CallbackBehaviorAttribute.ValidateMustUnderstand CallbackBehaviorAttribute.ValidateMustUnderstand CallbackBehaviorAttribute.ValidateMustUnderstand CallbackBehaviorAttribute.ValidateMustUnderstand Property


Gets or sets a value that specifies whether the system or the application enforces SOAP MustUnderstand header processing.

 property bool ValidateMustUnderstand { bool get(); void set(bool value); };
public bool ValidateMustUnderstand { get; set; }
member this.ValidateMustUnderstand : bool with get, set
Public Property ValidateMustUnderstand As Boolean

Property Value

true if the system is to perform SOAP header MustUnderstand processing; otherwise false, which indicates that the application performs this processing. The default value is true.


The following code example shows a CallbackBehaviorAttribute on a callback object that uses the SynchronizationContext object to determine which thread to marshal to, the ValidateMustUnderstand property to enforce message validation, and the IncludeExceptionDetailInFaults property to return exceptions as FaultException objects to the service for debugging purposes.

using System;
using System.ServiceModel;
using System.ServiceModel.Channels;
using System.Threading;

namespace Microsoft.WCF.Documentation
   IncludeExceptionDetailInFaults= true, 
  public class Client : SampleDuplexHelloCallback
    AutoResetEvent waitHandle;

    public Client()
      waitHandle = new AutoResetEvent(false);

    public void Run()
      // Picks up configuration from the configuration file.
      SampleDuplexHelloClient wcfClient
        = new SampleDuplexHelloClient(new InstanceContext(this), "WSDualHttpBinding_SampleDuplexHello");
        Console.ForegroundColor = ConsoleColor.White;
        Console.WriteLine("Enter a greeting to send and press ENTER: ");
        Console.Write(">>> ");
        Console.ForegroundColor = ConsoleColor.Green;
        string greeting = Console.ReadLine();
        Console.ForegroundColor = ConsoleColor.White;
        Console.WriteLine("Called service with: \r\n\t" + greeting);
        Console.WriteLine("Execution passes service call and moves to the WaitHandle.");
        Console.ForegroundColor = ConsoleColor.Blue;
        Console.WriteLine("Set was called.");
        Console.Write("Press ");
        Console.ForegroundColor = ConsoleColor.Red;
        Console.ForegroundColor = ConsoleColor.Blue;
        Console.Write(" to exit...");
      catch (TimeoutException timeProblem)
        Console.WriteLine("The service operation timed out. " + timeProblem.Message);
      catch (CommunicationException commProblem)
        Console.WriteLine("There was a communication problem. " + commProblem.Message);
    public static void Main()
      Client client = new Client();

    public void Reply(string response)
      Console.WriteLine("Received output.");
      Console.WriteLine("\r\n\t" + response);

Imports System.ServiceModel
Imports System.ServiceModel.Channels
Imports System.Threading

Namespace Microsoft.WCF.Documentation
  <CallbackBehaviorAttribute(IncludeExceptionDetailInFaults:= True, UseSynchronizationContext:=True, ValidateMustUnderstand:=True)> _
  Public Class Client
	  Implements SampleDuplexHelloCallback
	Private waitHandle As AutoResetEvent

	Public Sub New()
	  waitHandle = New AutoResetEvent(False)
	End Sub

	Public Sub Run()
	  ' Picks up configuration from the configuration file.
	  Dim wcfClient As New SampleDuplexHelloClient(New InstanceContext(Me), "WSDualHttpBinding_SampleDuplexHello")
		Console.ForegroundColor = ConsoleColor.White
		Console.WriteLine("Enter a greeting to send and press ENTER: ")
		Console.Write(">>> ")
		Console.ForegroundColor = ConsoleColor.Green
		Dim greeting As String = Console.ReadLine()
		Console.ForegroundColor = ConsoleColor.White
		Console.WriteLine("Called service with: " & Constants.vbCrLf & Constants.vbTab & greeting)
		Console.WriteLine("Execution passes service call and moves to the WaitHandle.")
		Console.ForegroundColor = ConsoleColor.Blue
		Console.WriteLine("Set was called.")
		Console.Write("Press ")
		Console.ForegroundColor = ConsoleColor.Red
		Console.ForegroundColor = ConsoleColor.Blue
		Console.Write(" to exit...")
	  Catch timeProblem As TimeoutException
		Console.WriteLine("The service operation timed out. " & timeProblem.Message)
	  Catch commProblem As CommunicationException
		Console.WriteLine("There was a communication problem. " & commProblem.Message)
	  End Try
	End Sub
	Public Shared Sub Main()
	  Dim client As New Client()
	End Sub

	Public Sub Reply(ByVal response As String) Implements SampleDuplexHelloCallback.Reply
	  Console.WriteLine("Received output.")
	  Console.WriteLine(Constants.vbCrLf & Constants.vbTab & response)
	End Sub
  End Class
End Namespace


Use the ValidateMustUnderstand property to turn off the enforced validation of arriving message headers. In normal execution, message headers are compared against the UnderstoodHeaders property to confirm that arriving headers are explicitly handled by the callback object runtime. Set this to false to disable this checking. When set to false, the application must check for headers marked with MustUnderstand="true" and return a fault if one or more of them were not understood. This is useful when the callback is supposed to accept any incoming SOAP messages (for example, when using typed or untyped messages) and perform custom header processing.

Applies to