抛弃EF，20分构建一个属于自己的ORM框架_.NET_编程开发_程序员俱乐部

相信EF大家都不陌生了，因为数据库表跟程序实体是一一对应的原因，我们能够通过lambda这种函数式的编程方式进行操作数据库，感觉非常清晰明了。与我们直接写SQL相比，lambda是强类型，拥有更好的扩展性，伸缩性，而且编程更加的方便，快捷。。下面我们就基于Expression和lambda来与大家构建一个属于自己的ORM框架。

思路的话很简单，就是将lambda转换成我们对应的数据库所需的查询条件，然后执行查询，再将结果以反射的方式封装成List<T>返回出去。

Expression

大家使用EF的时候多多少少会留意到有Expression这个东西。特别是查询时会看到要你传入Expression<Func<T,bool>>这样类型的参数，它又和Func<T,bool>有什么比同呢？

Expression<Func<T,bool>>是表达式树，我们可以通过它来分析我们的委托中的函数。当调用Compile方法后就会变成委托，才能执行。

Func<T,bool>只是一个普通的委托。

例如我们现在有个实体类Staff

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

我们还有一个这样的方法

  class Program
    {
        static void Main(string[] args)
        {
            FindAs<Staff>(x => x.Code == "张三" && x.Name.Contains("张"));
        }

        public static List<T> FindAs<T>(Expression<Func<T, bool>> func)
        {
            //将func转换成对应数据库的查询条件，然后执行查询
            return null;//将结果返回
        }
    }

我们希望通过 FindAs<Staff>(x => x.Age <50 && x.Name.Contains("张")); 就能查询出Staff表中Age<50并且Name包含有“张”字的人的信息。而生成的sql语句应该是select * from staff where Age<50 and Name like '%张%'。现在我们就来分析下这个func

从上面的图我们可以看到当前的Expression是一个lambda表达式，我们点开它的body看看。

我们可以看到body里分为左边和右边，还有NodeType。和我们的lambda对比下看看'x => x.Code =="张三" && x.Name.Contains("张")'是不是找到点灵感了？我们再继续把左边和右边拆开看看。

可以看到我们需要的信息都有了，看来转换成SQL已经不是什么难事了，动手开搞了。

 class Program
    {
        static void Main(string[] args)
        {
            FindAs<Staff>(x => x.Code == "张三" && x.Name.Contains("张"));
            FindAs<Staff>(x => x.Age <= 12 && x.Name.Contains("张"));
            Console.ReadKey();
        }

        public static List<T> FindAs<T>(Expression<Func<T, bool>> func)
        {
            BinaryExpression Binary = func.Body as BinaryExpression;
            string left = ResovleFunc(Binary.Left);
            string right = ResovleLinqToObject(Binary.Right);
            string oper = GetOperator(Binary.NodeType);
            string sql = string.Format("select * from {0} where {1}", typeof(T).Name, left + oper + right);
            Console.WriteLine(sql);
            return null;//将结果返回
        }

        //解析一般的条件，例如x=>x.name==xxxx   x.age==xxx
        public static string ResovleFunc(Expression express)
        {
            var inner = express as BinaryExpression;
            string Name = (inner.Left as MemberExpression).Member.Name;
            object Value = (inner.Right as ConstantExpression).Value;
            var Operator = GetOperator(inner.NodeType);
            string Result = string.Format("({0} {1} '{2}')", Name, Operator, Value);
            return Result;
        }

        //解析linq to object这类扩展方法
        public static string ResovleLinqToObject(Expression expression)
        {
            var MethodCall = expression as MethodCallExpression;
            var MethodName = MethodCall.Method.Name;
            if (MethodName == "Contains")
            {
                object Temp_Vale = (MethodCall.Arguments[0] as ConstantExpression).Value;
                string Value = string.Format("%{0}%", Temp_Vale);
                string Name = (MethodCall.Object as MemberExpression).Member.Name;
                string Result = string.Format("{0} like '{1}'", Name, Value);
                return Result;
            }
            return null;
        }

        public static 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));
            }
        }

    }

已经初步的达到了我们的目的了，但是我们的查询条件不可能固定是2个，有可能是N个，这时左边和右边又要继续再分下去，直到无法再分(想到递归了吧？)。而且我们还需要将查询条件参数化。而且我们的条件删除时也会用到。所以我们应该把它独立出来。传入一个lambda，生成sql where部分的语句，生成sqlparameter[]。这才是关键。。于是我们来构建一个解析Expresstion的类。。下面我就直接给出我自己写的实现代码了。。

 public class ResolveExpress
    {
        public Dictionary<string, object> Argument;
        public string SqlWhere;
        public SqlParameter[] Paras;

