diff --git a/csharp/src/Google.Protobuf.Test/ByteStringTest.cs b/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
index 4876e09..3810565 100644
--- a/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
+++ b/csharp/src/Google.Protobuf.Test/ByteStringTest.cs
@@ -41,9 +41,7 @@
 using System.Runtime.InteropServices;
 using System.Threading;
 using System.Runtime.CompilerServices;
-#if !NET35
 using System.Threading.Tasks;
-#endif
 
 namespace Google.Protobuf
 {
@@ -349,7 +347,6 @@
             Assert.AreEqual(expected, actual, $"{expected.ToBase64()} != {actual.ToBase64()}");
         }
 
-#if !NET35
         [Test]
         public async Task FromStreamAsync_Seekable()
         {
@@ -373,7 +370,6 @@
             ByteString expected = ByteString.CopyFrom(2, 3, 4);
             Assert.AreEqual(expected, actual, $"{expected.ToBase64()} != {actual.ToBase64()}");
         }
-#endif
 
         [Test]
         public void GetHashCode_Regression()
diff --git a/csharp/src/Google.Protobuf.Test/Collections/MapFieldTest.cs b/csharp/src/Google.Protobuf.Test/Collections/MapFieldTest.cs
index 1f7b8a3..abd19a2 100644
--- a/csharp/src/Google.Protobuf.Test/Collections/MapFieldTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Collections/MapFieldTest.cs
@@ -635,7 +635,6 @@
             Assert.IsTrue(input.IsAtEnd);
         }
 
-#if !NET35
         [Test]
         public void IDictionaryKeys_Equals_IReadOnlyDictionaryKeys()
         {
@@ -649,7 +648,6 @@
             var map = new MapField<string, string> { { "foo", "bar" }, { "x", "y" } };
             CollectionAssert.AreEquivalent(((IDictionary<string, string>)map).Values, ((IReadOnlyDictionary<string, string>)map).Values);
         }
-#endif
 
         private static KeyValuePair<TKey, TValue> NewKeyValuePair<TKey, TValue>(TKey key, TValue value)
         {
diff --git a/csharp/src/Google.Protobuf.Test/Compatibility/StreamExtensionsTest.cs b/csharp/src/Google.Protobuf.Test/Compatibility/StreamExtensionsTest.cs
deleted file mode 100644
index 48c0725..0000000
--- a/csharp/src/Google.Protobuf.Test/Compatibility/StreamExtensionsTest.cs
+++ /dev/null
@@ -1,67 +0,0 @@
-﻿#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2015 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.
-#endregion
-
-#if NET35
-using System;
-using System.IO;
-using NUnit.Framework;
-using Google.Protobuf.Compatibility;
-
-namespace Google.Protobuf.Test.Compatibility
-{
-    public class StreamExtensionsTest
-    {
-        [Test]
-        public void CopyToNullArgument()
-        {
-            var memoryStream = new MemoryStream();
-            Assert.Throws<ArgumentNullException>(() => memoryStream.CopyTo(null));
-        }
-
-        [Test]
-        public void CopyToTest()
-        {
-            byte[] bytesToStream = new byte[] { 0x31, 0x08, 0xFF, 0x00 };
-            Stream source = new MemoryStream(bytesToStream);
-            Stream destination = new MemoryStream((int)source.Length);
-            source.CopyTo(destination);
-            destination.Seek(0, SeekOrigin.Begin);
-
-            Assert.AreEqual(0x31, destination.ReadByte());
-            Assert.AreEqual(0x08, destination.ReadByte());
-            Assert.AreEqual(0xFF, destination.ReadByte());
-            Assert.AreEqual(0x00, destination.ReadByte());
-            Assert.AreEqual(-1, destination.ReadByte());
-        }
-    }
-}
-#endif
diff --git a/csharp/src/Google.Protobuf.Test/Compatibility/TypeExtensionsTest.cs b/csharp/src/Google.Protobuf.Test/Compatibility/TypeExtensionsTest.cs
index 5d86c20..1d69fcb 100644
--- a/csharp/src/Google.Protobuf.Test/Compatibility/TypeExtensionsTest.cs
+++ b/csharp/src/Google.Protobuf.Test/Compatibility/TypeExtensionsTest.cs
@@ -34,7 +34,6 @@
 using System.Collections.Generic;
 using System.Reflection;
 
