C# 你能教实体框架识别表达式吗?
我有一个使用实体框架的搜索功能。您可以搜索的内容之一是日期范围。您可能会说“开始日期在SearchStart和SearchEnd之间”。用linq语法编写并不困难,但当您有许多不同的日期参数可供搜索时,它可能会变得非常冗长 我在DateTime上有一个扩展方法,基本上检查日期是否包含在StartDate和EndDate之间。我在EF不是问题的其他地方使用它,但我也希望在EF查询中使用它。我通过在执行ToList(将尝试运行查询)之前应用额外的WHERE子句来动态创建查询 正如我所料,使用extension方法会引发异常: LINQ to Entities无法识别方法“Boolean IsBetween(System.DateTime,System.DateTime,System.DateTime)”方法,此方法无法转换为存储表达式 我知道LINQtoEntities无法知道在Sql中IsBetween转换为什么,但有没有一种方法可以让我给它提供指导?我试着在网上搜索答案,但没有多大帮助。如果有一些属性可以添加到扩展方法中,或者有一些方法可以更新EF配置 我猜不会,但我不想不问就假设 谢谢 更新:添加扩展方法代码C# 你能教实体框架识别表达式吗?,c#,entity-framework,extension-methods,C#,Entity Framework,Extension Methods,我有一个使用实体框架的搜索功能。您可以搜索的内容之一是日期范围。您可能会说“开始日期在SearchStart和SearchEnd之间”。用linq语法编写并不困难,但当您有许多不同的日期参数可供搜索时,它可能会变得非常冗长 我在DateTime上有一个扩展方法,基本上检查日期是否包含在StartDate和EndDate之间。我在EF不是问题的其他地方使用它,但我也希望在EF查询中使用它。我通过在执行ToList(将尝试运行查询)之前应用额外的WHERE子句来动态创建查询 正如我所料,使用exte
public static bool IsBetween(this DateTime date , DateTime start, DateTime end)
{
return (date >= start && date < end);
}
public static bool IsBetween(此日期时间日期、日期时间开始、日期时间结束)
{
返回(日期>=开始和日期<结束);
}
您可以使用DbFunctions
类在sql语句之间生成
在早于6.0的EF版本中称为EntityFunctions
。用法:
var start = new DateTime(2014, 01, 21);
var end = new DateTime(2014, 01, 22);
var queryable = dbContext.Set<MyEntity>();
queryable = queryable.InBetween(start, end);
var start=newdatetime(2014,01,21);
var end=新日期时间(2014年1月22日);
var queryable=dbContext.Set();
可查询=可查询。中间(开始、结束);
扩展方法
public static class EntityExtensions
{
public static IQueryable<MyEntity> InBetween(this IQueryable<MyEntity> queryable,
DateTime start, DateTime end)
{
return queryable.Where(x => x.DateColumn >= start && x.DateColumn < end);
}
}
公共静态类EntityExtensions
{
公共静态IQueryable中间(此IQueryable可查询,
日期时间开始,日期时间结束)
{
返回queryable.Where(x=>x.DateColumn>=start&&x.DateColumn
现在,您在这里无法完全重用其他扩展方法。但是,您可以在其他几个查询中重用这个新的扩展方法。只需在可查询项上调用.InBetween并传递参数
下面是另一种方法:
public static IQueryable<MyEntity> InBetween(this IQueryable<MyEntity> queryable,
DateTime start, DateTime end)
{
return queryable.Where(InBetween(start, end));
}
public static Expression<Func<MyEntity, bool>> InBetween(DateTime start,
DateTime end)
{
return x => x.DateColumn >= start && x.DateColumn < end;
}
公共静态IQueryable中间(此IQueryable可查询,
日期时间开始,日期时间结束)
{
返回可查询的。其中(介于(开始、结束)之间);
}
中间的公共静态表达式(日期时间开始,
日期时间(结束)
{
返回x=>x.DateColumn>=start&&x.DateColumn
这里是一种完全通用的方法,但深入研究并手工构建表达式树以使其工作
我们不是在教实体框架如何读取新表达式,而是将表达式分解成实体框架已经知道如何读取的部分
public static IQueryable<T> IsBetween<T>(this IQueryable<T> query, Expression<Func<T, DateTime>> selector, DateTime start, DateTime end)
{
//Record the start time and end time and turn them in to constants to be passed in to the query.
