expression动态拼接

发布时间：2020-07-26 14:36:03来源：本站阅读（1010）

参考地址：https://www.cnblogs.com/rengke2002/p/13047385.html




public static class ExpressionBuilder
    {
        public static Expression<Func<T, bool>> And<T>(
            this Expression<Func<T, bool>> first,
            Expression<Func<T, bool>> second)
        {
            return first.AndAlso<T>(second, Expression.AndAlso);
        }

        public static Expression<Func<T, bool>> Or<T>(
            this Expression<Func<T, bool>> first,
            Expression<Func<T, bool>> second)
        {
            return first.AndAlso<T>(second, Expression.OrElse);
        }

        private static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2, Func<Expression, Expression, BinaryExpression> func)
        {
            var parameter = Expression.Parameter(typeof(T));
            //var parameter2 = Expression.Parameter(typeof(T));
            var ddd= expr1.Parameters;
            var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);
            var left = leftVisitor.Visit(expr1.Body);

            var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);//不是用.Parameters[0]结果是不对的
            var right = rightVisitor.Visit(expr2.Body);

             return Expression.Lambda<Func<T, bool>>(func(left, right), parameter);
             ///return Expression.Lambda(func(left, right), parameter);
        }

        private class ReplaceExpressionVisitor
            : ExpressionVisitor
        {
            private readonly Expression _oldValue;
            private readonly Expression _newValue;

            public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
            {
                _oldValue = oldValue;
                _newValue = newValue;
            }

            public override Expression Visit(Expression node)
            {
                if (node == _oldValue)
                   return _newValue;
                return base.Visit(node);
            }
        }

    }