-#if !NET35
 namespace Google.Protobuf.Compatibility
 {
     public class TypeExtensionsTest
@@ -114,4 +113,3 @@
         }
     }
 }
-#endif
diff --git a/csharp/src/Google.Protobuf.Test/FieldCodecTest.cs b/csharp/src/Google.Protobuf.Test/FieldCodecTest.cs
index 7f36692..4f2a562 100644
--- a/csharp/src/Google.Protobuf.Test/FieldCodecTest.cs
+++ b/csharp/src/Google.Protobuf.Test/FieldCodecTest.cs
@@ -169,7 +169,6 @@
                 // WriteTagAndValue ignores default values
                 var stream = new MemoryStream();
                 CodedOutputStream codedOutput;
-#if !NET35
                 codedOutput = new CodedOutputStream(stream);
                 codec.WriteTagAndValue(codedOutput, codec.DefaultValue);
                 codedOutput.Flush();
@@ -179,7 +178,6 @@
                 {
                     Assert.AreEqual(default(T), codec.DefaultValue);
                 }
-#endif
 
                 // The plain ValueWriter/ValueReader delegates don't.
                 if (codec.DefaultValue != null) // This part isn't appropriate for message types.
diff --git a/csharp/src/Google.Protobuf/ByteString.cs b/csharp/src/Google.Protobuf/ByteString.cs
index 8c6eb5b..c615616 100644
--- a/csharp/src/Google.Protobuf/ByteString.cs
+++ b/csharp/src/Google.Protobuf/ByteString.cs
@@ -37,13 +37,8 @@
 using System.Runtime.InteropServices;
 using System.Security;
 using System.Text;
-#if !NET35
 using System.Threading;
 using System.Threading.Tasks;
-#endif
-#if NET35
-using Google.Protobuf.Compatibility;
-#endif
 
 namespace Google.Protobuf
 {
@@ -186,7 +181,6 @@
             return AttachBytes(bytes);
         }
 
-#if !NET35
         /// <summary>
         /// Constructs a <see cref="ByteString"/> from data in the given stream, asynchronously.
         /// </summary>
@@ -200,7 +194,6 @@
             ProtoPreconditions.CheckNotNull(stream, nameof(stream));
             return ByteStringAsync.FromStreamAsyncCore(stream, cancellationToken);
         }
-#endif
 
         /// <summary>
         /// Constructs a <see cref="ByteString" /> from the given array. The contents
diff --git a/csharp/src/Google.Protobuf/ByteStringAsync.cs b/csharp/src/Google.Protobuf/ByteStringAsync.cs
index 3465cc6..5aa92f0 100644
--- a/csharp/src/Google.Protobuf/ByteStringAsync.cs
+++ b/csharp/src/Google.Protobuf/ByteStringAsync.cs
@@ -43,7 +43,6 @@
     /// </summary>
     internal static class ByteStringAsync
     {
-#if !NET35
         internal static async Task<ByteString> FromStreamAsyncCore(Stream stream, CancellationToken cancellationToken)
         {
             int capacity = stream.CanSeek ? checked((int)(stream.Length - stream.Position)) : 0;
@@ -59,6 +58,5 @@
 #endif
             return ByteString.AttachBytes(bytes);
         }
-#endif
     }
 }
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Collections/MapField.cs b/csharp/src/Google.Protobuf/Collections/MapField.cs
index e5217f4..0f02d59 100644
--- a/csharp/src/Google.Protobuf/Collections/MapField.cs
+++ b/csharp/src/Google.Protobuf/Collections/MapField.cs
@@ -68,10 +68,7 @@
     /// in future versions.
     /// </para>
     /// </remarks>
-    public sealed class MapField<TKey, TValue> : IDeepCloneable<MapField<TKey, TValue>>, IDictionary<TKey, TValue>, IEquatable<MapField<TKey, TValue>>, IDictionary
-#if !NET35
-        , IReadOnlyDictionary<TKey, TValue>
-#endif
+    public sealed class MapField<TKey, TValue> : IDeepCloneable<MapField<TKey, TValue>>, IDictionary<TKey, TValue>, IEquatable<MapField<TKey, TValue>>, IDictionary, IReadOnlyDictionary<TKey, TValue>
     {
         private static readonly EqualityComparer<TValue> ValueEqualityComparer = ProtobufEqualityComparers.GetEqualityComparer<TValue>();
         private static readonly EqualityComparer<TKey> KeyEqualityComparer = ProtobufEqualityComparers.GetEqualityComparer<TKey>();
@@ -600,11 +597,8 @@
         #endregion
 
