how to write string to response.outputstream in c#

Response.OutputStream returns object of System.IO.Stream.

Have a look at below link.
http://msdn.microsoft.com/en-us/library/system.web.httpresponse.outputstream.aspx

You may convert object of System.IO.Stream to String as below.

using (StreamReader reader = new StreamReader(yourStream))
{
  string myString = reader.ReadToEnd();
}

Updated - Answer Updated based on your comment posted in Solution-1.

string s = "YourString";
byte[] bytes = System.Text.Encoding.UTF8.GetBytes(s);

Response.OutputStream.Write(bytes, 0, bytes.Length);

HttpResponse.OutputStream property is of type System.IO.Stream

System.IO.Stream has built in ToString method to convert the current object into string content. So, you can write the code as:

myResponse.OutputStream.ToString()