LINQ to SQL查詢中的通配符搜索未執行

Question

我正在嘗試使用帶有以下查詢的LINQ 2 SQL執行SQL“ Like”通配符搜索

public IQueryable<DbHeader> GetHeaders()
{
    return Work.Context.Headers
        .Include(m => m.Terms.Select(t => t.MetaDataPairs))
        .Include(m => m.States)
        .Include(m => m.Attachments);
}

var dbt = from th in _objectStoreRepository.GetHeaders()
                        where SqlMethods.Like(th.CreatedBy, "test")
                        select th;

foreach (var t in dbt) {
...

但是，當我運行它時，在foreach行上我得到

發生意外錯誤：LINQ to Entities無法識別方法'Boolean Like（System.String，System.String）'方法，並且該方法無法轉換為商店表達式。

為什么考慮將它視為LINQ to Entities查詢？ 我以為這是純LINQ 2 SQL

Answer 1

做你想做的..我通常使用這個Utlis ..（擴展方法，用通配符執行wherelike），....希望它能幫助你

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace Fwd.Repository.EF.Utils
{
    public static class LinqUtils
    {
        /// <summary>
        ///     Wheres the like.
        /// </summary>
        /// <typeparam name="TSource">The type of the source.</typeparam>
        /// <param name="source">The source.</param>
        /// <param name="valueSelector">The value selector.</param>
        /// <param name="value">The value.</param>
        /// <param name="wildcard">The wildcard.</param>
        /// <returns></returns>
        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));
        }

        /// <summary>
        ///     Builds the like expression.
        /// </summary>
        /// <typeparam name="TElement">The type of the element.</typeparam>
        /// <param name="valueSelector">The value selector.</param>
        /// <param name="value">The value.</param>
        /// <param name="wildcard">The wildcard.</param>
        /// <returns></returns>
        /// <exception cref="System.ArgumentNullException">valueSelector</exception>
        public 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);
        }

        /// <summary>
        ///     Gets the like method.
        /// </summary>
        /// <param name="value">The value.</param>
        /// <param name="wildcard">The wildcard.</param>
        /// <returns></returns>
        private static MethodInfo GetLikeMethod(string value, char wildcard)
        {
            var methodName = "Contains";

            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";
                textLength = value.Length;
            }

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

然后您可以在IQueryable實體上使用它，例如

var query = _dbContext.Users.WhereLike(xx=>xx.name,"fred*","*")

Answer 2

如錯誤消息所述，不是Linq to SQL，而是Linq to Entities，其中不支持SqlMethods類（ 請參閱Microsoft文檔 ）。 我猜是因為您針對實體框架的ObjectContext或DbContext運行查詢。

在Linq to Entities上，以下方法將起作用：

var dbt = from th in _objectStoreRepository.GetHeaders()
          where th.CreatedBy.Contains("test")
          select th;