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

        private string Resolve(Expression expression)
        {
            if (expression is LambdaExpression)
            {
                LambdaExpression lambda = expression as LambdaExpression;
                expression = lambda.Body;
                return Resolve(expression);
            }
            if (expression is BinaryExpression)
            {
                BinaryExpression binary = expression as BinaryExpression;
                if (binary.Left is MemberExpression && binary.Right is ConstantExpression)//解析x=>x.Name=="123" x.Age==123这类
                    return ResolveFunc(binary.Left, binary.Right, binary.NodeType);
                if (binary.Left is MethodCallExpression && binary.Right is ConstantExpression)//解析x=>x.Name.Contains("xxx")==false这类的
                {
                    object value = (binary.Right as ConstantExpression).Value;
                    return ResolveLinqToObject(binary.Left, value, binary.NodeType);
                }
                if (binary.Left is MemberExpression && binary.Right is MemberExpression)//解析x=>x.Date==DateTime.Now这种
                {
                    LambdaExpression lambda = Expression.Lambda(binary.Right);
                    Delegate fn = lambda.Compile();
                    ConstantExpression value = Expression.Constant(fn.DynamicInvoke(null), binary.Right.Type);
                    return ResolveFunc(binary.Left, value, binary.NodeType);
                }
            }
            if (expression is UnaryExpression)
            {
                UnaryExpression unary = expression as UnaryExpression;
                if (unary.Operand is MethodCallExpression)//解析!x=>x.Name.Contains("xxx")或!array.Contains(x.Name)这类
                    return ResolveLinqToObject(unary.Operand, false);
                if (unary.Operand is MemberExpression && unary.NodeType == ExpressionType.Not)//解析x=>!x.isDeletion这样的 
                {
                    ConstantExpression constant = Expression.Constant(false);
                    return ResolveFunc(unary.Operand, constant, ExpressionType.Equal);
                }
            }
            if (expression is MemberExpression && expression.NodeType == ExpressionType.MemberAccess)//解析x=>x.isDeletion这样的 
            {
                MemberExpression member = expression as MemberExpression;
                ConstantExpression constant = Expression.Constant(true);
                return ResolveFunc(member, constant, ExpressionType.Equal);
            }
            if (expression is MethodCallExpression)//x=>x.Name.Contains("xxx")或array.Contains(x.Name)这类
            {
                MethodCallExpression methodcall = expression as MethodCallExpression;
                return ResolveLinqToObject(methodcall, true);
            }
            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caozuofu.html" target="_blank">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, Expression right, ExpressionType expressiontype)
        {
            var Name = (left as MemberExpression).Member.Name;
            var Value = (right as ConstantExpression).Value;
            var Operator = GetOperator(expressiontype);
            string CompName = SetArgument(Name, Value.ToString());
            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))
            {
                int code = Guid.NewGuid().GetHashCode();
                if (code < 0)
                    code *= -1;
                temp = name + code;
            }
            Argument[temp] = value;
            return temp;
        }

        private string In(MethodCallExpression expression, object isTrue)
        {
            var Argument1 = (expression.Arguments[0] as MemberExpression).Expression as ConstantExpression;
            var Argument2 = expression.Arguments[1] as MemberExpression;
            var Field_Array = Argument1.Value.GetType().GetFields().First();
            object[] Array = Field_Array.GetValue(Argument1.Value) 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)
        {
            object Temp_Vale = (expression.Arguments[0] as ConstantExpression).Value;
            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;
        }

    }

 static void Main(string[] args)
        {
            string[] Names = { "Andy", "Amy", "Mike" };
            Expression<Func<Staff, bool>> func = x => (!Names.Contains(x.Name) && (x.Name == "A" || x.Name.Count() > 5));
            ResolveExpress resolve = new ResolveExpress();
            resolve.ResolveExpression(func);
            Console.WriteLine(resolve.SqlWhere);
            foreach (var item in resolve.Paras)
            {
                Console.WriteLine(item.ParameterName + ":" + item.Value);
            }
            Console.ReadKey();
        }

结果：

这里有几个重要的东西要给大家讲下

string[] Names={"Andy","Amy","Mike"};

1.）x => Names.Contains(x.Name);

2.）x => Names.Contains(x.Name)==false;

3.）x => !Names.Contains(x.Name);

这3种在Expression中的表现都不一样

1的话会看成是一个静态方法（MethodCallExpression）

2的话会看成是一个2元运算（BinaryExpression）

3的话会看成是一个1元运算（UnaryExpression）

所以我们都要支持，处理都有所不同。

还有

x=>x.Birthday<DateTime.Now;

string name="123";

x=>x.Name==name;

和

x=>x.Name=="123"

的处理也不一样。大家可以在例子中细细的看看。

这样的构造使得我们切换数据库变得非常简单。因为我们程序中的查询都是基于lambda。换了数据库只要添加一个对应的lamdba转数据库查询条件的实现就可以了。写得够多了。至于数据层怎么封装，到了这一步它已经变得没什么难度了。希望大家能从文章中有所启发和帮助

补充点东西

IEnumerable和IQueryable有什么比同？

为什么EF查询后返回的是IQueryable<T>而不是IEnumerable<T>。我们对着IQueryable<T>F12去看看。

原来里头有个Expression，为什么需要它呢？我大胆推测下，EF是不是也是通过解析Expression从而转换成SQL的呢？