protobuf/csharp/src/ProtocolBuffers/FieldAccess/SingleFieldAccessor.cs

#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/
//
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// 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
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#endregion
    
using System;
using System.Reflection;
using Google.Protobuf.Descriptors;

namespace Google.Protobuf.FieldAccess
{
    /// <summary>
    /// Accessor for single fields.
    /// </summary>
    internal sealed class SingleFieldAccessor : FieldAccessorBase
    {
        // All the work here is actually done in the constructor - it creates the appropriate delegates.
        // There are various cases to consider, based on the property type (message, string/bytes, or "genuine" primitive)
        // and proto2 vs proto3 for non-message types, as proto3 doesn't support "full" presence detection or default
        // values.

        private readonly Action<object, object> setValueDelegate;
        private readonly Action<object> clearDelegate;

        internal SingleFieldAccessor(Type type, string propertyName, FieldDescriptor descriptor) : base(type, propertyName, descriptor)
        {
            PropertyInfo property = type.GetProperty(propertyName);
            // We know there *is* such a property, or the base class constructor would have thrown. We should be able to write
            // to it though.
            if (!property.CanWrite)
            {
                throw new ArgumentException("Not all required properties/methods available");
            }
            setValueDelegate = ReflectionUtil.CreateActionObjectObject(property.GetSetMethod());

            var clrType = property.PropertyType;
            
            // TODO: What should clear on a oneof member do? Clear the oneof?

            // TODO: Validate that this is a reasonable single field? (Should be a value type, a message type, or string/ByteString.)
            object defaultValue =
                typeof(IMessage).IsAssignableFrom(clrType) ? null
                : clrType == typeof(string) ? ""
                : clrType == typeof(ByteString) ? ByteString.Empty
                : Activator.CreateInstance(clrType);
            clearDelegate = message => SetValue(message, defaultValue);
        }

        public override void Clear(object message)
        {
            clearDelegate(message);
        }

        public override void SetValue(object message, object value)
        {
            setValueDelegate(message, value);
        }
    }
}