Question

我有一个类（实体），如下所示：

Public Class Product

    Property ID As Integer
    Property Name As String
    Property IssueDate As Date
    Property ExpireDate As Date
    Property NextCheckDate As Date

End Class

我正在使用Entity Framework 5，我想使用 where 子句中的一些谓词来查询我的数据库。为了创建日期的谓词，我想使用如下函数：

Function GetDateIssuePredicate(fromDate As Date?, toDate As Date?) As Expression(Of Func(Of Product, Boolean))

    Dim predicate As Expressions.Expression(Of Func(Of Product, Boolean))

    If fromDate.hasValue AndAlso toDate.hasValue Then
        predicate = (Function(p) p.IssueDate >= fromDate.Value AndAlso p.IssueDate<= toDate.Value)
    ElseIf fromDate.hasValue Then
        predicate = (Function(p) p.IssueDate >= fromDate.Value)
    ElseIf toDate.hasValue Then
        predicate = (Function(p) p.IssueDate<= toDate.Value)
    End If

    Return predicate

End Function

此功能适用于特定实体字段 IssueDate 。我想避免为不同的实体字段创建更多功能完全相同的功能（即 ExpireDate ， NextCheckDate ）。有没有办法实现这个目标？

Answer 1

你可以使它成为DateTime的功能，而不是Product的功能。

Function GetDateIssuePredicate(fromDate As Date?, toDate As Date?) As Expression(Of Func(Of DateTime, DateTime)) 

    Dim predicate As Expressions.Expression(Of Func(Of DateTime, DateTime)) 

    If fromDate.hasValue AndAlso toDate.hasValue Then 
        predicate = (Function(d) d >= fromDate.Value AndAlso d <= toDate.Value) 
    ElseIf fromDate.hasValue Then 
        predicate = (Function(d) d >= fromDate.Value) 
    ElseIf toDate.hasValue Then 
        predicate = (Function(d) d <= toDate.Value) 
    End If 

    Return predicate 

End Function

使用泛型函数创建Linq表达式

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