构建属于自己的ORM框架之二--IQueryable的奥秘

上篇文章标题乱起，被吐槽了，这次学乖了。

上篇文章中介绍了如何解析Expression生成对应的SQL语句，以及IQueryable的一些概念，以及我们所搭建的框架的思想等。但还没把它们结合并应用起来。这一篇文章将更黄更暴力，揭露IQueryable在实际使用中延迟加载的实现原理，结合上篇对Expression的解析，我们来实现一个自己的“延迟加载”

如果还不太了解如何解析Expression和IQueryable的一些基本概念，可以先看看我的上篇文章

我们先来做些基本工作，定义一个IDataBase接口，里面可以定义些查询，删除，修改，新增等方法，为了节约时间，我们就定义一个查询和删除的方法，再定义一个获取IQueryable<T>实例的方法

 　　public interface IDataBase
    {
        List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere);
        int Remove<T>(Expression<Func<T, bool>> lambdawhere);
        IQueryable<T> Source<T>();
    }

再添加一个类DBSql，实现我们上面的IDataBase接口，这个类是负责提供对sql数据库的操作

 public class DBSql : IDataBase
    {
        public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
        {
            throw new NotImplementedException();
        }

        public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
        {
            throw new NotImplementedException();
        }

        public IQueryable<T> Source<T>()
        {
            throw new NotImplementedException();
        }
    }

IQueryable<T>

上篇文章中有个朋友的回复对IQueryable的解释十分到位，“IQueryable只存贮条件，不立即运行，从而可以实现延迟加载。”那它是如何存贮条件，如何延迟加载的？

这时我们为了提供 public IQueryable<T> Source<T>() 所需的对象。我们再来建一个SqlQuery类，实现IQueryable<T>。

 　　public class SqlQuery<T> : IQueryable<T>
    {
        public IEnumerator<T> GetEnumerator()
        {
            throw new NotImplementedException();
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }

        public Type ElementType
        {
            get { throw new NotImplementedException(); }
        }

        public Expression Expression
        {
            get { throw new NotImplementedException(); }
        }

        public IQueryProvider Provider
        {
            get { throw new NotImplementedException(); }
        }
    }

看到这里大家都不陌生吧？

GetEnumerator()是IEnumerable<T>里的。有了它我们就能foreach了。有泛型和非泛型版本，所以有2个

Type提供访问当前对象的类型（反正由你定义。。。）

Expression是贮存查询条件的

IQueryProvider简单的翻译过来就是查询提供者，它是负责创建查询条件和执行查询的。我们写一个SqlProvider类来实现它

  public class SqlProvider<T> : IQueryProvider
    {

        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            throw new NotImplementedException();
        }

        public IQueryable CreateQuery(Expression expression)
        {
            throw new NotImplementedException();
        }

        public TResult Execute<TResult>(Expression expression)
        {
            throw new NotImplementedException();
        }

        public object Execute(Expression expression)
        {
            throw new NotImplementedException();
        }
    }

CreateQuery是创建查询条件。。

平时我们

IQueryable query=xxx源;

query=query.Where(x=>x.Name=="123");

这时Where方法里做的其实就是将前面query的Expression属性和Where里的(x=>x.Name=="123")相并，并且调用Provider属性里的CreateQuery方法。我们可以把我们的代码改成这样，来看看到底是不是这么回事。

   public class DBSql : IDataBase
    {
        public IQueryable<T> Source<T>()
        {
            return new SqlQuery<T>();
        }

        public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
        {
            throw new NotImplementedException();
        }

        public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
        {
            throw new NotImplementedException();
        }
    }

    public class SqlQuery<T> : IQueryable<T>
    {

        private Expression _expression;
        private IQueryProvider _provider;

        public SqlQuery()
        {
            _provider = new SqlProvider<T>();
            _expression = Expression.Constant(this);
        }

        public SqlQuery(Expression expression, IQueryProvider provider)
        {
            _expression = expression;
            _provider = provider;
        }

        public IEnumerator<T> GetEnumerator()
        {
            throw new NotImplementedException();
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }

        public Type ElementType
        {
            get { return typeof(SqlQuery<T>); }
        }

        public Expression Expression
        {
            get { return _expression; }
        }

        public IQueryProvider Provider
        {
            get { return _provider; }
        }
    }

    public class SqlProvider<T> : IQueryProvider
    {
        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            IQueryable<TElement> query = new SqlQuery<TElement>(expression, this);
            return query;
        }

        public IQueryable CreateQuery(Expression expression)
        {
            throw new NotImplementedException();
        }

