动态查询LINQ到xml的VB.NET

Question

嘿， 我想编写一个查询的“凡”在查询是一个字符串像“

昏暗查询的String =” NAME = XXXX和日期> 10 “

昏暗的T =从书doc.Descendants（” 书“）选择_ [名] = book..value，[日期] = book..value .... 凡（查询）

我在运行时构建查询字符串

谢谢..

Answer 1

我并不是说这是你的情况，但我从ASP经典的人那里看到了很多，我们经常用它来构建动态SQL字符串。我们倾向于希望LINQ在部分代码中给我们更多的权力，但让我们在其他地方使用字符串。不幸的是，这不是事实。 Where需要一个布尔参数，并且没有办法。你可以编写自己的使用反射的解析器，并最终返回一个布尔值，但是你会写很多可能容易出错的代码。下面是你真的应该这样做：

假设这是我们的数据类：

Public Class TestObject 
    Public Property Name As String 
    Public Property Job As String 
End Class 

下面是我们的测试数据：

Dim Objects As New List(Of TestObject) 
    Objects.Add(New TestObject() With {.Name = "A", .Job = "Baker"}) 
    Objects.Add(New TestObject() With {.Name = "B", .Job = "President"}) 
    Objects.Add(New TestObject() With {.Name = "C", .Job = "Bus Driver"}) 
    Objects.Add(New TestObject() With {.Name = "D", .Job = "Trainer"}) 

你想要做什么是创建一个代表一个变量要搜索的数据：

''//This variable simulates our choice. Normally we would be parsing the querystring, form data, XML values, etc 
    Dim RandNum = New Random().Next(0, 3) 
    Dim LookForName As String = Nothing 
    Select Case RandNum 
     Case 0 : LookForName = "A" 
     Case 1 : LookForName = "B" 
     Case 2 : LookForName = "C" 
    End Select 

    ''//Query based on our name 
    Dim Subset = (From O In Objects Select O Where (O.Name = LookForName)).ToList() 

如果有时您需要在Job上进行搜索，有时甚至是metimes你没有你就可能需要编写几个查询：

Dim Subset As List(Of TestObject) 
    Select Case RandNum 
     Case 0 
      Subset = (From O In Objects Select O Where (O.Name = "A" And O.Job = "Baker")).ToList() 
     Case Else 
      Select Case RandNum 
       Case 1 : LookForName = "B" 
       Case 2 : LookForName = "C" 
      End Select 
      Subset = (From O In Objects Select O Where (O.Name = LookForName)).ToList() 
    End Select 

而只是为了解释编写自己的查询分析器（这是我建议你切勿往下走的路径），在这里是一个非常非常非常粗糙的开始。它仅支持=并且仅支持字符串，并且可以在多个点中断。

Public Shared Function QueryParser(ByVal obj As Object, ByVal ParamArray queries() As String) As Boolean 
    ''//Sanity check 
    If obj Is Nothing Then Throw New ArgumentNullException("obj") 
    If (queries Is Nothing) OrElse (queries.Count = 0) Then Throw New ArgumentNullException("queries") 

    ''//Array of property/value 
    Dim NameValue() As String 
    ''//Loop through each query 
    For Each Q In queries 
     ''//Remove whitespace around equals sign 
     Q = System.Text.RegularExpressions.Regex.Replace(Q, "\s+=\s+", "=") 
     ''//Break the query into two parts. 
     ''//NOTE: this only supports the equal sign right now 
     NameValue = Q.Split("="c) 
     ''//NOTE: if either part of the query also contains an equal sign then this exception will be thrown 
     If NameValue.Length <> 2 Then Throw New ArgumentException("Queries must be in the format X=Y") 

     ''//Grab the property by name 
     Dim P = obj.GetType().GetProperty(NameValue(0)) 
     ''//Make sure it exists 
     If P Is Nothing Then Throw New ApplicationException(String.Format("Cannot find property {0}", NameValue(0))) 
     ''//We only support strings right now 
     If Not P.PropertyType Is GetType(String) Then Throw New ApplicationException("Only string property types are support") 

     ''//Get the value of the property for the supplied object 
     Dim V = P.GetValue(obj, Nothing) 
     ''//Assumming null never equals null return false for a null value 
     If V Is Nothing Then Return False 
     ''//Compare the two strings, return false if something doesn't match. 
     ''//You could use String.Compare here, too, but this will use the current Option Compare rules 
     If V.ToString() <> NameValue(1) Then Return False 
    Next 

    ''//The above didn't fail so return true 
    Return True 
End Function 

此代码将允许你写：

Dim Subset = (From O In Objects Select O Where (QueryParser(O, "Name = A", "Job = Baker"))).ToList() 

Answer 2

没有，没有什么直接像你要找的内容，您可以在传递一个字符串。正如他们所说的，当你拥有的只是一把锤子时，一切都看起来像一个钉子......真正的问题是你需要学习LINQ擅长的东西并将它应用于你的代码（如果它很合适），而不是比尝试使用动态构建的SQL查询字符串做得更好。

你应该做的是让那些“Where”子句强制类型化。你目前的代码有很大的潜力可以炸毁并且很难调试。

你可以做什么，而不是是这样的（对不起，使用C＃，已经有一段时间，因为我已经感动VB.NET）：

var query = from book in doc.Descendants("books") 
      select book; 

if(needsNameComparison) 
{ 
    query = query.where(book.Name == nameToCompare); 
} 

if(needsDateComparison) 
{ 
    query = query.Where(book.Date > 10); 
} 

List<book> bookList = query.ToList(); 

使用LINQ，“查询”是不是真正运行直到“ToList（）”调用。由于它使用后期执行，所以查询是动态的，因为它正在建立，直到它实际需要运行。这与您在提前构建查询字符串之前希望使用的代码类似，然后在特定点执行它。