c#中ExpressionVistor的用法

本篇內容介紹了“c#中ExpressionVistor的用法”的有關知識，在實際案例的操作過程中，不少人都會遇到這樣的困境，接下來就讓小編帶領大家學習一下如何處理這些情況吧！希望大家仔細閱讀，能夠學有所成！

class MyVistor :ExpressionVisitor
        {
            private string tableName;
            private StringBuilder sbSql = new StringBuilder();
            public string getsbSql() { return sbSql.ToString(); }
            private string ExpressionTypeToSQL(ExpressionType type)
            {
                switch(type)
                {
                    case ExpressionType.Add:
                        return " + ";
                    case ExpressionType.And:
                        return " and ";
                    case ExpressionType.AndAlso:
                        return " and ";
                    case ExpressionType.Assign:
                        return " = ";
                    case ExpressionType.NotEqual:
                        return " != ";
                    case ExpressionType.Equal:
                        return " == ";
                    case ExpressionType.GreaterThan:
                        return " > ";
                    case ExpressionType.GreaterThanOrEqual:
                        return " >= ";
                    case ExpressionType.LessThan:
                        return " < ";
                    case ExpressionType.LessThanOrEqual:
                        return " <= ";
                    case ExpressionType.Multiply:
                        return " * ";
                    case ExpressionType.Or:
                    case ExpressionType.OrElse:
                        return " or ";
                    default:
                        return "";
                }
            }
            public override Expression Visit(Expression node)
            {
                return base.Visit(node);
            }
            protected override Expression VisitBinary(BinaryExpression node)
            {
                base.Visit(node.Left);
                sbSql.Append(ExpressionTypeToSQL(node.NodeType));
                base.Visit(node.Right);
                return node;
            }
            public string GetSqlString()
            {
                return "select * from " + tableName + " where " + sbSql.ToString();
            }
            protected override Expression VisitConstant(ConstantExpression node)
            {
                if(node.Type == typeof(int))
                {
                    sbSql.Append(node.Value);//ConstantExpression.Value是表示常量表達式的值
                }
                else
                {
                    sbSql.Append("'" + node.Value + "'");
                }
                return base.VisitConstant(node);
            }
            protected override Expression VisitParameter(ParameterExpression node)
            {
                if(tableName == null)
                {
                    tableName = "[" + node.Type.Name + "]";
                }
                return base.VisitParameter(node);
            }
            protected override Expression VisitMember(MemberExpression node)
            {
                sbSql.Append("[" + node.Member.Name + "]");
                return base.VisitMember(node);
            }
        }
        class RefClas
        {
            public int id;
            public int age;
            public RefClas(int id, int age)
            {
                this.id = id;
                this.age = age;
            }
        }
 Expression<Func<RefClas, bool>> expressionsql = a => a.id > 5 && a.age < 10;
            MyVistor myvistor = new MyVistor();
            Expression exp = myvistor.Visit(expressionsql);
            Console.WriteLine(myvistor.getsbSql());//[id] > 5 and [age] < 10
            //編譯
            var func2 = expressionsql.Compile();
            RefClas cls2 = new RefClas(10,20);
            bool res = func2(cls2);
            Console.WriteLine("func2返回結果：" + res); //func2返回結果：False
            Console.WriteLine(myvistor.GetSqlString());//select * from [RefClas] where [id] > 5 and [age] < 10

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