Lambda表达式树解析（下)包含自定义的provider和查询

概述

　　前面章节，总结了Lambda树的构建，那么怎么解析Lambda表达式树那？Lambda表达式是一种委托构造而成，如果能够清晰的解析Lambda表达式树，那么就能够理解Lambda表达式要传递的正式意图。解析Lambda表达式树意义很大，比如我们用的EF框架、Rafy框架，里面封装了大量的Lambda查询表达式，通过解析表达式转换成SQL语句，即可以查询数据库，将数据呈现给前台用户；

Lambda表达式树解析

　　下面代码块是标识Express各个节点的信息，ExpressionType.Call标识表达式中含有方法调用，常用到该类型的3个参数为:Method.DeclaringType和Argument和object。ExpressionType.Quote标识表达式中含有一元表达式，常用到的参数是Operand,里面放置了Lamble具体表达式。

public string AnalysisExpression(Expression exp)
        {
            string TextSql = "";
            switch (exp.NodeType)
            {
                case ExpressionType.Call:
                    {
                        MethodCallExpression mce = exp as MethodCallExpression;
                        Console.WriteLine("The Method Is {0}", mce.Method.Name);
                        Console.WriteLine("The Method TypeOf {0}", mce.Method.DeclaringType);
                        if (mce.Method.DeclaringType == typeof(string))
                        {
                            break;
                        }
                        if (mce.Method.DeclaringType != typeof(Queryable)){
                            break;
                        }
                        for (int i = 0; i < mce.Arguments.Count; i++)
                        {

                            TextSql+=AnalysisExpression(mce.Arguments[i]);
                        }
                    }
                    break;
                case ExpressionType.Quote:
                    {
                        UnaryExpression ue = exp as UnaryExpression;
                        TextSql += AnalysisExpression(ue.Operand);
                    }
                    break;
                case ExpressionType.Lambda:
                    {
                        LambdaExpression le = exp as LambdaExpression;
                        AnalysisExpression(le.Body);
                        TextSql+= ExpressTreeAnalysis.GetSqlByExpression(le.Body);
                    }
                    break;
                case ExpressionType.AndAlso:
                case ExpressionType.OrElse:
                case ExpressionType.Equal:
                    {
                        BinaryExpression be = exp as BinaryExpression;
                        Console.WriteLine("The Method Is {0}", exp.NodeType.ToString());
                        TextSql += AnalysisExpression(be.Left);
                        TextSql += AnalysisExpression(be.Right);
                    }
                    break;
                case ExpressionType.Constant:
                    {
                        ConstantExpression ce = exp as ConstantExpression;
                        Console.WriteLine("The Value Type Is {0}", ce.Value.ToString());
                    }
                    break;
                case ExpressionType.Parameter:
                    {
                        ParameterExpression pe = exp as ParameterExpression;
                        Console.WriteLine("The Parameter Is {0}", pe.Name);
                    }
                    break;
                default:
                    {
                        Console.Write("UnKnow");
                    }
                    break;
            }
            return TextSql;
        }

由于自定义集合实现IQueryable和IQueryProvider来自定义查询项

实现IQuable定义的类：

public class BlogQueryable<T> : IQueryable<T>
    {
        BlogQueryProvider provider;
        Expression expression;

        public BlogQueryable(BlogQueryProvider provider)
        {
            if (provider == null)
            {
                throw new ArgumentNullException("provider");
            }
            this.provider = provider;
            this.expression = Expression.Constant(this);
        }

        public BlogQueryable(BlogQueryProvider provider, Expression expression)
        {
            this.provider = provider;
            this.expression = expression;
        }
        public Type ElementType
        {
            get
            {
                return typeof(T);
            }
        }

        public Expression Expression
        {
            get
            {
                return expression;
            }
        }

        public IQueryProvider Provider
        {
            get
            {
                return provider;
            }
        }

        public IEnumerator<T> GetEnumerator()
        {
            return ((IEnumerable<T>)this.provider.Execute(this.expression)).GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return ((IEnumerable)this.provider.Execute(this.expression)).GetEnumerator();
        }

        public void Where2(Expression<Func<T, bool>> func)
        {
            string TextSql = ExpressTreeAnalysis.GetSqlByExpression(func.Body);
        }
    }

实现IQueryProvider接口的类，CreateQuery解析表达式树而Execute则是执行解析后的SQL语句，查询数据，填充list集合：

public class BlogQueryProvider : IQueryProvider
{        public IQueryable CreateQuery(Expression expression)
        {
            Type elementType = expression.Type;
            try
            {
                return (IQueryable)Activator.CreateInstance(typeof(BlogQueryable<>).MakeGenericType(elementType), new object[] { this, expression });
            }
            catch(TargetInvocationException tie)
            {
                throw tie.InnerException;
            }
        }

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

            //string TextSql=  ExpressTreeAnalysis.GetSqlByExpression(((Expression)(expression as MethodCallExpression).Arguments[1]).bo);
            //Console.WriteLine(TextSql);
            AnalysisExpression(expression);
            return new BlogQueryable<TElement>(this, expression);
        }

        public object Execute(Expression expression)
        {
            return this.Execute2<object>(expression);
        }

        public TResult Execute<TResult>(Expression expression)
        {
            return (TResult)this.Execute2<TResult>(expression);
        }

        public TResult Execute2<TResult>(Expression expression)
        {
            return (TResult)Activator.CreateInstance(typeof(TResult));
        }
    }

具体下面即可使用查询

        public void MyTestMethod()
        {
            Catagory cag = new Catagory("四大名著点评");

            IList<Blog> Blogs = new List<Blog>();
            Blog bok = new Blog() { Title = "红楼梦", Conent = "红楼梦书籍不错",
　　　　　　　　 User = new Author() { UserName = "曹雪芹", BirefInfor = "中国清代著名作家" } };
            bok.Catagories.Add(cag);
            bok.Catagories.Add(new Catagory("红楼梦专栏"));
            Blogs.Add(bok);

            Blog bok2 = new Blog() { Title = "三国演义", Conent = "三国鼎立时期的描述",
　　　　　　　 User = new Author() { UserName = "罗贯中", BirefInfor = "中国元末明初著名作家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("三国演义专栏"));
            Blogs.Add(bok2);

            Blog bok3 = new Blog() { Title = "西游记", Conent = "西游记讲述四个徒弟和一个师傅取经事情", 
              User = new Author() { UserName = "吴承恩", BirefInfor = "中国明代杰出的小说家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("西游记专栏"));
            Blogs.Add(bok3);

            Blog bok4 = new Blog() { Title = "水浒传", Conent = "描述梁山好汉劫富济贫的故事", 
　　　　　　　　　　User = new Author() { UserName = "施耐庵", BirefInfor = "中国明代杰出的小说家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("水浒传专栏"));
            Blogs.Add(bok4);

            BlogQueryProvider Provider = new BlogQueryProvider();
            BlogQueryable<Blog> Quble = new BlogQueryable<Blog>(Provider);

            var t2 = Quble.Where(p => p.Title == "123" &&  p.Conent.Contains("水浒"));
        }

参考代码下载

后续：针对解析Lambda表达式树成SQL，本人也是刚刚入门，至于完全要解析成SQL，还需要研究......

https://www.cnblogs.com/xibei666/p/6858299.html