编写这个linq查询的更好方法是什么？
目前我正在使用开关操作的组合来生成linq查询，我认为代码有点臃肿。
有没有什么方法可以优化这段代码，或许可以动态构建它？
public string[] GetPeopleAutoComplete(string filter, int maxResults, string searchType, string searchOption) { var query = from people in _context.People select people; switch (searchOption) { case "StartsWith": switch (searchType) { case "IdentityCode": query = query.Where(o => o.IdentityCode.StartsWith(filter)); return query.Select(o => o.IdentityCode).Take(maxResults).ToArray(); case "Firstname": query = query.Where(o => o.Firstname.StartsWith(filter)); return query.Select(o => o.Firstname).Distinct().Take(maxResults).ToArray(); case "Surname": query = query.Where(o => o.Surname.StartsWith(filter)); return query.Select(o => o.Surname).Distinct().Take(maxResults).ToArray(); } break; case "EndsWith": switch (searchType) { case "IdentityCode": query = query.Where(o => o.IdentityCode.EndsWith(filter)); return query.Select(o => o.IdentityCode).Take(maxResults).ToArray(); case "Firstname": query = query.Where(o => o.Firstname.EndsWith(filter)); return query.Select(o => o.Firstname).Distinct().Take(maxResults).ToArray(); case "Surname": query = query.Where(o => o.Surname.EndsWith(filter)); return query.Select(o => o.Surname).Distinct().Take(maxResults).ToArray(); } break; case "Contains": switch (searchType) { case "IdentityCode": query = query.Where(o => o.IdentityCode.Contains(filter)); return query.Select(o => o.IdentityCode).Take(maxResults).ToArray(); case "Firstname": query = query.Where(o => o.Firstname.Contains(filter)); return query.Select(o => o.Firstname).Distinct().Take(maxResults).ToArray(); case "Surname": query = query.Where(o => o.Surname.Contains(filter)); return query.Select(o => o.Surname).Distinct().Take(maxResults).ToArray(); } break; } return query.Select(o => o.IdentityCode).Take(maxResults).ToArray(); } 这正是动态构建表达式有用的地方：
public string[] GetPeopleAutoComplete( string filter, int maxResults, string searchType, string searchOption) { IQueryable query = _context.People; var property = typeof(Person).GetProperty(searchType); var method = typeof(string).GetMethod(searchOption, new[] { typeof(string) }); query = query.Where(WhereExpression(property, method, filter)); var resultQuery = query.Select(SelectExpression(property)); if (searchType == "Firstname" || searchType == "Lastname") resultQuery = resultQuery.Distinct(); return resultQuery.Take(maxResults).ToArray(); } Expression> WhereExpression( PropertyInfo property, MethodInfo method, string filter) { var param = Expression.Parameter(typeof(Person), "o"); var propExpr = Expression.Property(param, property); var methodExpr = Expression.Call(propExpr, method, Expression.Constant(filter)); return Expression.Lambda>(methodExpr, param); } Expression> SelectExpression(PropertyInfo property) { var param = Expression.Parameter(typeof(Person), "o"); var propExpr = Expression.Property(param, property); return Expression.Lambda>(propExpr, param); }
这并不能解决您的默认情况，但这应该相对容易添加。 此外，使用这样的reflection可能会很慢，因此您可能希望缓存GetProperty()和GetMethod() 。
另外需要注意的是，选择是否使用Distinct()仍取决于属性名称，但也许您有更好的条件（或者您可以使用属性上的属性）。
并且这两个辅助方法不需要知道关于Person任何内容，因此将它们设为generics将是微不足道的。
使用Dynamic Linq to SQL库可以轻松解决您的问题。
博文： 使用Linq进行动态查询
谓词生成器
谓词构建器与动态linq库的工作方式相同，但主要区别在于它允许轻松编写更多类型安全的查询。
使用动态LINQ库
动态LINQ库允许构建查询具有不同的where子句或orderby。 要使用动态LINQ库，您需要在项目中下载并安装文件。
查看Scott GU的这篇文章 ： http ： //weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
您可以使用上面描述的动态linq选项，或者如果您想要更简单的东西，可以将一些切换逻辑重构为更小的部分，然后进行简单的查询
public string[] GetPeopleAutoComplete(string filter, int maxResults, string searchType, string searchOption) { var query = (from person in _context.People where MatchesSearchCriteria(searchType, searchOption, filter) select SelectAttribute(person,searchType,searchOption)); if (RequiresDistinct(filter,searchType, searchOption)) query = query.Distinct(); return query.Take(maxResults).ToArray(); } private bool MatchesSearchCriteria(string searchType, string searchOption, string filter) { //Implement some switching here... } private string SelectAttribute(Person person, string searchType, string searchOption) { //Implement some switching here to select the correct value from the person } private bool RequiresDistinct(string searchType, string searchOption) { //Return true if you need to select distinct values for this type of search }
我制作了2个新类，一个用于测试，另一个用于比较…
你希望通过名字得到明显的..
