Dynamic query Linq to xml VB.NET - vb.net

Question

Hey,
I want to write a query that the "where" in the query is a string something like"
Dim query as string= "Name =xxxx and Date > 10 "
Dim t = from book in doc.Descendants("books") Select _
[Name] = book..value, [Date] = book..value....
Where (query)
I build the query string on run time
Thanks...

Answer 1

I'm not saying this is your case but I see this a lot from people that came from ASP classic where we used to build dynamic SQL strings all of the time. We tend to hope that LINQ will give us some more power in part of the code but let us use strings elsewhere. Unfortunately this isn't true. Where takes a boolean argument and there's no way around that. You can write your own parser that uses reflection and eventually returns a boolean but you'd be writing a lot of code that could be error prone. Here's how you really should do it:
Assuming this is our data class:
Public Class TestObject
Public Property Name As String
Public Property Job As String
End Class
And here's our test data:
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"})
What you want to do is create a variable that represents the data to search for:
''//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()
If sometimes you need to search on Job and sometimes and sometimes you don't you just might have to write a couple of queries:
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
And just to explain writing your own query parser (which is a path that I recommend you DO NOT go down), here is a very, very, very rough start. It only supports = and only strings and can break at multiple points.
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
This code would allow you to write:
Dim Subset = (From O In Objects Select O Where (QueryParser(O, "Name = A", "Job = Baker"))).ToList()

Answer 2

No, there is nothing directly like what you're looking for where you can pass in a string. As they say, when all you have is a hammer, everything looks like a nail...The real problem is that you need to learn what LINQ is good at and apply it to your code (if it is a good fit), rather than try and make it do what you could with a dynamically built SQL query string.
What you should probably be doing is making those "Where" clauses strongly typed anyway. Your current code has a lot of potential to blow up and be hard to debug.
What you could do instead is something like this (sorry, using C#, been a while since I've touched 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();
With LINQ, "query" isn't actually run until the "ToList()" call. Since it uses late execution, the query is dynamic in that it's being built on until it actually needs to run. This is similar to the code you were looking to use before since you were building a query string ahead of time, then executing it at a specific point.