znyc.cloudcar.admin.api/Znyc.Cloudcar.Admin.Commons/Helpers/EntityMapper.cs

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Data;
using System.Data.Common;
using System.Reflection;
using System.Reflection.Emit;

namespace Znyc.Cloudcar.Admin.Commons.Helpers
{
    /// <summary>
    ///     实体类映射，源自SqlSugar(http://www.codeisbug.com/Doc/3/1113)
    /// </summary>
    public static class EntityMapper
    {
        //用于构造Emit的DataRow中获取字段的方法信息
        private static readonly MethodInfo getValueMethod =
            typeof(DataRow).GetMethod("get_Item", new[] { typeof(int) });

        //用于构造Emit的DataRow中判断是否为空行的方法信息
        private static readonly MethodInfo isDBNullMethod =
            typeof(DataRow).GetMethod("IsNull", new[] { typeof(int) });

        private static readonly ConcurrentDictionary<Type, Delegate> CachedMappers =
            new();

        public static IEnumerable<T> ToList<T>(this DbDataReader dr)
        {
            // If a mapping function from dr -> T does not exist, create and cache one
            if (!CachedMappers.ContainsKey(typeof(T)))
            {
                // Our method will take in a single parameter, a DbDataReader
                Type[] methodArgs = { typeof(DbDataReader) };

                // The MapDR method will map a DbDataReader row to an instance of type T
                DynamicMethod dm = new DynamicMethod("MapDR", typeof(T), methodArgs,
                    Assembly.GetExecutingAssembly().GetType().Module);
                ILGenerator il = dm.GetILGenerator();

                // We'll have a single local variable, the instance of T we're mapping
                il.DeclareLocal(typeof(T));

                // Create a new instance of T and save it as variable 0
                il.Emit(OpCodes.Newobj, typeof(T).GetConstructor(Type.EmptyTypes));
                il.Emit(OpCodes.Stloc_0);

                foreach (PropertyInfo pi in typeof(T).GetProperties())
                {
                    // Load the T instance, SqlDataReader parameter and the field name onto the stack
                    il.Emit(OpCodes.Ldloc_0);
                    il.Emit(OpCodes.Ldarg_0);
                    il.Emit(OpCodes.Ldstr, pi.Name);

                    // Push the column value onto the stack
                    il.Emit(OpCodes.Callvirt, typeof(DbDataReader).GetMethod("get_Item", new[] { typeof(string) }));

                    // Depending on the type of the property, convert the datareader column value to the type
                    switch (pi.PropertyType.Name)
                    {
                        case "Int16":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToInt16", new[] { typeof(object) }));
                            break;

                        case "Int32":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToInt32", new[] { typeof(object) }));
                            break;

                        case "Int64":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToInt64", new[] { typeof(object) }));
                            break;

                        case "Boolean":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToBoolean", new[] { typeof(object) }));
                            break;

                        case "String":
                            il.Emit(OpCodes.Callvirt, typeof(string).GetMethod("ToString", new Type[] { }));
                            break;

                        case "DateTime":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToDateTime", new[] { typeof(object) }));
                            break;

                        case "Decimal":
                            il.Emit(OpCodes.Call, typeof(Convert).GetMethod("ToDecimal", new[] { typeof(object) }));
                            break;

                        default:
                            // Don't set the field value as it's an unsupported type
                            continue;
                    }

                    // Set the T instances property value
                    il.Emit(OpCodes.Callvirt, typeof(T).GetMethod("set_" + pi.Name, new[] { pi.PropertyType }));
                }

                // Load the T instance onto the stack
                il.Emit(OpCodes.Ldloc_0);

                // Return
                il.Emit(OpCodes.Ret);

                // Cache the method so we won't have to create it again for the type T
                CachedMappers.TryAdd(typeof(T), dm.CreateDelegate(typeof(MapEntity<T>)));
            }

            // Get a delegate reference to the dynamic method
            MapEntity<T> invokeMapEntity = (MapEntity<T>)CachedMappers[typeof(T)];

            // For each row, map the row to an instance of T and yield return it
            while (dr.Read())
            {
                yield return invokeMapEntity(dr);
            }
        }

        /// <summary>
        ///     将DataTable转换成泛型对象列表
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="dt"></param>
        /// <returns></returns>
        public static List<T> ToList<T>(this DataTable dt)
        {
            List<T> list = new List<T>();
            if (dt == null)
            {
                return list;
            }

            //声明 委托Load<T>的一个实例rowMap
            Load<T> rowMap = null;

            //从rowMapMethods查找当前T类对应的转换方法，没有则使用Emit构造一个。
            if (!CachedMappers.ContainsKey(typeof(T)))
            {
                DynamicMethod method = new DynamicMethod("DynamicCreateEntity_" + typeof(T).Name, typeof(T),
                    new[] { typeof(DataRow) }, typeof(T), true);
                ILGenerator generator = method.GetILGenerator();
                LocalBuilder result = generator.DeclareLocal(typeof(T));
                generator.Emit(OpCodes.Newobj, typeof(T).GetConstructor(Type.EmptyTypes));
                generator.Emit(OpCodes.Stloc, result);

                for (int index = 0; index < dt.Columns.Count; index++)
                {
                    PropertyInfo propertyInfo = typeof(T).GetProperty(dt.Columns[index].ColumnName);
                    Label endIfLabel = generator.DefineLabel();
                    if (propertyInfo != null && propertyInfo.GetSetMethod() != null)
                    {
                        generator.Emit(OpCodes.Ldarg_0);
                        generator.Emit(OpCodes.Ldc_I4, index);
                        generator.Emit(OpCodes.Callvirt, isDBNullMethod);
                        generator.Emit(OpCodes.Brtrue, endIfLabel);
                        generator.Emit(OpCodes.Ldloc, result);
                        generator.Emit(OpCodes.Ldarg_0);
                        generator.Emit(OpCodes.Ldc_I4, index);
                        generator.Emit(OpCodes.Callvirt, getValueMethod);
                        generator.Emit(OpCodes.Unbox_Any, propertyInfo.PropertyType);
                        generator.Emit(OpCodes.Callvirt, propertyInfo.GetSetMethod());
                        generator.MarkLabel(endIfLabel);
                    }
                }

                generator.Emit(OpCodes.Ldloc, result);
                generator.Emit(OpCodes.Ret);

                //构造完成以后传给rowMap
                rowMap = (Load<T>)method.CreateDelegate(typeof(Load<T>));
            }
            else
            {
                rowMap = (Load<T>)CachedMappers[typeof(T)];
            }

            //遍历Datatable的rows集合，调用rowMap把DataRow转换为对象（T）
            foreach (DataRow info in dt.Rows)
            {
                list.Add(rowMap(info));
            }

            return list;
        }

        public static void ClearCachedMapperMethods()
        {
            CachedMappers.Clear();
        }

        private delegate T MapEntity<T>(DbDataReader dr);

        //把DataRow转换为对象的委托声明
        private delegate T Load<T>(DataRow dataRecord);
    }
}