“喜欢"实体框架中的查询 [英] "like" queries in Entity Framework

问题描述

如何使用 edo 实体框架在 ASP.net MVC 中获取通配符文本搜索(如 SQL 的like"语句)?

How do I get wildcard text searches (like SQL's "like" statement) in ASP.net MVC using the edo entity framework?

我认为这会奏效:

var elig = (from e in _documentDataModel.Protocol_Eligibility_View
            where e.criteria.Contains(query)
            select e);

但是即使搜索肯定在数据库中的查询字符串，它也不会返回任何结果.我做错了什么?

But it returns no results even when searching for a query string that's definitely in the database. What am I doing wrong?

推荐答案

这个家伙为 Linq 做了一个非常好的WhereLike"扩展，它接受任何通配符并将两个值(其中一个来自表达式)与一个泛型进行比较从通配符的位置派生的方法.

This guy made a very nice "WhereLike" extension for Linq that accepts any wildcard character and compares two values (one of which comes from an expression) with a generic method derived from the location of the wildcard.

• x% -> 开头
• %x -> 结尾
• %x% -> 包含

http://trentacular.com/2010/08/linq-to-entities-wild-card-like-extension-method/

文章好像挂了.我将在下面粘贴扩展代码:

The article seems to be down. I will paste the extention code below:

public static class LinqHelper
    {
        //Support IQueryable (Linq to Entities)
        public static IQueryable<TSource> WhereLike<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, string>> valueSelector, string value, char wildcard)
        {
            return source.Where(BuildLikeExpression(valueSelector, value, wildcard));
        }

        //Support IEnumerable (Linq to objects)
        public static IEnumerable<TSource> WhereLike<TSource>(this IEnumerable<TSource> sequence, Func<TSource, string> expression, string value, char wildcard)
        {
            var regEx = WildcardToRegex(value, wildcard);

            //Prevent multiple enumeration:
            var arraySequence = sequence as TSource[] ?? sequence.ToArray();

            try
            {
                return arraySequence.Where(item => Regex.IsMatch(expression(item), regEx));
            }
            catch (ArgumentNullException)
            {
                return arraySequence;
            }
        }

        //Used for the IEnumerable support
        private static string WildcardToRegex(string value, char wildcard)
        {
            return "(?i:^" + Regex.Escape(value).Replace("\" + wildcard, "." + wildcard) + "$)";
        }

        //Used for the IQueryable support
        private static Expression<Func<TElement, bool>> BuildLikeExpression<TElement>(Expression<Func<TElement, string>> valueSelector, string value, char wildcard)
        {
            if (valueSelector == null) throw new ArgumentNullException("valueSelector");

            var method = GetLikeMethod(value, wildcard);

            value = value.Trim(wildcard);
            var body = Expression.Call(valueSelector.Body, method, Expression.Constant(value));

            var parameter = valueSelector.Parameters.Single();
            return Expression.Lambda<Func<TElement, bool>>(body, parameter);
        }

        private static MethodInfo GetLikeMethod(string value, char wildcard)
        {
            var methodName = "Equals";

            var textLength = value.Length;
            value = value.TrimEnd(wildcard);
            if (textLength > value.Length)
            {
                methodName = "StartsWith";
                textLength = value.Length;
            }

            value = value.TrimStart(wildcard);
            if (textLength > value.Length)
            {
                methodName = (methodName == "StartsWith") ? "Contains" : "EndsWith";
            }

            var stringType = typeof(string);
            return stringType.GetMethod(methodName, new[] { stringType });
        }
    }
}

这篇关于“喜欢"实体框架中的查询的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！