.net - 使用泛型函数创建 Linq 表达式

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，我想在where子句中使用一些谓词来查询我的数据库。为了为日期创建谓词，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

此函数适用于特定实体字段IssueDate。我想避免为不同的实体字段（即ExpireDate、NextCheckDate ）创建更多执行完全相同的操作的函数。有什么办法可以做到这一点？

Answer 1

您可以使它成为 DateTime 的函数，而不是 Product 的函数。

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