C# Properties getting null values back instead of actual values

I am trying to have a form which gets submitted and passes various text box parameters down along the way.  So I have properties in class for that with private strings (didn't use the auto-implemented properties but tried both).

Here is where the values originate from a WPF form:

            var exe = new DataReadWrite();
            var uv = new UserValues();

            uv.strFirstName = tbxFirstName.Text;
            uv.strLastName = tbxLastName.Text;
            uv.strEmailAddress = tbxEmailAddress.Text;

            exe.exeInsUser();

In the debugger the values above are populated...

Now when exeInsUser() chokes, it is due to the values being null as here:

separate class to hold the values:

    public class UserValues
    {
        private String firstname;
        private String lastname;
        private String emailaddress;

        public string strFirstName
        {
            get
            {
                return firstname;
            }

            set
            {
                firstname = value;
            }
        }

        public string strLastName
        {
            get
            {
                return lastname;
            }

            set
            {
                lastname = value;
            }
        }

        public string strEmailAddress
        {
            get
            {
                return emailaddress;
            }

            set
            {
                emailaddress = value;
            }
        }


The execution fails here with a null reference exception:

separate class to execute:

            var oPa = new DataParameters();
            var str = new UserValues();

            StoredProcParameter[] oPL = oPa.insUsersParm( str.strFirstName
                                                        , str.strLastName
                                                        , str.strEmailAddress);

I even tried implementing an interface (although the code above has this removed in the implementation) but everything is null!?

Any help is greatly appreciated!