Описание тега streamwriter

In.Net StreamWriter (under System.IO) is designed for character output in a particular encoding, whereas classes derived from Stream are designed for byte input and output.

StreamWriter defaults to using an instance of UTF8Encoding unless specified otherwise. This instance of UTF8Encoding is constructed without a byte order mark (BOM), so its GetPreamble method returns an empty byte array. The default UTF-8 encoding for this constructor throws an exception on invalid bytes. This behavior is different from the behavior provided by the encoding object in the Encoding.UTF8 property. To specify a BOM and determine whether an exception is thrown on invalid bytes, use a constructor that accepts an encoding object as a parameter, such as StreamWriter(String, Boolean, Encoding) or StreamWriter.

By default, a StreamWriter is not thread safe
See official documentation from Microsoft.