C# Emit动态代理生成一个实体对象

   /// <summary>
     /// 使用Emit动态代理收集实体信息
     /// </summary>
     /// <typeparam name="T">实体类</typeparam>
     public class TDynamicBuilder<T>
     {
         #region 变量区域
         private static readonly MethodInfo getValueMethod = typeof(IDataRecord).GetMethod("get_Item", new[] { typeof(int) });

         private static readonly MethodInfo isDBNullMethod = typeof(IDataRecord).GetMethod("IsDBNull", new[] { typeof(int) });

         private delegate T Load(IDataRecord dataRecord);

         /// <summary>
         /// 代理
         /// </summary>
         private Load Handler;//最终执行动态方法的一个委托 参数是IDataRecord接口
         #endregion

         #region 构造函数
         private TDynamicBuilder() { }
         #endregion

         #region +Build Emit动态代理生成一个实体对象
         /// <summary>
         /// Emit动态代理生成一个实体对象
         /// </summary>
         /// <param name="dataRecord">DataReader接口实现对象</param>
         /// <returns></returns>
         public T Build(IDataRecord dataRecord)
         {
             return Handler(dataRecord);//执行CreateBuilder里创建的DynamicCreate动态方法的委托
         }
         #endregion

         #region + CreateBuilder 创建一个Builder器
         /// <summary>
         /// 创建一个Builder器
         /// </summary>
         /// <param name="dataRecord">DataReader接口实现对象</param>
         /// <returns></returns>
         public static TDynamicBuilder<T> CreateBuilder(IDataRecord dataRecord)
         {
             var dynamicBuilder = new TDynamicBuilder<T>();
             var type = typeof(T);
             //定义一个名为DynamicCreate的动态方法，返回值typof(T)，参数typeof(IDataRecord)
             var method = new DynamicMethod("DynamicCreate", type, new[] { typeof(IDataRecord) }, typeof(T), true);
             var generator = method.GetILGenerator();//创建一个MSIL生成器，为动态方法生成代码
             var result = generator.DeclareLocal(type);//声明指定类型的局部变量 可以T t;这么理解
             //The next piece of code instantiates the requested type of object and stores it in the local variable. 可以t=new T();这么理解
             generator.Emit(OpCodes.Newobj, type.GetConstructor(Type.EmptyTypes));
             generator.Emit(OpCodes.Stloc, result);
             var propertys = type.GetProperties();
             //数据集合
             for (var i = ; i < dataRecord.FieldCount; i++)
             {
                 //查找属性
                 var propertyInfo = propertys.FirstOrDefault(a => string.Equals(a.Name.ToLower(), dataRecord.GetName(i).ToLower()));
                 if (propertyInfo == null) continue;
                 if (!string.Equals(propertyInfo.Name, dataRecord.GetName(i), StringComparison.CurrentCultureIgnoreCase)) continue;
                 //根据列名取属性  原则上属性和列是一一对应的关系
                 var endIfLabel = generator.DefineLabel();

                 /*The code then loops through the fields in the data reader, finding matching properties on the type passed in.
                  * When a match is found, the code checks to see if the value from the data reader is null.
                  */
                 generator.Emit(OpCodes.Ldarg_0);
                 generator.Emit(OpCodes.Ldc_I4, i);
                 //就知道这里要调用IsDBNull方法 如果IsDBNull==true contine
                 generator.Emit(OpCodes.Callvirt, isDBNullMethod);
                 generator.Emit(OpCodes.Brtrue, endIfLabel);

                 /*If the value in the data reader is not null, the code sets the value on the object.*/
                 generator.Emit(OpCodes.Ldloc, result);
                 generator.Emit(OpCodes.Ldarg_0);
                 generator.Emit(OpCodes.Ldc_I4, i);
                 generator.Emit(OpCodes.Callvirt, getValueMethod); //调用get_Item方法
                 generator.Emit(OpCodes.Unbox_Any, dataRecord.GetFieldType(i));
                 generator.Emit(OpCodes.Callvirt, propertyInfo.GetSetMethod()); //给该属性设置对应值

                 generator.MarkLabel(endIfLabel);

             }

             /*The last part of the code returns the value of the local variable*/
             generator.Emit(OpCodes.Ldloc, result);
             generator.Emit(OpCodes.Ret);//方法结束，返回

             //完成动态方法的创建，并且创建执行该动态方法的委托，赋值到全局变量handler,handler在Build方法里Invoke
             dynamicBuilder.Handler = (Load)method.CreateDelegate(typeof(Load));
             return dynamicBuilder;
         }
         #endregion

     }