         #region IReadOnlyDictionary explicit interface implementation
-#if !NET35
         IEnumerable<TKey> IReadOnlyDictionary<TKey, TValue>.Keys => Keys;
-
         IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values => Values;
-#endif
         #endregion
 
         private class DictionaryEnumerator : IDictionaryEnumerator
diff --git a/csharp/src/Google.Protobuf/Collections/ReadOnlyDictionary.cs b/csharp/src/Google.Protobuf/Collections/ReadOnlyDictionary.cs
deleted file mode 100644
index 28530a2..0000000
--- a/csharp/src/Google.Protobuf/Collections/ReadOnlyDictionary.cs
+++ /dev/null
@@ -1,147 +0,0 @@
-#region Copyright notice and license
-// 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.
-#endregion
-
-using System;
-using System.Collections;
-using System.Collections.Generic;
-
-namespace Google.Protobuf.Collections
-{
-    /// <summary>
-    /// Read-only wrapper around another dictionary.
-    /// </summary>
-    internal sealed class ReadOnlyDictionary<TKey, TValue> : IDictionary<TKey, TValue>
-    {
-        private readonly IDictionary<TKey, TValue> wrapped;
-
-        public ReadOnlyDictionary(IDictionary<TKey, TValue> wrapped)
-        {
-            this.wrapped = wrapped;
-        }
-
-        public void Add(TKey key, TValue value)
-        {
-            throw new InvalidOperationException();
-        }
-
-        public bool ContainsKey(TKey key)
-        {
-            return wrapped.ContainsKey(key);
-        }
-
-        public ICollection<TKey> Keys
-        {
-            get { return wrapped.Keys; }
-        }
-
-        public bool Remove(TKey key)
-        {
-            throw new InvalidOperationException();
-        }
-
-        public bool TryGetValue(TKey key, out TValue value)
-        {
-            return wrapped.TryGetValue(key, out value);
-        }
-
-        public ICollection<TValue> Values
-        {
-            get { return wrapped.Values; }
-        }
-
-        public TValue this[TKey key]
-        {
-            get { return wrapped[key]; }
-            set { throw new InvalidOperationException(); }
-        }
-
-        public void Add(KeyValuePair<TKey, TValue> item)
-        {
-            throw new InvalidOperationException();
-        }
-
-        public void Clear()
-        {
-            throw new InvalidOperationException();
-        }
-
-        public bool Contains(KeyValuePair<TKey, TValue> item)
-        {
-            return wrapped.Contains(item);
-        }
-
-        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
-        {
-            wrapped.CopyTo(array, arrayIndex);
-        }
-
-        public int Count
-        {
-            get { return wrapped.Count; }
-        }
-
-        public bool IsReadOnly
-        {
-            get { return true; }
-        }
-
-        public bool Remove(KeyValuePair<TKey, TValue> item)
-        {
-            throw new InvalidOperationException();
-        }
-
-        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
-        {
-            return wrapped.GetEnumerator();
-        }
-
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            return ((IEnumerable) wrapped).GetEnumerator();
-        }
-
-        public override bool Equals(object obj)
-        {
-            return wrapped.Equals(obj);
-        }
-
-        public override int GetHashCode()
-        {
-            return wrapped.GetHashCode();
-        }
-
-        public override string ToString()
-        {
-            return wrapped.ToString();
-        }
-    }
-}
\ No newline at end of file
diff --git a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
index 9269c74..cd6f5eb 100644
--- a/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
+++ b/csharp/src/Google.Protobuf/Collections/RepeatedField.cs
@@ -48,10 +48,7 @@
     /// supported by Protocol Buffers but nor does it guarantee that all operations will work in such cases.
     /// </remarks>
     /// <typeparam name="T">The element type of the repeated field.</typeparam>
-    public sealed class RepeatedField<T> : IList<T>, IList, IDeepCloneable<RepeatedField<T>>, IEquatable<RepeatedField<T>>
-#if !NET35
-        , IReadOnlyList<T>
-#endif
+    public sealed class RepeatedField<T> : IList<T>, IList, IDeepCloneable<RepeatedField<T>>, IEquatable<RepeatedField<T>>, IReadOnlyList<T>
     {
         private static readonly EqualityComparer<T> EqualityComparer = ProtobufEqualityComparers.GetEqualityComparer<T>();
         private static readonly T[] EmptyArray = new T[0];
diff --git a/csharp/src/Google.Protobuf/Compatibility/MethodInfoExtensions.cs b/csharp/src/Google.Protobuf/Compatibility/MethodInfoExtensions.cs
deleted file mode 100644
index 7b946cb..0000000
--- a/csharp/src/Google.Protobuf/Compatibility/MethodInfoExtensions.cs
+++ /dev/null
@@ -1,47 +0,0 @@
-﻿#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2017 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.
-#endregion
-
-#if NET35
-using System;
-using System.Reflection;
-
-namespace Google.Protobuf.Compatibility
-{
-    // .NET Core (at least netstandard1.0) doesn't have Delegate.CreateDelegate, and .NET 3.5 doesn't have
-    // MethodInfo.CreateDelegate. Proxy from one to the other on .NET 3.5...
-    internal static class MethodInfoExtensions
-    {
-        internal static Delegate CreateDelegate(this MethodInfo method, Type type) =>
-            Delegate.CreateDelegate(type, method);
-    }
-}
-#endif
diff --git a/csharp/src/Google.Protobuf/Compatibility/PropertyInfoExtensions.cs b/csharp/src/Google.Protobuf/Compatibility/PropertyInfoExtensions.cs
index 95a02c7..8a6fefa 100644
--- a/csharp/src/Google.Protobuf/Compatibility/PropertyInfoExtensions.cs
+++ b/csharp/src/Google.Protobuf/Compatibility/PropertyInfoExtensions.cs
@@ -47,11 +47,7 @@
         /// </summary>
         internal static MethodInfo GetGetMethod(this PropertyInfo target)
         {
-#if NET35
-            var method = target.GetGetMethod();
-#else
             var method = target.GetMethod;
-#endif
             return method != null && method.IsPublic ? method : null;
         }
 
