【Expression笔记】ExpressionVisitor介绍及案例

介绍：

案例1：使用已有表达式树构建新的表达式树

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

将上面两个表达式合并成一个 item=>item.Length>2 && item.Length<4 

不能直接使用下面这种方法构建新的表达式树，因为Body中带着之前的Parameter信息

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();
        }

只能使用ExpressionVisitor修改表达式，将之前的Parameter替换成新的

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();
        }

调用：

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

 有了上面的基础，可以写几个扩展方法 And()、Or()、Not() 

public static class ExpressionExtension
    {
        #region Expression<Func<T>>扩展
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> exp1, Expression<Func<T, bool>> exp2)
        {
            return exp1.Compose(exp2, Expression.AndAlso);
        }
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> exp1, Expression<Func<T, bool>> exp2)
        {
            return exp1.Compose(exp2, Expression.OrElse);
        }
        public static Expression<Func<T, bool>> Compose<T>(this Expression<Func<T, bool>> exp1, Expression<Func<T, bool>> exp2, Func<Expression, Expression, Expression> merge)
        {
            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 = merge(newExp1, newExp2);
            return Expression.Lambda<Func<T, bool>>(newBodyExp, parameter);
        }
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> exp)
        {
            return Expression.Lambda<Func<T, bool>>(Expression.Not(exp.Body), exp.Parameters[0]);
        }
        #endregion

        #region Expression<Predicate<T>>扩展
        public static Expression<Predicate<T>> And<T>(this Expression<Predicate<T>> exp1, Expression<Predicate<T>> exp2)
        {
            return exp1.Compose(exp2, Expression.AndAlso);
        }
        public static Expression<Predicate<T>> Or<T>(this Expression<Predicate<T>> exp1, Expression<Predicate<T>> exp2)
        {
            return exp1.Compose(exp2, Expression.OrElse);
        }
        public static Expression<Predicate<T>> Compose<T>(this Expression<Predicate<T>> exp1, Expression<Predicate<T>> exp2, Func<Expression, Expression, Expression> merge)
        {
            var parameter = Expression.Parameter(typeof(T), "name");
            SetParamExpressionVisitor visitor = new SetParamExpressionVisitor(parameter);
            var newExp1 = visitor.Modify(exp1.Body);
            var newExp2 = visitor.Modify(exp2.Body);
            var newBodyExp = merge(newExp1, newExp2);
            return Expression.Lambda<Predicate<T>>(newBodyExp, parameter);
        }
        public static Expression<Predicate<T>> Not<T>(this Expression<Predicate<T>> exp)
        {
            return Expression.Lambda<Predicate<T>>(Expression.Not(exp.Body), exp.Parameters[0]);
        }
        #endregion

    }

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