How do I append a 'where' clause using VB.NET and LINQ?

Question

I am pretty new to VB.NET and am having a bit of trouble here with something I thought should be simple.

Keeping it simple, let's say I have a Document table with "Name" that I want to search on (in reality there are several other tables, joins, etc. ..). I need to be able to build the query using a where clause based on string values passed in.

Example - the user may pass in "ABC", "ABC DEF", "ABC DEF GHI".

The final query would be (the syntax is not correct, I know):

Select * from Documents Where Name Like %ABC% AND Name Like %DEF% AND Name like %GHI%

So, I thought I could do something like this.

Dim query = From document In _context.Documents

<< loop based on number of strings passed in >>
query = query.Where( ... what goes here?? )

For some reason, being brain-dead or something, I can't figure out how to make this work in VB.NET, or if I'm doing it correctly.

Solution 1

I believe this is how you would do it in VB (I'm a C# developer):

query = query.Where(Function(s) s = "ABC")

See LINQ - Sample Queries for some examples.

Solution 2

I think the tricky part here is the unknown number of query parameters. You can use the underlying LINQ IQueryable(Of T) here to help.

I think the following would work (it's not compiled, just notepad code here):

Public Function GetDocuments(criteria as String)
    Dim splitCriteria = SplitTheCriteria(criteria)

    dim query = from document in _context.Documents

    For Each item in splitCriteria
        Dim localItem = item
        query = AddCriteriaToQuery(query, localItem)
    Next

    dim matchingDocuments = query.ToList()
End Function

Private Function AddCriteriaToQuery(query as IQueryable(Of Document), criteria as string) as IQueryable(Of Document)
     return query.Where(Function(doc) doc.Name = criteria)
End Function

Since LINQ will delay-execute the query you can append where clauses onto your query in the loop and then call .ToList() at the end to execute the query.

Solution 3

In LINQ to SQL you can add WHERE clauses to your query using the .Where method of the query object, as you noted in your question. To use the LIKE operator, try using the .Contains method of the object you're querying in the Lambda expression of your call to the Where method.

Here's a simplified example in a console application. Hopefully it will lead you in the correct direction.

Public Class Doc

    Private _docName As String
    Public Property DocName() As String
        Get
            Return _docName
        End Get
        Set(ByVal value As String)
            _docName = value
        End Set
    End Property

    Public Sub New(ByVal newDocName As String)
        _docName = newDocName
    End Sub
End Class

Sub Main()
    Dim Documents As New List(Of Doc)
    Documents.Add(New Doc("ABC"))
    Documents.Add(New Doc("DEF"))
    Documents.Add(New Doc("GHI"))
    Documents.Add(New Doc("ABC DEF"))
    Documents.Add(New Doc("DEF GHI"))
    Documents.Add(New Doc("GHI LMN"))

    Dim qry = From docs In Documents

    qry = qry.Where(Function(d) d.DocName.Contains("GHI"))

    Dim qryResults As List(Of Doc) = qry.ToList()

    For Each d As Doc In qryResults
        Console.WriteLine(d.DocName)
    Next

End Sub

Note the .Contains("GHI") call in the Lambda expression of the .Where method. I'm referencing the parameter of the expression, "d", which exposes the DocName property, which further exposes the .Contains method. This should produce the LIKE query you're expecting.

This method is additive, i.e. the call to the .Where method could be enclosed in a loop to make additional LIKE operators added to the WHERE clause of your query.

Solution 4

Dim query = From document In _context.Documents where document.name = 'xpto' select document 

Or

Dim query = From document In _context.Documents where document.name.contains('xpto') select document 

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