        public TResult Execute<TResult>(Expression expression)
        {
            throw new NotImplementedException();
        }

        public object Execute(Expression expression)
        {
            throw new NotImplementedException();
        }
    }

　　　　 public class Staff
        {
            public int ID { get; set; }
            public string Code { get; set; }
            public string Name { get; set; }
            public DateTime? Birthday { get; set; }
            public bool Deletion { get; set; }
        }

        static void Main(string[] args)
        {
            IDataBase db = new DBSql();
            IQueryable<Staff> query = db.Source<Staff>();
            string name = "张三";
            Expression express = null;
            query = query.Where(x => x.Name == "赵建华");
            express = query.Expression;
            query = query.Where(x => x.Name == name);
            express = query.Expression;
        }

段点打在 

public IQueryable<TElement> CreateQuery<TElement>(Expression expression)

每次query.Where都会跑这里来。并且Expression都是前后相并的结果。

到了这一步，相信大家都明白了IQueryable只存贮条件这个概念了吧。

那延迟加载呢？什么时候加载啊！当我们foreach或者ToList/ToArray时啊。这时你想到了什么？GetEnumerator()。在调用GetEnumerator()时。我们再调用Provider里的Execute(Expression)。里面解析Expression，生成SQL语句，通过反射的方式生成实例，再一个个返回回去。完成！下面我直接给代码了。解析Expression的类我也改了，这个更黄更暴力。

  public class ResolveExpression
    {
        public Dictionary<string, object> Argument;
        public string SqlWhere;
        public SqlParameter[] Paras;
        private int index = 0;
        /// <summary>
        /// 解析lamdba，生成Sql查询条件
        /// </summary>
        /// <param name="expression"></param>
        /// <returns></returns>
        public void ResolveToSql(Expression expression)
        {
            this.index = 0;
            this.Argument = new Dictionary<string, object>();
            this.SqlWhere = Resolve(expression);
            this.Paras = Argument.Select(x => new SqlParameter(x.Key, x.Value)).ToArray();
        }

        private object GetValue(Expression expression)
        {
            if (expression is ConstantExpression)
                return (expression as ConstantExpression).Value;
            if (expression is UnaryExpression)
            {
                UnaryExpression unary = expression as UnaryExpression;
                LambdaExpression lambda = Expression.Lambda(unary.Operand);
                Delegate fn = lambda.Compile();
                return fn.DynamicInvoke(null);
            }
            if (expression is MemberExpression)
            {
                MemberExpression member = expression as MemberExpression;
                string name = member.Member.Name;
                var constant = member.Expression as ConstantExpression;
                if (constant == null)
                    throw new Exception("取值时发生异常" + member);
                return constant.Value.GetType().GetFields().First(x => x.Name == name).GetValue(constant.Value);
            }
            throw new Exception("无法获取值" + expression);
        }

        private string Resolve(Expression expression)
        {
            if (expression is LambdaExpression)
            {
                LambdaExpression lambda = expression as LambdaExpression;
                expression = lambda.Body;
                return Resolve(expression);
            }
            if (expression is BinaryExpression)//解析二元运算符
            {
                BinaryExpression binary = expression as BinaryExpression;
                if (binary.Left is MemberExpression)
                {
                    object value = GetValue(binary.Right);
                    return ResolveFunc(binary.Left, value, binary.NodeType);
                }
                if (binary.Left is MethodCallExpression && (binary.Right is UnaryExpression || binary.Right is MemberExpression))
                {
                    object value = GetValue(binary.Right);
                    return ResolveLinqToObject(binary.Left, value, binary.NodeType);
                }
            }
            if (expression is UnaryExpression)//解析一元运算符
            {
                UnaryExpression unary = expression as UnaryExpression;
                if (unary.Operand is MethodCallExpression)
                {
                    return ResolveLinqToObject(unary.Operand, false);
                }
                if (unary.Operand is MemberExpression)
                {
                    return ResolveFunc(unary.Operand, false, ExpressionType.Equal);
                }
            }
            if (expression is MethodCallExpression)//解析扩展方法
            {
                return ResolveLinqToObject(expression, true);
            }
            if (expression is MemberExpression)//解析属性。。如x.Deletion
            {
                return ResolveFunc(expression, true, ExpressionType.Equal);
            }
            var body = expression as BinaryExpression;
            if (body == null)
                throw new Exception("无法解析" + expression);
            var Operator = GetOperator(body.NodeType);
            var Left = Resolve(body.Left);
            var Right = Resolve(body.Right);
            string Result = string.Format("({0} {1} {2})", Left, Operator, Right);
            return Result;
        }

