Error Using SQL Datatype Text As Output Parameter From C# Data Access Layer

Hello,

My datalayer of C# code is invoking a stored procedure that returns a varchar(max) SQL data type. In my ASP.NET code, I have:

SqlCommand myCommand = new SqlCommand("usp_GetTestString", myConnection);
myCommand.Parameters.Add(new SqlParameter("@TestString", SqlDbType.Text));
myCommand.Parameters["@TestString"].Direction = ParameterDirection.Output;
myConnection.Open();
myCommand.ExecuteNonQuery();
return Convert.ToString(myCommand.Parameters["@TestString"].Value);

The query fails to execute and returns an error: String[1]: the Size property has an invalid size of 0. If I change the SqlDbType.Text parameter type to SqlDBType.Varchar, 100 (or any other fixed varchar length), it works but limits the length my unlimited field text. Any suggestions on how I can use db type text or varchar(max)? The field I need to retrieve is string characters of unlimited length and
hence the datatype varchar(max).