protobuf/csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs

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

using System;
using System.Reflection;

namespace Google.Protobuf.Reflection
{
    /// <summary>
    /// Reflection access for a oneof, allowing clear and "get case" actions.
    /// </summary>
    public sealed class OneofAccessor
    {
        private readonly Func<object, int> caseDelegate;
        private readonly Action<object> clearDelegate;
        private OneofDescriptor descriptor;

        internal OneofAccessor(PropertyInfo caseProperty, MethodInfo clearMethod, OneofDescriptor descriptor) 
        {
            if (!caseProperty.CanRead)
            {
                throw new ArgumentException("Cannot read from property");
            }
            this.descriptor = descriptor;
            caseDelegate = ReflectionUtil.CreateFuncObjectT<int>(caseProperty.GetGetMethod());

            this.descriptor = descriptor;
            clearDelegate = ReflectionUtil.CreateActionObject(clearMethod);
        }

        public OneofDescriptor Descriptor { get { return descriptor; } }

        /// <summary>
        /// Clears the oneof in the specified message.
        /// </summary>
        public void Clear(object message)
        {
            clearDelegate(message);
        }

        /// <summary>
        /// Indicates which field in the oneof is set for specified message
        /// </summary>
        public FieldDescriptor GetCaseFieldDescriptor(object message)
        {
            int fieldNumber = caseDelegate(message);
            if (fieldNumber > 0)
            {
                return descriptor.ContainingType.FindFieldByNumber(fieldNumber);
            }
            return null;
        }
    }
}