third_party/protobuf/3.6.1/csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs

#region Copyright notice and license
// Protocol Buffers - Google's data interchange format
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#endregion

using System;
using System.Reflection;
using Google.Protobuf.Compatibility;

namespace Google.Protobuf.Reflection
{
    /// <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<IMessage, object> setValueDelegate;
        private readonly Action<IMessage> clearDelegate;

        internal SingleFieldAccessor(PropertyInfo property, FieldDescriptor descriptor) : base(property, descriptor)
        {
            if (!property.CanWrite)
            {
                throw new ArgumentException("Not all required properties/methods available");
            }
            setValueDelegate = ReflectionUtil.CreateActionIMessageObject(property.GetSetMethod());

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

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

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