.net - LINQ Dynamic Method Call Using Expressions?
May 3, 2011
I'm trying to implement multicolumn filtering using LINQ expressions in a class that extends BindingList(Of T). Here is the relevant code:
Public Function GetFilterPredicate() As Func(Of T, Boolean)
Dim expressionList As List(Of Expression) = New List(Of Expression)
For Each item as FilterInfo in _FilterList
[code]....
However, an exception is thrown at the Expression.Call statement. I can't quite figure out the right arguments to supply. As it is now, I am getting this error when I run the code:
InvalidOperationException was unhandled:No generic method 'Equal' on type 'System.Linq.Expressions.Expression' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.
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Jan 5, 2011
I want to create a simple function that does the following:
Sub SetValue(Of TInstance As Class, TProperty)(
ByVal instance As TInstance,
ByVal [property] As Expression(Of Func(Of TInstance, TProperty)),
[code].....
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Mar 16, 2009
I have an untyped dataset returned from my WebService. The user wants to dynamically construct a query referencing tables and columns and specifying values to test for.I have looked at Lambda Expressions and the Expressions.Expression Namespace. I think these provide the answer I'm looking for, but I'm not certain how to go about contructing the linq expressions to extract the datarows I'm looking for.
I plan on limiting the result to one datatable that the query will result in and i"ll provide the joins from the user's constructs.For a simple example I have a DataTable with (n) rows called "Table", and in it there is a computed column called "b_IsActive" that has the expression "Convert(IsActive, 'System.Boolean')". The user wants to retrieve all rows in "Table" where field "b_IsActive" is true.
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May 23, 2012
The following LINQ query resp. call of the extension method Select in Visual Basic 2010 is working fine:
Dim qAvSalary = qJobData.Select(Function(e) e.AvSalary) But doing so I am not able to specify the name of property I want the query (e.g. AvSalary) using a string variable. This should be possible if I use a LINQ expression tree. Searching and trying a long time on how to translate the query to a corresponding expression tree was not successful. My final approach is:
[Code]...
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Feb 28, 2012
The pattern I'm trying to avoid is checking if a string (normally a control's text value) is null/empty, and if it is, comparing it using Contains to a field in my data. Obviously the field isn't hard-coded into my extension, neither is the object type.What I've got works perfectly in Linq to Objects, but I get the generic run-time error "LINQ to Entities does not recognize the method 'System.String Invoke(GenericQueryHelper.Customer)' method, and this method cannot be translated into a store expression." when using an entity framework model.
Here's what I have:
<System.Runtime.CompilerServices.Extension()>
Public Function CompareAndFilter(Of T)(source As System.Linq.IQueryable(Of T), expressionField As System.Linq.Expressions.Expression(Of System.Func(Of T, String)), compareTo As String)
[code]....
I want my usage to look something like this:
Dim results = repository.Customers.CompareAndFilter(Function(c) c.FirstName, searchText)
I do need to get this running against a SQL database really, as it is filtering results, so I don't want to be doing that in memory.
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Feb 5, 2010
I would like to bind a DataGridView to a different LINQ query every time (in order to reuse the same form/DGV for different queries), like this:
[Code]....
but the "Qry", and the number of comboboxes, and their field names would change every time.
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Mar 22, 2012
I want to create an ExpressionTree where a Func(of Vector, Vector, Vector, Vector, Double, Double, Vektor) should be called. So I use
Expression.Call(Forces(0).Function.Method, {Vec1, Vec2, Vec3, Vec4, Double1, Double2})(Vec1-4, Double1-2 are declared as ParameterExpression and Forces(0).Function as Func(of Vector, Vector, Vector, Vector, Double, Double, Vector). But I get an AgrumentException, because Forces(0).Function.Method has seven Arguments. (System.Runtime.CompilerServices.Closure as seventh argument at Index 0).
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Jul 12, 2011
I converted the following query from C#:
src.Select((c, i) => src.Substring(i)).Count(sub => sub.StartsWith(target))
To the VB.NET query:
src.[Select](Function(c, i) src.Substring(i)).Count(Function([sub]) [sub].StartsWith(target))
Using Developer Fusion. I was just wondering why the VB.NET version has [] throughout.
