接口
interface ILog
    {
        bool Write(string message);
        bool Write(Exception ex);
    }

TextFileLog
class TextFileLog : ILog
    {
        public bool Write(string message)
        {
            string fileDir = ConfigurationManager.AppSettings["LogTarget"].ToString();
            using (StreamWriter w = File.AppendText(fileDir))
            {
                // w.Write(" Log Entry : ");
                w.WriteLine("发生时间{0}", DateTime.Now.ToLocalTime().ToString());
                w.WriteLine("日志内容为:{0}", message);
                w.WriteLine("-------------------------------");
                // Update the underlying file.
                w.Flush();
                w.Close();
            }
            return true;
        }
        public bool Write(Exception ex)
        {

Write(ex.Message);
            return true;
        }
    }

XmlFileLog
class XmlFileLog : ILog
    {
        public bool Write(string message)
        {
            string xmlFilePath = ConfigurationManager.AppSettings["LogTarget"].ToString();
            if (File.Exists(xmlFilePath))
            {
                XmlDocument doc = new XmlDocument();
                doc.Load(xmlFilePath);
                XmlDocumentFragment docFrag = doc.CreateDocumentFragment();
                XmlNode nod = doc.SelectSingleNode("Logs");
                docFrag.InnerXml = "<Log><Time>" + DateTime.Now.ToLocalTime().ToString()
                    + "</Time><Message>" + message + "</Message></Log>";
                nod.AppendChild(docFrag);

doc.Save(xmlFilePath);
                return true;
            }
            else
            {
                XmlWriterSettings settings = new XmlWriterSettings();
                settings.Indent = true;     //设置缩进       
                settings.ConformanceLevel = ConformanceLevel.Auto;
                settings.IndentChars = " ";
                settings.OmitXmlDeclaration = false;
                using (XmlWriter writer = XmlWriter.Create(xmlFilePath, settings))
                {
                    //Start writing the XML document
                    writer.WriteStartDocument(false);
                    //Start with the root element
                    writer.WriteStartElement("Logs");
                    writer.WriteStartElement("Log");
                    writer.WriteStartElement("Time");
                    writer.WriteString(DateTime.Now.ToLocalTime().ToString());
                    writer.WriteEndElement();
                    writer.WriteStartElement("Message");
                    writer.WriteString(message);
                    writer.WriteEndElement();
                    writer.WriteEndElement();
                    writer.WriteEndDocument();
                    //Flush the object and write the XML data to the file
                    writer.Flush();
                    return true;
                }

}
        }
        public bool Write(Exception ex)
        {
            Write(ex.Message);
            return true;

}
    }

App.config配置
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="LogType" value="LogClassLibrary.TextFileLog"/>
    <!--
    本程序集配置
    <add key="LogType" value="ConsoleApplication2.Log例子.TextFileLog"/>
    -->
    <!-- XmlFileLog  TextFileLog-->
    <add key="LogTarget" value="c:\log.txt"/>
  </appSettings>
</configuration>

主程序
 public static void Main()
        {
            #region 同程序集下
            System.Type type = System.Type.GetType(ConfigurationManager.AppSettings["LogType"].ToString());
            ILog log = (ILog)Activator.CreateInstance(type);
            log.Write(new Exception("异常测试"));
            #endregion
        }

不同程序集主函数
 public static void Main()
        {

#region 不同程序集
            string assemblyPath = Path.Combine(Environment.CurrentDirectory, "LogClassLibrary.dll");
            Assembly a = Assembly.LoadFrom(assemblyPath);
            Type type = a.GetType(ConfigurationManager.AppSettings["LogType"].ToString());
            LogClassLibrary.ILog log = (LogClassLibrary.ILog)type.InvokeMember(null,BindingFlags.CreateInstance,null,null,null);
            log.Write(new Exception("异常测试"));
            #endregion

}

接口
public interface IHost
    {
        List<ILog> Plugins { get; }
        int LoadPlugins(string path);
        ILog GetLog(string name);

}
 public interface ILog
    {
        bool Write(string message);
        bool Write(Exception ex);

}

宿主实现
 public class Host : IHost
    {
        private List<ILog> plugins = new List<ILog>();
        #region IHost 成员

public List<ILog> Plugins
        {
            get { return plugins; }
        }

public int LoadPlugins(string path)
        {
            string[] assemblyFiles = Directory.GetFiles(path, "*.dll");
            foreach (var file in assemblyFiles)
            {
                Assembly assembly = Assembly.LoadFrom(file);
                foreach (var type in assembly.GetExportedTypes())
                {
                    if (type.IsClass && typeof(ILog).IsAssignableFrom(type))
                    {
                        ILog plugin = Activator.CreateInstance(type) as ILog;
                        plugins.Add(plugin);

}
                }
            }
            return plugins.Count;
        }

public ILog GetLog(string name)
        {
            foreach (var item in plugins)
            {
                if (item.GetType().ToString()==name)
                {
                    return item;
                }
                
            }
            return null;
        }

#endregion
    }

主程序代码
static void Main(string[] args)
        {
            Host.Host host = new Host.Host();
            host.LoadPlugins(".");
            InterfaceLayer.ILog log = host.GetLog(ConfigurationManager.AppSettings["LogType"].ToString());
            log.Write(new Exception("异常测试"));
        }

Code
public class DDlControl 
    {
        private ListControl underlyingList;

public DDlControl(ListControl underlyingList)
        {
            this.underlyingList = underlyingList;
        }

public void Add(IDDL ddl)
        {
            underlyingList.Items.Add(new ListItem(ddl.Name, ddl.Value));
        }
        public void Add<T>(T t, string nameStr, string valueStr)
        {
            string name = Convert.ToString(t.GetType().InvokeMember
                   (nameStr, System.Reflection.BindingFlags.GetProperty, null, t, null));
            string value = Convert.ToString(t.GetType().InvokeMember
                (valueStr, System.Reflection.BindingFlags.GetProperty, null, t, null));
            Add(new DDLStruct(name,value));
            
        }
        public void Clear()
        {
            underlyingList.Items.Clear();
        }

public IDDL SelectedItem
        {
            get
            {
                ListItem item = underlyingList.SelectedItem;
                return new DDLStruct(item.Text, item.Value);
            }
        }

public void BindTo<T>(IEnumerable<T> list, string nameStr, string valueStr)
        {
            Clear();
            foreach (var item in list)
            {
                Add<T>(item, nameStr, valueStr);
            }
        }

public string SelectValue
        {
            get
            {
                return underlyingList.SelectedValue;
            }
            set
            {
                underlyingList.SelectedValue=value;
            }
        }
    }
public struct DDLStruct 
    {
        public DDLStruct(string name, string value)
        {
            this.name = name;
            this.value = value;
        }
        private string name;
        private string value;
        public string Name
        {
            get { return name; }
        }

public string Value
        {
            get { return value; }
        }
    }