web service error ' it does not have a parameterless constructor '

Question

I've created a web service , which can a method to set the user credential using Microsoft.Web.Services3.WebServicesClientProtocol. The sample code is :

<WebMethod()> _
    Public Sub ClientCredential1(Of TSecurityToken As SecurityToken)_
         (ByVal UserCred As Microsoft.Web.Services3.Security.Tokens.UsernameToken)

        Dim cProxy As New Microsoft.Web.Services3.WebServicesClientProtocol()
        cProxy.SetClientCredential(UserCred)
    End Sub

When I run the web service it gives this error:

"Microsoft.Web.Services3.Security.Tokens.UsernameToken cannot be serialized because it does not have a parameterless constructor."

Does any one know where is the problem ?

Hey! Welcome to StackOverflow! If you'd like to format you posts so they are more readable and more likely to be answered, please read the [StackOverflow Markup Guide](http://stackoverflow.com/editing-help). Good Luck with your Question! — Tyler Carter, Aug 07 '10 at 05:35
I take it to mean that to initialize or create a new variable of type UserCred, you must send something along with the New() constructor. IE, Dim myCreds as new userCred(param1, param2). Since I don't know for sure, I am not posting this as an answer, but I would start there. — Tommy, Aug 07 '10 at 06:09

score 2 · Accepted Answer · answered Aug 07 '10 at 06:29

The root of the problem here is that the class Microsoft.Web.Services3.Security.Tokens.UsernameToken doesn't have a parameter-less constructor. It's got 3 of them, but they all demand a parameter. UsernameToken constructors on MSDN.

UsernameToken (XmlElement)
UsernameToken (String, String)
UsernameToken (String, String, PasswordOption)

The problem is that during deserialization, XmlSerializer calls the parameterless constructor to create an instance of that class. It can't deserialize a type that doesn't have a parameterless constructor.

I get the sense there's not much you can do to work around this problem. I'd only suggest creating a partial class, and implementing that zero-param constructor yourself.

'ensure namespacing is correct.
Public Partial Class UsernameToken
    Public Sub New()
    End Sub    
End Class

Since UsernameToken is defined in a referenced assembly, I don't think you can add a constructor using a partial class definition... I think that only works when you have the source code for all parts of the class. — Richard Beier, Aug 07 '10 at 06:36

score 0 · Answer 2 · answered Aug 07 '10 at 06:43

p. campbell is right, it's because the XmlSerializer requires a parameterless constructor.

I don't know WSE, but from looking at this post on Aleem's Weblog, I don't think the UsernameToken is supposed to be passed as a regular argument to a web method - it's supposed to be passed in the WS-Security SOAP headers. You get the proxy to pass it in the headers by calling SetClientCredential(). Here's the example from the above blog post:

Dim oService As New WSETestService.ServiceWse

Dim U As New UsernameToken(“<User_Name>”, “<Password>”, PasswordOption.SendHashed)
oService.SetClientCredential(U)

score 0 · Answer 3 · answered Aug 07 '10 at 06:53

You can't use a parameter of the type Microsoft.Web.Services3.Security.Tokens.UsernameToken in a web service, as it's not possible to serialise (or more specifically not possible to deserialise).

Create a class that just contains the data that you need to create a UsernameToken and use as parameter type. The client side would not create a real UsernameToken object anyway, there is a proxy class created from the WSDL information.

web service error ' it does not have a parameterless constructor '

3 Answers3