DataServiceContext.BeginExecute(Of TElement) Method (Uri, AsyncCallback, Object)


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Asynchronously sends the request so that this call does not block processing while waiting for the results from the service.

Namespace:   System.Data.Services.Client
Assembly:  System.Data.Services.Client (in System.Data.Services.Client.dll)

Public Function BeginExecute(Of TElement) (
	requestUri As Uri,
	callback As AsyncCallback,
	state As Object
) As IAsyncResult


Type: System.Uri

The URI to which the query request will be sent. The URI may be any valid data service URI; it can contain $ query parameters.

Type: System.AsyncCallback

Delegate to invoke when results are available for client consumption.

Type: System.Object

User-defined state object passed to the callback.

Return Value

Type: System.IAsyncResult

An object that is used to track the status of the asynchronous operation.

Type Parameters


The type returned by the query.

The returned IAsyncResult object is used to determine when the asynchronous operation has completed. For more information, see Asynchronous Operations (WCF Data Services).

The method BeginExecute(Of TElement) uses the same semantics as Execute(Of TElement), however this method asynchronously sends the request so that this call does not block processing while waiting for the results from the service. According to the standard begin-end asynchronous pattern, the provided callback is invoked when query results are retrieved.

The following example shows how to execute an asynchronous query by calling the BeginExecute method to start the query. The inline delegate calls the EndExecute method to display the query results. This example uses the DataServiceContext generated by the Add Service Reference tool based on the Northwind data service, which is created when you complete the WCF Data Services .

Public Shared Sub BeginExecuteCustomersQuery()
    ' Create the DataServiceContext using the service URI.
    Dim context = New NorthwindEntities(svcUri)

    ' Define the delegate to callback into the process
    Dim callback As AsyncCallback = AddressOf OnCustomersQueryComplete

    ' Define the query to execute asynchronously that returns 
    ' all customers with their respective orders.
    Dim query As DataServiceQuery(Of Customer) = _

        ' Begin query execution, supplying a method to handle the response
        ' and the original query object to maintain state in the callback.
        query.BeginExecute(callback, query)
    Catch ex As DataServiceQueryException
        Throw New ApplicationException( _
                "An error occurred during query execution.", ex)
    End Try
End Sub
' Handle the query callback.
Private Shared Sub OnCustomersQueryComplete(ByVal result As IAsyncResult)
    ' Get the original query from the result.
    Dim query As DataServiceQuery(Of Customer) = _
        CType(result.AsyncState, DataServiceQuery(Of Customer))

    ' Complete the query execution.
    For Each customer As Customer In query.EndExecute(result)
        Console.WriteLine("Customer Name: {0}", customer.CompanyName)
        For Each order As Order In customer.Orders
            Console.WriteLine("Order #: {0} - Freight $: {1}", _
                    order.OrderID, order.Freight)
End Sub

.NET Framework
Available since 3.5
Available since 2.0
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