//There may be a better way to pass them as parameters instead of constants but I don't have the skill with expression trees to know how to do it.
var startTime = Expression.Constant(start);
var endTime = Expression.Constant(end);
//We get the body of the expression that was passed in that selects the DateTime column in the row for us.
var selectorBody = selector.Body;
//We need to pass along the parametres from that original selector.
var selectorParameters = selector.Parameters;
// Represents the "date >= start"
var startCompare = Expression.GreaterThanOrEqual(selectorBody, startTime);
// Represents the "date < end"
var endCompare = Expression.LessThan(selectorBody, endTime);
// Represents the "&&" between the two statements.
var combinedExpression = Expression.AndAlso(startCompare, endCompare);
//Reform the new expression in to a lambada to be passed along to the Where clause.
var lambada = Expression.Lambda<Func<T, bool>>(combinedExpression, selectorParameters);
//Perform the filtering and return the filtered query.
return query.Where(lambada);
}
public static IQueryable IsBetween(此IQueryable查询、表达式选择器、日期时间开始、日期时间结束)
{
//记录开始时间和结束时间,并将它们转换为要传递给查询的常量。
//可能有更好的方法将它们作为参数而不是常量传递,但我不知道如何使用表达式树。
var startTime=表达式常数(开始);
var endTime=表达式常数(结束);
//我们得到传入的表达式的主体,该表达式为我们选择行中的DateTime列。
var selectorBody=selector.Body;
//我们需要传递原始选择器的参数。
var selectorParameters=选择器参数;
//表示“日期>=开始”
var startCompare=Expression.GreaterThanOrEqual(selectorBody,startTime);
//表示“日期<结束”
var endCompare=Expression.LessThan(selectorBody,endTime);
//表示两条语句之间的“&&”。
var combinedExpression=Expression.AndAlso(开始比较,结束比较);
//将中的新表达式改为lambada,以传递给Where子句。
var lambada=Expression.Lambda(组合表达式、选择器或参数);
//执行筛选并返回筛选后的查询。
返回query.Where(lambada);
}
它生成以下SQL语句
SELECT
[Extent1].[TestId] AS [TestId],
[Extent1].[Example] AS [Example]
FROM [dbo].[Tests] AS [Extent1]
WHERE ([Extent1].[Example] >= convert(datetime2, '2013-01-01 00:00:00.0000000', 121)) AND ([Extent1].[Example] < convert(datetime2, '2014-01-01 00:00:00.0000000', 121))
选择
[Extent1][TestId]作为[TestId],
[Extent1].[Example]作为[Example]
从[dbo].[Tests]到[Extent1]
其中([Extent1].[Example]>=convert(datetime2,'2013-01-01 00:00:00.0000000',121))和([Extent1].[Example]
使用下面的程序
private static void Main(string[] args)
{
using (var context = new TestContext())
{
context.SaveChanges();
context.Tests.Add(new Test(new DateTime(2013, 6, 1)));
context.Tests.Add(new Test(new DateTime(2014, 6, 1)));
context.SaveChanges();
DateTime start = new DateTime(2013, 1, 1);
DateTime end = new DateTime(2014, 1, 1);
var query = context.Tests.IsBetween(row => row.Example, start, end);
var traceString = query.ToString();
var result = query.ToList();
Debugger.Break();
}
}
public class Test
{
public Test()
{
Example = DateTime.Now;
}
public Test(DateTime time)
{
Example = time;
}
public int TestId { get; set; }
public DateTime Example { get; set; }
}
public class TestContext : DbContext
{
public DbSet<Test> Tests { get; set; }
}
private static void Main(字符串[]args)
{
使用(var context=newtestcontext())
{
SaveChanges();
添加(新测试(新日期时间(2013,6,1));
添加(新测试(新日期时间(2014,6,1));
SaveChanges();
日期时间开始=新的日期时间(2013,1,1);
DateTime end=新的日期时间(2014,1,1);
var query=context.Tests.IsBetween(row=>row.Example,start,end);
var traceString=query.ToString();
var result=query.ToList();
Debugger.Break();
}
}
公开课考试
{
公开考试()
{
示例=DateTime.Now;
}
公共测试(日期时间)
{
示例=时间;
}
公共int TestId{get;set;}
公共日期时间示例{get;set;}
}
公共类TestContext:DbContext
{
公共数据库集测试{get;set;}
}
下面是一个可以转换通用表达式并将其映射到特定对象的实用程序。这允许您将表达式编写为date=>date>=start&&date
,并将其传递到转换器进行映射
public static class LambadaConverter
{
/// <summary>
/// Converts a many parametered expression in to a single paramter expression using a set of mappers to go from the source type to mapped source.