@@ -61,11 +57,7 @@
         /// </summary>
         internal static MethodInfo GetSetMethod(this PropertyInfo target)
         {
-#if NET35
-            var method = target.GetSetMethod();
-#else
             var method = target.SetMethod;
-#endif
             return method != null && method.IsPublic ? method : null;
         }
     }
diff --git a/csharp/src/Google.Protobuf/Compatibility/StreamExtensions.cs b/csharp/src/Google.Protobuf/Compatibility/StreamExtensions.cs
deleted file mode 100644
index bf4bf22..0000000
--- a/csharp/src/Google.Protobuf/Compatibility/StreamExtensions.cs
+++ /dev/null
@@ -1,66 +0,0 @@
-﻿#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2015 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.
-#endregion
-
-#if NET35
-using System;
-using System.IO;
-
-namespace Google.Protobuf.Compatibility
-{
-    /// <summary>
-    /// Extension methods for <see cref="Stream"/> in order to provide
-    /// backwards compatibility with .NET 3.5
-    /// </summary>
-    public static class StreamExtensions
-    {
-        // 81920 seems to be the default buffer size used in .NET 4.5.1
-        private const int BUFFER_SIZE = 81920;
-
-        /// <summary>
-        /// Write the contents of the current stream to the destination stream
-        /// </summary>
-        public static void CopyTo(this Stream source, Stream destination)
-        {
-            if (destination == null)
-            {
-                throw new ArgumentNullException(nameof(destination));
-            }
-
-            byte[] buffer = new byte[BUFFER_SIZE];
-            int numBytesRead;
-            while ((numBytesRead = source.Read(buffer, 0, buffer.Length)) > 0) {
-                destination.Write(buffer, 0, numBytesRead);
-            }
-        }
-    }
-}
-#endif
diff --git a/csharp/src/Google.Protobuf/Compatibility/TypeExtensions.cs b/csharp/src/Google.Protobuf/Compatibility/TypeExtensions.cs
index b3acda2..5db5dca 100644
--- a/csharp/src/Google.Protobuf/Compatibility/TypeExtensions.cs
+++ b/csharp/src/Google.Protobuf/Compatibility/TypeExtensions.cs
@@ -34,7 +34,6 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 
-#if !NET35
 namespace Google.Protobuf.Compatibility
 {
     /// <summary>
@@ -112,4 +111,3 @@
         }
     }
 }
-#endif
diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index 40a6ff8..415b7be 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -36,10 +36,6 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
-#if NET35
-// Needed for ReadOnlyDictionary, which does not exist in .NET 3.5
-using Google.Protobuf.Collections;
-#endif
 
 namespace Google.Protobuf.Reflection
 {
diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs
index 58a33cb..dd00e2d 100644
--- a/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs
+++ b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs
@@ -63,12 +63,7 @@
             if (firstInvalid == null)
             {
                 var writer = new StringWriter();
-#if NET35
-                var query = paths.Select(JsonFormatter.ToJsonName);
-                JsonFormatter.WriteString(writer, string.Join(",", query.ToArray()));
-#else
                 JsonFormatter.WriteString(writer, string.Join(",", paths.Select(JsonFormatter.ToJsonName)));
-#endif
                 return writer.ToString();
             }
             else