Getting "Object is read only" error when setting ClientCredentials in WCF

I noticed that after creating an instance of the proxy class for the service, I can set the Username and Password once without errors and do a successful call to my webservice. When I then try to set the Username and Password again on the existing instance (unnecessary of course) I get the 'Object is Read-Only' error you mentioned. Setting the values once per instance lifetime worked for me.

It appears that you can only access these properties pretty early in the instanciation cycle. If I override the constructor in the proxy class (ServerClient), I'm able to set these properties:

base.ClientCredentials.UserName.UserName = "Sample";

I'm beginning to appreciate the people who suggest not using the automatically built proxies provided by VS.