediaTypeHeaderValue("text/plain");
+            var mediaType1 = mediaType0.CopyAsReadOnly();
+            Assert.NotSame(mediaType0, mediaType1);
+            Assert.Same(mediaType0.MediaType, mediaType1.MediaType);
+            Assert.NotSame(mediaType0.Parameters, mediaType1.Parameters);
+            Assert.Equal(mediaType0.Parameters.Count, mediaType1.Parameters.Count);
+
+            Assert.False(mediaType0.IsReadOnly);
+            Assert.True(mediaType1.IsReadOnly);
+            Assert.Throws<InvalidOperationException>(() => { mediaType1.MediaType = "some/value"; });
+        }
+
+        [Fact]
+        public void Copy_WithParameters_Copied()
         {
             var mediaType0 = new MediaTypeHeaderValue("text/plain");
             mediaType0.Parameters.Add(new NameValueHeaderValue("name", "value"));
-            var mediaType1 = mediaType0.Clone();
+            var mediaType1 = mediaType0.Copy();
             Assert.NotSame(mediaType0, mediaType1);
             Assert.Same(mediaType0.MediaType, mediaType1.MediaType);
             Assert.NotSame(mediaType0.Parameters, mediaType1.Parameters);
@@ -92,6 +107,34 @@ namespace Microsoft.Net.Http.Headers
             Assert.Same(pair0.Value, pair1.Value);
         }
 
+        [Fact]
+        public void CopyAsReadOnly_WithParameters_CopiedAndReadOnly()
+        {
+            var mediaType0 = new MediaTypeHeaderValue("text/plain");
+            mediaType0.Parameters.Add(new NameValueHeaderValue("name", "value"));
+            var mediaType1 = mediaType0.CopyAsReadOnly();
+            Assert.NotSame(mediaType0, mediaType1);
+            Assert.False(mediaType0.IsReadOnly);
+            Assert.True(mediaType1.IsReadOnly);
+            Assert.Same(mediaType0.MediaType, mediaType1.MediaType);
+
+            Assert.NotSame(mediaType0.Parameters, mediaType1.Parameters);
+            Assert.False(mediaType0.Parameters.IsReadOnly);
+            Assert.True(mediaType1.Parameters.IsReadOnly);
+            Assert.Equal(mediaType0.Parameters.Count, mediaType1.Parameters.Count);
+            Assert.Throws<InvalidOperationException>(() => mediaType1.Parameters.Add(new NameValueHeaderValue("name")));
+            Assert.Throws<InvalidOperationException>(() => mediaType1.Parameters.Remove(new NameValueHeaderValue("name")));
+            Assert.Throws<InvalidOperationException>(() => mediaType1.Parameters.Clear());
+
+            var pair0 = mediaType0.Parameters.First();
+            var pair1 = mediaType1.Parameters.First();
+            Assert.NotSame(pair0, pair1);
+            Assert.False(pair0.IsReadOnly);
+            Assert.True(pair1.IsReadOnly);
+            Assert.Same(pair0.Name, pair1.Name);
+            Assert.Same(pair0.Value, pair1.Value);
+        }
+
         [Fact]
         public void MediaType_SetAndGetMediaType_MatchExpectations()
         {
diff --git a/test/Microsoft.Net.Http.Headers.Tests/NameValueHeaderValueTest.cs b/test/Microsoft.Net.Http.Headers.Tests/NameValueHeaderValueTest.cs
index a1e1a935f9..a5bf8c9b66 100644
--- a/test/Microsoft.Net.Http.Headers.Tests/NameValueHeaderValueTest.cs
+++ b/test/Microsoft.Net.Http.Headers.Tests/NameValueHeaderValueTest.cs
@@ -60,29 +60,71 @@ namespace Microsoft.Net.Http.Headers
         }
 
         [Fact]
-        public void Clone_NameOnly_SuccesfullyCopied()
+        public void Copy_NameOnly_SuccesfullyCopied()
         {
             var pair0 = new NameValueHeaderValue("name");
-            var pair1 = pair0.Clone();
+            var pair1 = pair0.Copy();
             Assert.NotSame(pair0, pair1);
             Assert.Same(pair0.Name, pair1.Name);
             Assert.Null(pair0.Value);
             Assert.Null(pair1.Value);
+
+            // Change one value and verify the other is unchanged.
+            pair0.Value = "othervalue";
+            Assert.Equal("othervalue", pair0.Value);
+            Assert.Equal(null, pair1.Value);
         }
 
         [Fact]
-        public void Clone_NameAndValue_SuccesfullyCopied()
+        public void CopyAsReadOnly_NameOnly_CopiedAndReadOnly()
+        {
+            var pair0 = new NameValueHeaderValue("name");
+            var pair1 = pair0.CopyAsReadOnly();
+            Assert.NotSame(pair0, pair1);
+            Assert.Same(pair0.Name, pair1.Name);
+            Assert.Null(pair0.Value);
+            Assert.Null(pair1.Value);
+            Assert.False(pair0.IsReadOnly);
+            Assert.True(pair1.IsReadOnly);
+
+            // Change one value and verify the other is unchanged.
+            pair0.Value = "othervalue";
+            Assert.Equal("othervalue", pair0.Value);
+            Assert.Equal(null, pair1.Value);
+            Assert.Throws<InvalidOperationException>(() => { pair1.Value = "othervalue"; });
+        }
+
+        [Fact]
+        public void Copy_NameAndValue_SuccesfullyCopied()
         {
             var pair0 = new NameValueHeaderValue("name", "value");
-            var pair1 = pair0.Clone();
+            var pair1 = pair0.Copy();
             Assert.NotSame(pair0, pair1);
             Assert.Same(pair0.Name, pair1.Name);
             Assert.Same(pair0.Value, pair1.Value);
 
             // Change one value and verify the other is unchanged.
-            pair1.Value = "othervalue";
-            Assert.Equal("value", pair0.Value);
-            Assert.Equal("othervalue", pair1.Value);
+            pair0.Value = "othervalue";
+            Assert.Equal("othervalue", pair0.Value);
+            Assert.Equal("value", pair1.Value);
+        }
+
+        [Fact]
+        public void CopyAsReadOnly_NameAndValue_CopiedAndReadOnly()
+        {
+            var pair0 = new NameValueHeaderValue("name", "value");
+            var pair1 = pair0.CopyAsReadOnly();
+            Assert.NotSame(pair0, pair1);
+            Assert.Same(pair0.Name, pair1.Name);
+            Assert.Same(pair0.Value, pair1.Value);
+            Assert.False(pair0.IsReadOnly);
+            Assert.True(pair1.IsReadOnly);
+
+            // Change one value and verify the other is unchanged.
+            pair0.Value = "othervalue";
+            Assert.Equal("othervalue", pair0.Value);
+            Assert.Equal("value", pair1.Value);
+            Assert.Throws<InvalidOperationException>(() => { pair1.Value = "othervalue"; });
         }
 
         [Fact]