Unsure how to use a C# StreamWriter in the decorator pattern

I have a StreamOutput class that looks like this:

class StreamOutput : Output
{
    private StreamWriter sink;

    public StreamOutput(StreamWriter stream)
    {
        sink = stream;
    }
    public string write(Object o) 
    {
        return writeString(o.ToString());
    }
    public string writeString(String s)
    {
        try 
        {
            sink.Write(s);
        }
        catch (IOException) 
        {
            Console.WriteLine("IO Exception");
        }
        return s;
    }
}

The interface from which it inherits is this:

public interface Output
{
    string writeString(String s);
}

And I have a few decorators that look like this one:

class LineOutput : Output
{
    Output theOutput;

    public LineOutput(Output theOutput)
    {
        this.theOutput = theOutput;
    }
    public string writeString(String s)
    {
        return theOutput.writeString(s) + "\n";
    }
}

In my Main function, I have an Output object called output, created like this:

StreamWriter writer = new StreamWriter("output.txt");
Output output = new StreamOutput(writer);

that does the decorating by the line

output = new LineOutput(output);

The objective of this decorator is to get a bunch of lines of text from a file, which happens in Main, and then LineOutput double-spaces it all. The problem is, when I write it to a file, it doesn't get double-spaced and, in fact, loses its line formatting altogether.

Any thoughts on if I need to fix the StreamOutput class, and in what way, or if I need to fix the decorators to return something else?

Thanks.