试试这样

public void Write(TextWriter tw, object o) {
    if(tw == null) {
        throw new ArgumentNullException("tw");
    }
    if(o == null) {
        throw new ArgumentNullException("o");
    }
    tw.WriteLine(o.ToString());
}

你应该用

public void Write(TextWriter tw, object o) {
    Contract.Requires(tw != null);
    Contract.Requires(o != null);
    tw.WriteLine(o.ToString());
}