Lambda表達式樹解析（下）

概述

　　前面章節，總結了Lambda樹的構建，那么怎么解析Lambda表達式樹那？Lambda表達式是一種委托構造而成，如果能夠清晰的解析Lambda表達式樹，那么就能夠理解Lambda表達式要傳遞的正式意圖。解析Lambda表達式樹意義很大，比如我們用的EF框架、Rafy框架，里面封裝了大量的Lambda查詢表達式，通過解析表達式轉換成SQL語句，即可以查詢數據庫，將數據呈現給前台用戶；

Lambda表達式樹解析

　　下面代碼塊是標識Express各個節點的信息，ExpressionType.Call標識表達式中含有方法調用，常用到該類型的3個參數為:Method.DeclaringType和Argument和object。ExpressionType.Quote標識表達式中含有一元表達式，常用到的參數是Operand,里面放置了Lamble具體表達式。

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;
        }

由於自定義集合實現IQueryable和IQueryProvider來自定義查詢項

實現IQuable定義的類：

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);
        }
    }

實現IQueryProvider接口的類，CreateQuery解析表達式樹而Execute則是執行解析后的SQL語句，查詢數據，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));
        }
    }

具體下面即可使用查詢

        public void MyTestMethod()
        {
            Catagory cag = new Catagory("四大名著點評");

            IList<Blog> Blogs = new List<Blog>();
            Blog bok = new Blog() { Title = "紅樓夢", Conent = "紅樓夢書籍不錯",
　　　　　　　　 User = new Author() { UserName = "曹雪芹", BirefInfor = "中國清代著名作家" } };
            bok.Catagories.Add(cag);
            bok.Catagories.Add(new Catagory("紅樓夢專欄"));
            Blogs.Add(bok);

            Blog bok2 = new Blog() { Title = "三國演義", Conent = "三國鼎立時期的描述",
　　　　　　　 User = new Author() { UserName = "羅貫中", BirefInfor = "中國元末明初著名作家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("三國演義專欄"));
            Blogs.Add(bok2);

            Blog bok3 = new Blog() { Title = "西游記", Conent = "西游記講述四個徒弟和一個師傅取經事情", 
              User = new Author() { UserName = "吳承恩", BirefInfor = "中國明代傑出的小說家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("西游記專欄"));
            Blogs.Add(bok3);

            Blog bok4 = new Blog() { Title = "水滸傳", Conent = "描述梁山好漢劫富濟貧的故事", 
　　　　　　　　　　User = new Author() { UserName = "施耐庵", BirefInfor = "中國明代傑出的小說家" } };
            bok2.Catagories.Add(cag);
            bok2.Catagories.Add(new Catagory("水滸傳專欄"));
            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("水滸"));
        }

參考代碼下載

后續：針對解析Lambda表達式樹成SQL，本人也是剛剛入門，至於完全要解析成SQL，還需要研究......