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Jul 4, 2011
Is there a way to escape all metacharacters in a string with the exception of "*"?
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May 24, 2010
I need to represent the following query using LINQ:
DECLARE @PurchasedInventoryItemID Int = 2
DECLARE @PurchasedInventorySectionID Int = 0
DECLARE @PurchasedInventoryItem_PurchasingCategoryID Int = 3
DECLARE @PurchasedInventorySection_PurchasingCategoryID Int = 0
[code]....
Now, I know that a query in .NET doesnt look like this, this is my test in the SQL Design Studio. Naturally VB.NET variables will be used in place of the SQL local variables.My problem is this: All of the conditions after "WHERE" are optional. In that a query might be made that uses one, some, all, or none of the conditions. V.PropertyID and V.Value can also appear any number of times.In VB.NET I can make this query easy enough by simply concatenating strings, and using a loop to append the "V.PropertyID/V.Value" conditions.I can also make a Stored Procedure in MS SQL, which is easy enough.However, I want to accomplish this using LINQ.
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Nov 16, 2009
I'm still trying to get my head around the whole "Lambda Expressions" thing.
Can anyone here give me an example ordering by multiple columns using VB.Net and Linq-to-SQL using a lambda expression?
Here is my existing code, which returns an ordered list using a single-column to order the results:
Return _dbContext.WebCategories.OrderBy(Function(c As WebCategory) c.DisplayOrder).ToList
Note: The WebCategory object has a child WebPage object (based on a foreign key). I'd like to order by WebPage.DisplayOrder first, then by WebCategory.DisplayOrder.
I tried chaining the order bys, like below, and though it compiled and ran, it didn't seem to return the data in the order I wanted.
Return _dbContext.WebCategories.OrderBy(Function(c As WebCategory) c.DisplayOrder).OrderBy(Function(c As WebCategory) c.WebPage.DisplayOrder).ToList
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Jan 30, 2010
how to create a lamdba expression instead of using a regular comprehension linq queries. All of the examples i see are not query a database using linq to sql. i want to able to construct a lambda expression that get my the orders from the northwind database where the country equals "us". I know how to construct this using the standard linq query, but just not using lambdas in vb.net.
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May 20, 2009
I'm looking for an advanced level VB.NET book which covers LINQ and Lambda Expressions.Generally I read C# .NET books due to lack of good VB.NET books when it comes to generic .NET Framework related subjects. However Lambda and LINQ is quite different in C# and VB.NET I'm looking for an advanced level VB.NET book on this subjec
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Nov 12, 2009
Does calling the Dispose method on a Windows.Forms.Timer call it's Stop method? Or should I stop the timer before I dispose it?
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Sep 10, 2009
Example-I have a person class
Public Class Person
Private _fname As String
Public Property Fname() As String
[Code]...
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Mar 31, 2011
Forgive my ignorance on this.I have this LINQ Query:Dim ngBikersDataContext As New CarBikeWalkDataContext
bikersList = (From c In ngBikersDataContext.Reg_Bikers _
Order By c.L_Name _
Select New Bikers() With { _
.BikerID = c.BikerID, _
.F_Name = c.F_Name, _
[Code]...
with the error "Overload resolution failed because no accesible 'Select' accepts this number of arguments."
Over the "NEW" I get an error " ')'expected."
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Feb 16, 2010
I will try to explain what I need.Let's say that I have a class like this:
Public Class Example1 Public Sub ToBeCalled()
[Code]...
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Jan 29, 2012
I have a Application that Crypts all Files in a Directory and the Subdirectories
Public Shared Sub CryptAllFiles(ByVal crypt As Object)
'check if this dir exists
Dim vDirInfo As New DirectoryInfo(vPath)
If Not vDirInfo.Exists Then Exit Sub
'get all files' sizes in current path
[Code]...
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Nov 24, 2010
I have a list of properties and values that i'd like to use to dynamically build an Expression(Of Func(Of MyClass,MyClass))
I can run through the list and create each Expression by itself, but the only way I know how to combine them would be to use Expression.And or Expression.AndAlso, but that returns a BinaryExpression rather than my original Expression(Of Func(Of MyClass,MyClass)).