public class PeopleCollection { public people[] People; public class people { public string IdentityCode; public string Firstname; public string Surname; } } public class ForCompare : IEqualityComparer { string _fieldName = ""; public ForCompare(string fieldName) { _fieldName = fieldName; } public bool Equals(PeopleCollection.people a, PeopleCollection.people b) { return "IdentityCode".Equals(_fieldName) ? true : a.GetType().GetProperty(_fieldName).GetValue(a, null).Equals(b.GetType().GetProperty(_fieldName).GetValue(b, null)); } public int GetHashCode(PeopleCollection.people a) { return a.GetHashCode(); } }
然后，方法就像↓
public static string[] GetPeopleAutoComplete(string filter, int maxResults, string searchType, string searchOption) { var property = typeof(PeopleCollection.people).GetProperty(searchType); var method = typeof(string).GetMethod(searchOption, new[] { typeof(string) }); var query = from people in _context.People select people; return query.Distinct(new ForCompare(searchType)) .Select(o => (string)property.GetValue(o, null)) .Where(value => (bool)method.Invoke(value, new object[] { filter })) .Take(maxResults).ToArray(); }
我希望这对你有用……
一般来说你想要这个：
query.Where(o => o.PropertyName.MethodName(keyword)); .Select(o => o.PropertyName).Take(maxResults).ToArray();
这是一个例子：
public class Person { public string FirstName { get; set; } } static void Main(string[] args) { string propertyName = "FirstName"; string methodName = "StartsWith"; string keyword = "123"; Type t = typeof(Person); ParameterExpression paramExp = Expression.Parameter(t, "p"); // the parameter: p MemberExpression memberExp = Expression.MakeMemberAccess(paramExp, t.GetMember(propertyName).FirstOrDefault()); // part of the body: p.FirstName MethodCallExpression callExp = Expression.Call(memberExp, typeof(string).GetMethod(methodName, new Type[] { typeof(string) }), Expression.Constant(keyword)); // the body: p.FirstName.StartsWith("123") Expression> whereExp = Expression.Lambda>(callExp, paramExp); Expression> selectExp = Expression.Lambda>(memberExp, paramExp); Console.WriteLine(whereExp); // p => p.FirstName.StartsWith("123") Console.WriteLine(selectExp); // p => p.FirstName List people = new List (); List firstNames = people.Where(whereExp.Compile()).Select(selectExp.Compile()).ToList(); Console.Read(); }
我想这就是你想要的……
动态LINQ
我现在已经全部接通了这个并且正在查看生成的sql，这是我得到的：
SELECT [Project1].[Id] AS [Id], [Project1].[Firstname] AS [Firstname], [Project1].[LevelGroup] AS [LevelGroup], [Project1].[IdentityCode] AS [IdentityCode], [Project1].[C1] AS [C1], [Project1].[Surname] AS [Surname] FROM ( SELECT [Extent1].[Id] AS [Id], [Extent1].[IdentityCode] AS [IdentityCode], [Extent1].[Firstname] AS [Firstname], [Extent1].[Surname] AS [Surname], [Extent1].[LevelGroup] AS [LevelGroup], (SELECT COUNT(1) AS [A1] FROM [dbo].[Loans] AS [Extent2] WHERE [Extent1].[Id] = [Extent2].[PersonId]) AS [C1] FROM [dbo].[People] AS [Extent1] WHERE [Extent1].[IdentityCode] LIKE N'a%' ) AS [Project1] ORDER BY [Project1].[Surname] ASC
此查询现在没有参数化！ 我如何解决这个问题，我觉得使用这段代码感觉不安全：