Windows Media SDK and stringbuilder marshalling

[superdawgie]

Hi,

Im have implemented the IWMCredentialCallback for a windows media pushsink in my application.

My pushsink works fine for publishing to the windows media server when no authentication is required, but when credentials are required, my implentation of AcquireCredentials is called, but i cannot seem to copy my user details into the stringbuilder buffers successfully.


I am using this ComImport...

..
 
    [ComImport]
    [Guid("342e0eb7-e651-450c-975b-2ace2c90c48e")]
        [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IWMCredentialCallback
    {
        void AcquireCredentials(
          [In, MarshalAs(UnmanagedType.LPWStr)] string bstrRealm,
          [In, MarshalAs(UnmanagedType.LPWStr)] string bstrSite,
          [In, Out, MarshalAs(UnmanagedType.LPWStr)] StringBuilder pwszUser,
          [In] uint cchUser,
          [In, Out, MarshalAs(UnmanagedType.LPWStr)] StringBuilder pwszPassword,
          [In] uint cchPassword,
          [In] IntPtr hrStatus,
          [In, Out] ref uint pdwFlags);
    }
 
..

And this is my method implementation...

..
 
        public void AcquireCredentials(string bstrRealm, string bstrSite, StringBuilder pwszUser, uint cchUser, StringBuilder pwszPassword, uint cchPassword, IntPtr hrStatus, ref uint pdwFlags)
        {
 
        // the bstrRealm here is correct as the name of the server i am publishing to
 
 
        pwszUser.Append("username");        // these lines throw "System.ArgumentOutOfRangeException"
        pwszPassword.Append("password");    // capacity was less than the current size
 
 
 
 
        pdwFlags = 0x00;
 
        }
 
..

i have tried various things to copy the username into the stringbuilder
the pwszUser has a maxcapacity of zero, and so i cannot write anything into it
i cannot reset the capacity of it either, as it throws "capacity exceeds maxcapacity"

pwszPassword also shows zero capacity, however when the method is called a second and third time trying to authenticate,
its buffer shows a capacity of 8 and prints '*' chars to the debug output

my understanding of using stringbuilder for marshal is to make sure the capacity is set first.
This method is initially called by the media server when the pushsink connects, so i have no control over this.

From Microsoft description of the SDK AcquireCredentials, the value of cchUser is supposed to be the buffer size allocated to pwszUser.
This is clearly not happening.

Perhaps i have implemented this interface the wrong way?


Can anyone help me with this?