Expression introduction and case notes -ExpressionVisitor

Introduction:

 

 

Case 1: Use the existing expression tree to build a new expression tree

Expression<Func<string, bool>> lambda0 = item => item.Length > 2;
Expression<Func<string, bool>> lambda1 = item => item.Length < 4;

The above two expressions are merged into one  Item => item.Length> 2  && item.Length < . 4 

You can not directly use the following method of constructing this new expression tree, as the information before the Body Parameter with

public Func<string, bool> ReBuildExpression(Expression<Func<string, bool>> lambd0, Expression<Func<string, bool>> lambd1)
        {
            parameter = Expression.Parameter(typeof(string), "name");
            Expression left = lambd0.Body;
            Expression right = lambd1.Body;
            BinaryExpression expression = Expression.AndAlso(left, right);
            Expression<Func<string, bool>> lambda = Expression.Lambda<Func<string, bool>>(expression, parameter);
            return lambda.Compile();
        }

Use only ExpressionVisitor modify the expression will be replaced with a new Parameter before

public class SetParamExpressionVisitor : ExpressionVisitor
    {
        public ParameterExpression Parameter { get; set; }
        public SetParamExpressionVisitor() { }
        public SetParamExpressionVisitor(ParameterExpression parameter) {
            this.Parameter = parameter;
        }
        public Expression Modify(Expression exp)
        {
            return this.Visit(exp);
        }
        protected override Expression VisitParameter(ParameterExpression parameter)
        {
            return this.Parameter;
        }
    }

public static Func<string, bool> AndAlsoExpression(Expression<Func<string,bool>> exp1,Expression<Func<string, bool>> exp2)
        {
            var parameter = Expression.Parameter(typeof(string), "name");
            SetParamExpressionVisitor visitor = new SetParamExpressionVisitor(parameter);
            var newExp1 = visitor.Modify(exp1.Body);
            var newExp2 = visitor.Modify(exp2.Body);
            var newBodyExp = Expression.AndAlso(newExp1, newExp2);
            return Expression.Lambda<Func<string, bool>>(newBodyExp, parameter).Compile();
        }

transfer:

Expression<Func<string, bool>> exp1 = item => item.Length > 2;
Expression<Func<string, bool>> exp2 = item => item.Length < 4;
Func<string,bool> func = AndAlsoExpression(exp1, exp2);
bool b = func("aaaa");//false

 

 

 

 

 

 

 

 

 

 

 

 

Reference: https://www.cnblogs.com/FlyEdward/archive/2010/12/06/Linq_ExpressionTree7.html

To be continued ...