        /// <summary>
        /// 根据条件生成对应的sql查询操作符
        /// </summary>
        /// <param name="expressiontype"></param>
        /// <returns></returns>
        private string GetOperator(ExpressionType expressiontype)
        {
            switch (expressiontype)
            {
                case ExpressionType.And:
                    return "and";
                case ExpressionType.AndAlso:
                    return "and";
                case ExpressionType.Or:
                    return "or";
                case ExpressionType.OrElse:
                    return "or";
                case ExpressionType.Equal:
                    return "=";
                case ExpressionType.NotEqual:
                    return "<>";
                case ExpressionType.LessThan:
                    return "<";
                case ExpressionType.LessThanOrEqual:
                    return "<=";
                case ExpressionType.GreaterThan:
                    return ">";
                case ExpressionType.GreaterThanOrEqual:
                    return ">=";
                default:
                    throw new Exception(string.Format("不支持{0}此种运算符查找！" + expressiontype));
            }
        }


        private string ResolveFunc(Expression left, object value, ExpressionType expressiontype)
        {
            string Name = (left as MemberExpression).Member.Name;
            string Operator = GetOperator(expressiontype);
            string Value = value.ToString();
            string CompName = SetArgument(Name, Value);
            string Result = string.Format("({0} {1} {2})", Name, Operator, CompName);
            return Result;
        }

        private string ResolveLinqToObject(Expression expression, object value, ExpressionType? expressiontype = null)
        {
            var MethodCall = expression as MethodCallExpression;
            var MethodName = MethodCall.Method.Name;
            switch (MethodName)//这里其实还可以改成反射调用，不用写switch
            {
                case "Contains":
                    if (MethodCall.Object != null)
                        return Like(MethodCall);
                    return In(MethodCall, value);
                case "Count":
                    return Len(MethodCall, value, expressiontype.Value);
                case "LongCount":
                    return Len(MethodCall, value, expressiontype.Value);
                default:
                    throw new Exception(string.Format("不支持{0}方法的查找！", MethodName));
            }
        }

        private string SetArgument(string name, string value)
        {
            name = "@" + name;
            string temp = name;
            while (Argument.ContainsKey(temp))
            {
                temp = name + index;
                index = index + 1;
            }
            Argument[temp] = value;
            return temp;
        }

        private string In(MethodCallExpression expression, object isTrue)
        {
            var Argument1 = expression.Arguments[0];
            var Argument2 = expression.Arguments[1] as MemberExpression;
            var fieldValue = GetValue(Argument1);
            object[] array = fieldValue as object[];
            List<string> SetInPara = new List<string>();
            for (int i = 0; i < array.Length; i++)
            {
                string Name_para = "InParameter" + i;
                string Value = array[i].ToString();
                string Key = SetArgument(Name_para, Value);
                SetInPara.Add(Key);
            }
            string Name = Argument2.Member.Name;
            string Operator = Convert.ToBoolean(isTrue) ? "in" : " not in";
            string CompName = string.Join(",", SetInPara);
            string Result = string.Format("{0} {1} ({2})", Name, Operator, CompName);
            return Result;
        }

        private string Like(MethodCallExpression expression)
        {
            Expression argument = expression.Arguments[0];
            object Temp_Vale = GetValue(argument);
            string Value = string.Format("%{0}%", Temp_Vale);
            string Name = (expression.Object as MemberExpression).Member.Name;
            string CompName = SetArgument(Name, Value);
            string Result = string.Format("{0} like {1}", Name, CompName);
            return Result;
        }

        private string Len(MethodCallExpression expression, object value, ExpressionType expressiontype)
        {
            object Name = (expression.Arguments[0] as MemberExpression).Member.Name;
            string Operator = GetOperator(expressiontype);
            string CompName = SetArgument(Name.ToString(), value.ToString());
            string Result = string.Format("len({0}){1}{2}", Name, Operator, CompName);
            return Result;
        }

