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Where Clause

Where Clause (Visual Basic)

Specifies the filtering condition for a query.

Where condition

condition

Required. An expression that determines whether the values for the current item in the collection are included in the output collection. The expression must evaluate to a Boolean value or the equivalent of a Boolean value. If the condition evaluates to True, the element is included in the query result; otherwise, the element is excluded from the query result.

The Where clause enables you to filter query data by selecting only elements that meet certain criteria. Elements whose values cause the Where clause to evaluate to True are included in the query result; other elements are excluded. The expression that is used in a Where clause must evaluate to a Boolean or the equivalent of a Boolean, such as an Integer that evaluates to False when its value is zero. You can combine multiple expressions in a Where clause by using logical operators such as And, Or, AndAlso, OrElse, Is, and IsNot.

By default, query expressions are not evaluated until they are accessed—for example, when they are data-bound or iterated through in a For loop. As a result, the Where clause is not evaluated until the query is accessed. If you have values external to the query that are used in the Where clause, ensure that the appropriate value is used in the Where clause at the time the query is executed. For more information about query execution, see Writing Your First LINQ Query (Visual Basic).

You can call functions within a Where clause to perform a calculation or operation on a value from the current element in the collection. Calling a function in a Where clause can cause the query to be executed immediately when it is defined instead of when it is accessed. For more information about query execution, see Writing Your First LINQ Query (Visual Basic).

The following query expression uses a From clause to declare a range variable cust for each Customer object in the customers collection. The Where clause uses the range variable to restrict the output to customers from the specified region. The For Each loop displays the company name for each customer in the query result.

Sub DisplayCustomersForRegion(ByVal customers As List(Of Customer), _
                              ByVal region As String)

  Dim customersForRegion = From cust In customers _
                           Where cust.Region = region

  For Each cust In customersForRegion
    Console.WriteLine(cust.CompanyName)
  Next 
End Sub

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