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Pass LINQ where clause as Parameter
It really is a simple question yet finding so much trouble with Google finding how to do it. After the where clause I want to return the results from my strongly typed dataset table with a parameter passed from a parameter with vb.net and how to call it. If you can assist I would appreciate it.
Bolded is where I think it should go. But I clearly don't know how to say it. "test" would be something like
"row.Item(columnname) IsNot DBNull.Value AndAlso row.Item(columnname) = filter AndAlso row.TT_Rec_ID = Template_ID" just for example. But the way I see it we should be able to pass anything through that is LINQ compatible.
Bolded is where I think it should go. But I clearly don't know how to say it. "test" would be something like
"row.Item(columnname) IsNot DBNull.Value AndAlso row.Item(columnname) = filter AndAlso row.TT_Rec_ID = Template_ID" just for example. But the way I see it we should be able to pass anything through that is LINQ compatible.
Function Global_Table_Query([b]ByVal test As Predicate(Of ds2.JOINEDDATADataTable[/b])) As DataTable
ErrorMessage = ""
Dim dt As New DataTable
Dim Is_Good As Byte = 1
Dim thread As New Task(
Function()
Try
Dim lowNums = From row In Global_Table
[b]Where test
[/b] Select row
dt = lowNums.CopyToDataTable
Return dt
Catch ex As Exception
ErrorMessage = ex.Message
Return Nothing
Finally
End Try
End Function
)
Dim tasks() = {thread}
Array.ForEach(tasks, Sub(tx) tx.Start())
Task.WaitAll(tasks)
For Each tsk In tasks
Select Case True
Case tsk.IsCanceled
Is_Good = 1
Case tsk.IsFaulted
Is_Good = 1
Case tsk.IsCompleted
Is_Good = 0
Case Else
Is_Good = 1
End Select
Next
tasks = Nothing
thread = Nothing
'checking if something went wrong
If ErrorMessage <> "" Then
Is_Good = Nothing
Return Nothing
End If
Return dt
End Function
Who is Participating?
Hi Kelly;
In answer to your question is that this can not be done right out of the box with VB.Net or any .Net language. The reason is that .Net is a type safe system and objects like this are checked at compile time so "This means you get compile-time checking of your LINQ queries, and full intellisense and refactoring support over your code:". If you want to do something like this you can use a library such as Dynamic LINQ which has been imported into a NuGet package called System.Linq.Dynamic.Core. Using this library the Where clause becomes a string it also means that you do not get compile time checking of the Linq query and can now get run time exceptions, so you will need to validate the string to make sure that it is correct.
In answer to your question is that this can not be done right out of the box with VB.Net or any .Net language. The reason is that .Net is a type safe system and objects like this are checked at compile time so "This means you get compile-time checking of your LINQ queries, and full intellisense and refactoring support over your code:". If you want to do something like this you can use a library such as Dynamic LINQ which has been imported into a NuGet package called System.Linq.Dynamic.Core. Using this library the Where clause becomes a string it also means that you do not get compile time checking of the Linq query and can now get run time exceptions, so you will need to validate the string to make sure that it is correct.
You could pass the LINQ expression as a parameter of your function, thus you can still reuse the function logic for different LINQ expressions that provide the same result type.
Miguel is correct, you could pass an expression that is generated during run-time; refer to this previous EE PAQ.
That being said, in order for the expression to be generated in the way you want to use it, you will need to collect the meta-data/schema of the database in order to dynamically build your expressions. If you are dealing with stronly-typed classes which represent your meta-data/schema, this shouldn't pose a problem.
-saige-
That being said, in order for the expression to be generated in the way you want to use it, you will need to collect the meta-data/schema of the database in order to dynamically build your expressions. If you are dealing with stronly-typed classes which represent your meta-data/schema, this shouldn't pose a problem.
-saige-
Miguel and it-sage .... can you show me how you would do the different approaches you have talked about here? Fernando I will check out your idea ....
Thank you all for your efforts. This is the answer I came up with if your interested....
https://kellyschronicles.wordpress.com/2017/12/16/dynamic-predicate-for-a-linq-query/
https://kellyschronicles.wordpress.com/2017/12/16/dynamic-predicate-for-a-linq-query/
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Form1.cs -
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Form1.Designer.cs -Open in new window
Which produces the following output -Initial Load -
-saige-