    }

　　public interface IDataBase
    {
        List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere);
        int Remove<T>(Expression<Func<T, bool>> lambdawhere);
        IQueryable<T> Source<T>();
    }

namespace Data.DataBase
{
    public class DBSql : IDataBase
    {
        private readonly static string ConnectionString = @"Data Source=.;Initial Catalog=btmmcms-Standard;Persist Security Info=True;User ID=sa;Password=sa;";

        public IQueryable<T> Source<T>()
        {
            return new SqlQuery<T>();
        }

        public List<T> FindAs<T>(Expression<Func<T, bool>> lambdawhere)
        {
            using (SqlConnection Conn = new SqlConnection(ConnectionString))
            {
                using (SqlCommand Command = new SqlCommand())
                {
                    try
                    {
                        Command.Connection = Conn;
                        Conn.Open();
                        string sql = string.Format("select * from {0}", typeof(T).Name);
                        if (lambdawhere != null)
                        {
                            ResolveExpression resolve = new ResolveExpression();
                            resolve.ResolveToSql(lambdawhere);
                            sql = string.Format("{0} where {1}", sql, resolve.SqlWhere);
                            Command.Parameters.AddRange(resolve.Paras);
                        }
                        //为了测试，就在这里打印出sql语句了
                        Console.WriteLine(sql);
                        Command.CommandText = sql;
                        SqlDataReader dataReader = Command.ExecuteReader();
                        List<T> ListEntity = new List<T>();
                        while (dataReader.Read())
                        {
                            var constructor = typeof(T).GetConstructor(new Type[] { });
                            T Entity = (T)constructor.Invoke(null);
                            foreach (var item in Entity.GetType().GetProperties())
                            {
                                var value = dataReader[item.Name];
                                if (value == null)
                                    continue;
                                if (value is DBNull)
                                    value = null;
                                item.SetValue(Entity, value, null);
                            }
                            ListEntity.Add(Entity);
                        }
                        if (ListEntity.Count == 0)
                            return null;
                        return ListEntity;
                    }
                    catch (Exception ex)
                    {
                        throw ex;
                    }
                    finally
                    {
                        Conn.Close();
                    }
                }
            }
        }

        public int Remove<T>(Expression<Func<T, bool>> lambdawhere)
        {
            throw new NotImplementedException();
        }
    }

    public class SqlQuery<T> : IQueryable<T>
    {

        private Expression _expression;
        private IQueryProvider _provider;

        public SqlQuery()
        {
            _provider = new SqlProvider<T>();
            _expression = Expression.Constant(this);
        }

        public SqlQuery(Expression expression, IQueryProvider provider)
        {
            _expression = expression;
            _provider = provider;
        }

        public IEnumerator<T> GetEnumerator()
        {
            var result = _provider.Execute<List<T>>(_expression);
            if (result == null)
                yield break;
            foreach (var item in result)
            {
                yield return item;
            }
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            throw new NotImplementedException();
        }

        public Type ElementType
        {
            get { return typeof(SqlQuery<T>); }
        }

        public Expression Expression
        {
            get { return _expression; }
        }

        public IQueryProvider Provider
        {
            get { return _provider; }
        }
    }

    public class SqlProvider<T> : IQueryProvider
    {

        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            IQueryable<TElement> query = new SqlQuery<TElement>(expression, this);
            return query;
        }

        public IQueryable CreateQuery(Expression expression)
        {
            throw new NotImplementedException();
        }

        public TResult Execute<TResult>(Expression expression)
        {
            MethodCallExpression methodCall = expression as MethodCallExpression;
            Expression<Func<T, bool>> result = null;
            while (methodCall != null)
            {
                Expression method = methodCall.Arguments[0];
                Expression lambda = methodCall.Arguments[1];
                LambdaExpression right = (lambda as UnaryExpression).Operand as LambdaExpression;
                if (result == null)
                {
                    result = Expression.Lambda<Func<T, bool>>(right.Body, right.Parameters);
                }
                else
                {
                    Expression left = (result as LambdaExpression).Body;
                    Expression temp = Expression.And(right.Body, left);
                    result = Expression.Lambda<Func<T, bool>>(temp, result.Parameters);
                }
                methodCall = method as MethodCallExpression;
            }
            var source = new DBSql().FindAs<T>(result);
            dynamic _temp = source;
            TResult t = (TResult)_temp;
            return t;
        }

        public object Execute(Expression expression)
        {
            throw new NotImplementedException();
        }
    }
}

搞定，这时可以改下数据库连接，连到自己的数据库，然后像下面这样，添加一个实体类（要与数据库表对应），就可以使用了

   class Program
    {
        public class Staff
        {
            public int ID { get; set; }
            public string Code { get; set; }
            public string Name { get; set; }
            public DateTime? Birthday { get; set; }
            public bool Deletion { get; set; }
        }

        static void Main(string[] args)
        {
            IDataBase db = new DBSql();
            IQueryable<Staff> query = db.Source<Staff>();
            query = query.Where(x => x.Name == "张三");
            foreach (var item in query)
            {

            }
        }
    }

是不是很简单？

虽然信息量有点大，但慢慢理清并消化，我相信会对你又很大帮助！