.net - 显式实现接口(interface)时 coSTLy 是如何装箱的

Question

当前的显式成员实现指南建议:

• 使用显式成员来近似私有(private)接口(interface)实现。 如果您仅出于基础架构原因需要实现一个接口(interface)，并且您从不期望开发人员直接从该类型调用该接口(interface)上的方法，那么显式实现成员以从公众 View 中“隐藏”它们.
• 公开一种替代方法来访问允许子类覆盖的任何显式实现的成员。

一个很好的例子就是当你想实现 IXmlSerializable界面。 ReadXml 和 WriteXml 方法应由 XmlSerializer 调用，开发人员通常不会直接调用。

当提供一种替代方法来显式访问您希望允许被覆盖的成员时，调用显式实现的成员以避免代码重复似乎是有意义的。请考虑以下事项:

using System;
using System.Xml;
using System.Xml.Schema;
using System.Xml.Serialization;

namespace Demo
{
    /// <summary>
    /// Demonstrates explicit implementation of the IXmlSerializable interface.
    /// </summary>
    [Serializable(), XmlRoot(ElementName = "foo")]
    public class Foo : IXmlSerializable
    {
        //============================================================
        //  IXmlSerializable Implementation
        //============================================================
        #region GetSchema()
        /// <summary>
        /// Returns an <see cref="XmlSchema"/> that describes the XML representation of the object.
        /// </summary>
        /// <returns>
        /// An <see cref="XmlSchema"/> that describes the XML representation of the object that is 
        /// produced by the <see cref="IXmlSerializable.WriteXml(XmlWriter)"/> method and consumed by the <see cref="IXmlSerializable.ReadXml(XmlReader)"/> method.
        /// </returns>
        /// <remarks>This method is reserved and should not be used.</remarks>
        XmlSchema IXmlSerializable.GetSchema()
        {
            return null;
        }
        #endregion

        #region ReadXml(XmlReader reader)
        /// <summary>
        /// Generates an object from its XML representation.
        /// </summary>
        /// <param name="reader">The <see cref="XmlReader"/> stream from which the object is deserialized.</param>
        /// <exception cref="ArgumentNullException">The <paramref name="reader"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
        void IXmlSerializable.ReadXml(XmlReader reader)
        {
            // Class state values read from supplied XmlReader
        }
        #endregion

        #region WriteXml(XmlWriter writer)
        /// <summary>
        /// Converts an object into its XML representation.
        /// </summary>
        /// <param name="writer">The <see cref="XmlWriter"/> stream to which the object is serialized.</param>
        /// <exception cref="ArgumentNullException">The <paramref name="writer"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
        void IXmlSerializable.WriteXml(XmlWriter writer)
        {
            // Current class state values written using supplied XmlWriter
        }
        #endregion

        //============================================================
        //  Public Methods
        //============================================================
        #region WriteTo(XmlWriter writer)
        /// <summary>
        /// Saves the current <see cref="Foo"/> to the specified <see cref="XmlWriter"/>.
        /// </summary>
        /// <param name="writer">The <see cref="XmlWriter"/> stream to which the <see cref="Foo"/> is serialized.</param>
        /// <exception cref="ArgumentNullException">The <paramref name="writer"/> is a <b>null</b> reference (Nothing in Visual Basic).</exception>
        public void WriteTo(XmlWriter writer)
        {
            writer.WriteStartElement("foo");

            ((IXmlSerializable)this).WriteXml(writer);

            writer.WriteEndElement();
        }
        #endregion
    }
}

我的问题是关于此实现中 WriteXml 方法的装箱成本是多少。 ((IXmlSerializable)this).WriteXml(writer) 会显着影响性能吗？

Answer 1

您的示例中没有装箱...它只是一个转换，并且在编译时可解析，因此它根本不会对性能产生任何影响。

编辑:用 ILDASM 来看，接口(interface)转换会给你一个虚拟方法调用而不是一个常规方法调用，但这可以忽略不计(仍然没有涉及装箱)。

编辑 2:如果您使用结构而不是类，那么您将得到一个通过界面的框，性能损失更大。