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Mar 17, 2012
i want to which way is better when concentration is on processing speed. i want to call a method that compute some data that method is on another form in my windows form application then what you prefer is better way 1) to call that method in another form and use returned value or create similar method in form where it is required.i want to make it's processing fast.
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May 2, 2012
How do you select a chart based on its name? I have a number of charts in a form and I need to assign data to particular charts at particular times. I would like to have a simple code sequence to assign data to the appropriate chart depending[code]....
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Oct 28, 2011
Names of entities have been altered to protect their identities...
I've created a class called AnimalSearch(Of AnimalType As Animal(Of Int32))
Public Class AnimalSearch(Of AnimalType As Animal(Of Int32))[code]...
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Sep 14, 2010
How would I get something like this to work so that I can dynamically alter the where-clause in this linq to sql query?
Dim AccountID = 1234
Dim AccountList
Select Case Types
[Code]....
With the above code I get this error:
Late binding operations cannot be converted to an expression tree
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May 16, 2012
I am looking to make a database call based on user input. The line I am using is:general_read.CommandText = "SELECT * FROM general WHERE Business_Name = '" & business_name & "';"
business_name is dynamically loaded from:Protected Sub btnLoad_Click(sender As Object, e As System.EventArgs) Handles btnLoad.Click business_name = txtName.Text
[Code]...
The error that is being shown is: Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.I'm assuming it has something to do with the variable not being set before the database call is made, but I'm not entirely sure.
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Mar 12, 2009
I work with Access and VB6 for quite a while. I have read the thread of Phillip Maerz, but there was still the problem that the Function names had to be written in the delegate array. Thats why I started a new thread.Following Problem:How can I call a funktion dymacally, something like thisPublic Function Test1()Returns 1End Function Public Function Test2()Returns 2End Function Sub Main()dim i, r as integeri=2
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Mar 14, 2011
I have been trying the following but it returns unexpected results:
Dim xd As XDocument = _
<?xml version="1.0" encoding="utf-8"?>
<root>
[code].....
The above result returns both 'element' nodes however it should only return the first where element/subelement@id=1/subsubelement@id=3 However if results.Ancestors. is used it returns the correct 'subelement' and if that line is not included it returns a single 'subsubelement' whih is also correct I don't understand why when mvoing to the 'element' it returns both (I realise both have a subelement with id=1 but I thought each further query would filter out the presvious results)?
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Feb 24, 2012
UPDATE: Groo's answer was marked correct as it would be a good alternative. I decided to stick with the select/switch statements to avoid the performance issues of using reflection. As far as I can tell there is no way to do what I want without using Dynamic LINQ (and reflection) or a decompiler (to manually code each LINQ statement, which I am already doing by hand). Note: to make this update DropDownList16 through DropDownList20 were removed (needed the characters).Is there a way to abstract the building of a dynamic Linq to SQL query?
I am trying to build a Linq to SQL query with a dynamic where clause, based on user supplied filters. The user needs to be able to filter strings, integers, and dates using advanced options (equal, not equal, contains, starts with, etc). The user needs to be able to use these filters with as many or few columns as desired.
[Code]...
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Jan 5, 2010
Dim query=Form f in BBSDb.Forms order by f.FormID Descending Select f where I want the f.FormID to be able to replace with a variable..What I mean is.. it doesn't have to f.FormID .. I may want to set f.FormName to be in descending for example using the code dynamically.. Is there anyway to achive?
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Mar 16, 2011
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
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Oct 17, 2011
This is an extension of my last question which was answered, so I am starting a new thread. I have a query in LINQ that I was given to trim down the columns of a datatable. This works well, however, I need to make the LINQ query dynamic. So the query below:
Dim qColumnsIWant = From row As DataRow In inDT.AsEnumerable() _
Select _
EPN = row.Field(Of Integer)("EPNID"), _
EPNID = row.Field(Of String)("EPNNumber"), _
Ingot = row.Field(Of String)("Ingot"), _
ShopOrder = row.Field(Of String)("ShopOrder")
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