/// </summary>
/// <typeparam name="TNewSourceType">The datatype for the new soruce type</typeparam>
/// <typeparam name="TResult">The return type of the old lambada return type.</typeparam>
/// <param name="query">The query to convert.</param>
/// <param name="parameterMapping">The mappers to go from the single source class to a set of </param>
/// <returns></returns>
public static Expression<Func<TNewSourceType, TResult>> Convert<TNewSourceType, TResult>(Expression query, params Expression[] parameterMapping)
{
//Doing some pre-condition checking to make sure everything was passed in correctly.
var castQuery = query as LambdaExpression;
if (castQuery == null)
throw new ArgumentException("The passed in query must be a lambada expression", "query");
if (parameterMapping.Any(expression => expression is LambdaExpression == false) ||
parameterMapping.Any(expression => ((LambdaExpression)expression).Parameters.Count != 1) ||
parameterMapping.Any(expression => ((LambdaExpression)expression).Parameters[0].Type != typeof(TNewSourceType)))
{
throw new ArgumentException("Each pramater mapper must be in the form of \"Expression<Func<TNewSourceType,TResut>>\"",
"parameterMapping");
}
//We need to remap all the input mappings so they all share a single paramter variable.
var inputParameter = Expression.Parameter(typeof(TNewSourceType));
//Perform the mapping-remapping.
var normlizer = new ParameterNormalizerVisitor(inputParameter);
var mapping = normlizer.Visit(new ReadOnlyCollection<Expression>(parameterMapping));
//Perform the mapping on the expression query.
var customVisitor = new LambadaVisitor<TNewSourceType, TResult>(mapping, inputParameter);
return (Expression<Func<TNewSourceType, TResult>>)customVisitor.Visit(query);
}
/// <summary>
/// Causes the entire series of input lambadas to all share the same
/// </summary>
private class ParameterNormalizerVisitor : ExpressionVisitor
{
public ParameterNormalizerVisitor(ParameterExpression parameter)
{
_parameter = parameter;
}
private readonly ParameterExpression _parameter;
protected override Expression VisitParameter(ParameterExpression node)
{
if(node.Type == _parameter.Type)
return _parameter;
else
throw new InvalidOperationException("Was passed a parameter type that was not expected.");
}
}
/// <summary>
/// Rewrites the output query to use the new remapped inputs.
/// </summary>
private class LambadaVisitor<TSource,TResult> : ExpressionVisitor
{
public LambadaVisitor(ReadOnlyCollection<Expression> parameterMapping, ParameterExpression newParameter)
{
_parameterMapping = parameterMapping;
_newParameter = newParameter;
}
private readonly ReadOnlyCollection<Expression> _parameterMapping;
private readonly ParameterExpression _newParameter;
private ReadOnlyCollection<ParameterExpression> _oldParameteres = null;
protected override Expression VisitParameter(ParameterExpression node)
{
//Check to see if this is one of our known parameters, and replace the body if it is.
var index = _oldParameteres.IndexOf(node);
if (index >= 0)
{
return ((LambdaExpression)_parameterMapping[index]).Body;
}
//Not one of our known parameters, process as normal.
return base.VisitParameter(node);
}
protected override Expression VisitLambda<T>(Expression<T> node)
{
if (_oldParameteres == null)
{
_oldParameteres = node.Parameters;
var newBody = this.Visit(node.Body);
return Expression.Lambda<Func<TSource, TResult>>(newBody, _newParameter);
}
else
throw new InvalidOperationException("Encountered more than one Lambada, not sure how to handle this.");
}
}
}
private static void Main(string[] args)
{
using (var context = new TestContext())
{
DateTime start = new DateTime(2013, 1, 1);
DateTime end = new DateTime(2014, 1, 1);
var query = context.Tests.IsBetween(row => row.Example, start, end);
var traceString = query.ToString(); // Generates the where clause: WHERE ([Extent1].[Example] >= @p__linq__0) AND ([Extent1].[Example] < @p__linq__1)
var query2 = context.Tests.ComplexTest(row => row.Param1, row => row.Param2);
var traceString2 = query2.ToString(); //Generates the where clause: WHERE (N'Foo' = [Extent1].[Param1]) AND ([Extent1].[Param1] IS NOT NULL) AND (2 = [Extent1].[Param2])
Debugger.Break();
}
}
public class Test
{
public int TestId { get; set; }
public DateTime Example { get; set; }
public string Param1 { get; set; }
public int Param2 { get; set; }
}
public class TestContext : DbContext
{
public DbSet<Test> Tests { get; set; }
}
public static IQueryable<T> IsBetween<T>(this IQueryable<T> query, Expression<Func<T, DateTime>> dateSelector, DateTime start, DateTime end)
{
Expression<Func<DateTime, bool>> testQuery = date => date >= start && date < end;
var newQuery = LambadaConverter.Convert<T, bool>(testQuery, dateSelector);
return query.Where(newQuery);
}
public static IQueryable<T> ComplexTest<T>(this IQueryable<T> query, Expression<Func<T, string>> selector1, Expression<Func<T, int>> selector2)
{
Expression<Func<string, int, bool>> testQuery = (str, num) => str == "Foo" && num == 2;
var newQuery = LambadaConverter.Convert<T, bool>(testQuery, selector1, selector2);
return query.Where(newQuery);
}