創建使用LINQ表達式泛型函數

Question

我有一個類（實體）像下面這樣：

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 

我使用實體框架5，我想在其中子句使用一些謂詞來查詢我的數據庫。爲了創建一個謂語的日期我想使用的功能如下所示：

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 

此功能適用於特定的實體領域IssueDate。我想避免爲不同的實體字段創建完全相同的功能（即到期日期,NextCheckDate）。有什麼辦法可以做到這一點？

Answer 1

您可以使其成爲DateTime的函數而不是Product的函數。

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