In the C# code below, I'm having trouble when I try to use a bind variable
for the column name in an update command.  The example below works fine when
I use the first sqlstr spelling out the column name.  When I try to use the
second sqlstr using @columnName I get an error saying string or binary data
would be truncated.  Below the code I have also pasted the command as sql
profiler shows it.  It looks ok to me.

Anyone know the cause?
thanks,
T
 public static string UOrderField(string columnname, string value, int key)
   {
       SqlConnection conn = new
SqlConnection((string)HttpContext.Current.Session["VTDBConnectionString"]);
       conn.Open();
       int rowsAffected;
       //string sqlstr = "update orders set problemdesc = @value where
(ordernumber = @key)";
       string sqlstr = "update orders set @columnname = @value where
(ordernumber = @key)";
       SqlCommand cmd = new SqlCommand(sqlstr, conn);
       cmd.Parameters.Add(new SqlParameter("@columnname", columnname));
       cmd.Parameters.Add(new SqlParameter("@key", key));
       cmd.Parameters.Add(new SqlParameter("@value", value));
       try
       {
           rowsAffected = cmd.ExecuteNonQuery();
           if (rowsAffected == 0)
           {
               return "Zero user emails were changed";
           }
           return "";
       }
       catch (Exception ex)
       {
           return ex.Message;
       }
       finally
       {
           conn.Close();
       }

   }

exec sp_executesql N'update orders set @columnname = @value where
(ordernumber = @key)',N'@columnname nvarchar(11),@key int,@value
nvarchar(12)',@columnname=N'problemdesc',@key=124063,@value=N'anothervalue'