diff --git a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
index e1d4d7809ad6..718c3edcb80e 100644
--- a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
+++ b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
@@ -261,6 +261,18 @@ public void RequiredFields()
             Assert.True(message.IsInitialized());
         }
 
+        /// <summary>
+        /// Code was accidentally left in message parser that threw exceptions when missing required fields after parsing.
+        /// We've decided to not throw exceptions on missing fields, instead leaving it up to the consumer how they
+        /// want to check and handle missing fields.
+        /// </summary>
+        [Test]
+        public void RequiredFieldsNoThrow()
+        {
+            Assert.DoesNotThrow(() => TestRequired.Parser.ParseFrom(new byte[0]));
+            Assert.DoesNotThrow(() => (TestRequired.Parser as MessageParser).ParseFrom(new byte[0]));
+        }
+
         [Test]
         public void RequiredFieldsInExtensions()
         {
diff --git a/csharp/src/Google.Protobuf/CodedOutputStream.cs b/csharp/src/Google.Protobuf/CodedOutputStream.cs
index f9ad29086bda..1d76d276029d 100644
--- a/csharp/src/Google.Protobuf/CodedOutputStream.cs
+++ b/csharp/src/Google.Protobuf/CodedOutputStream.cs
@@ -89,7 +89,7 @@ public CodedOutputStream(byte[] flatArray) : this(flatArray, 0, flatArray.Length
         private CodedOutputStream(byte[] buffer, int offset, int length)
         {
             this.output = null;
-            this.buffer = buffer;
+            this.buffer = ProtoPreconditions.CheckNotNull(buffer, nameof(buffer));
             this.position = offset;
             this.limit = offset + length;
             leaveOpen = true; // Simple way of avoiding trying to dispose of a null reference
diff --git a/csharp/src/Google.Protobuf/JsonFormatter.cs b/csharp/src/Google.Protobuf/JsonFormatter.cs
index 2b4c5f0a8038..5aaefe7a890f 100644
--- a/csharp/src/Google.Protobuf/JsonFormatter.cs
+++ b/csharp/src/Google.Protobuf/JsonFormatter.cs
@@ -133,7 +133,7 @@ static JsonFormatter()
         /// <param name="settings">The settings.</param>
         public JsonFormatter(Settings settings)
         {
-            this.settings = settings;
+            this.settings = ProtoPreconditions.CheckNotNull(settings, nameof(settings));
         }
 
         /// <summary>
diff --git a/csharp/src/Google.Protobuf/JsonParser.cs b/csharp/src/Google.Protobuf/JsonParser.cs
index fb594f2a4343..3f88ea38f177 100644
--- a/csharp/src/Google.Protobuf/JsonParser.cs
+++ b/csharp/src/Google.Protobuf/JsonParser.cs
@@ -110,7 +110,7 @@ private static void MergeWrapperField(JsonParser parser, IMessage message, JsonT
         /// <param name="settings">The settings.</param>
         public JsonParser(Settings settings)
         {
-            this.settings = settings;
+            this.settings = ProtoPreconditions.CheckNotNull(settings, nameof(settings));
         }
 
         /// <summary>
diff --git a/csharp/src/Google.Protobuf/MessageParser.cs b/csharp/src/Google.Protobuf/MessageParser.cs
index 76a350ce4c61..06d0f1059c7a 100644
--- a/csharp/src/Google.Protobuf/MessageParser.cs
+++ b/csharp/src/Google.Protobuf/MessageParser.cs
@@ -72,7 +72,6 @@ public IMessage ParseFrom(byte[] data)
         {
             IMessage message = factory();
             message.MergeFrom(data, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -87,7 +86,6 @@ public IMessage ParseFrom(byte[] data, int offset, int length)
         {
             IMessage message = factory();
             message.MergeFrom(data, offset, length, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -100,7 +98,6 @@ public IMessage ParseFrom(ByteString data)
         {
             IMessage message = factory();
             message.MergeFrom(data, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -113,7 +110,6 @@ public IMessage ParseFrom(Stream input)
         {
             IMessage message = factory();
             message.MergeFrom(input, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -130,7 +126,6 @@ public IMessage ParseDelimitedFrom(Stream input)
         {
             IMessage message = factory();
             message.MergeDelimitedFrom(input, DiscardUnknownFields, Extensions);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -143,7 +138,6 @@ public IMessage ParseFrom(CodedInputStream input)
         {
             IMessage message = factory();
             MergeFrom(message, input);
-            CheckMergedRequiredFields(message);
             return message;
         }
 
@@ -176,12 +170,6 @@ internal void MergeFrom(IMessage message, CodedInputStream codedInput)
             }
         }
 
-        internal static void CheckMergedRequiredFields(IMessage message)
-        {
-            if (!message.IsInitialized())
-                throw new InvalidOperationException("Parsed message does not contain all required fields");
-        }
-
         /// <summary>
         /// Creates a new message parser which optionally discards unknown fields when parsing.
         /// </summary>
diff --git a/csharp/src/Google.Protobuf/Reflection/ExtensionCollection.cs b/csharp/src/Google.Protobuf/Reflection/ExtensionCollection.cs
index 38e33d7ff2bd..9664559df249 100644
--- a/csharp/src/Google.Protobuf/Reflection/ExtensionCollection.cs
+++ b/csharp/src/Google.Protobuf/Reflection/ExtensionCollection.cs
@@ -107,13 +107,14 @@ internal void CrossLink()
             {
                 descriptor.CrossLink();
 
-                IList<FieldDescriptor> _;
-                if (!declarationOrder.TryGetValue(descriptor.ExtendeeType, out _))
+                IList<FieldDescriptor> list;
+                if (!declarationOrder.TryGetValue(descriptor.ExtendeeType, out list))
                 {
-                    declarationOrder.Add(descriptor.ExtendeeType, new List<FieldDescriptor>());
+                    list = new List<FieldDescriptor>();
+                    declarationOrder.Add(descriptor.ExtendeeType, list);
                 }
 
-                declarationOrder[descriptor.ExtendeeType].Add(descriptor);
+                list.Add(descriptor);
             }
 
             extensionsByTypeInDeclarationOrder = declarationOrder