Expert Advisor Programming by Andrew R. Young

Advanced Order Placement
Here's how we check the trading execution thread using IsTradeContextBusy():
while(IsTradeContextBusy()) Sleep(10);
int Ticket = OrderSend(Symbol(),OP_BUY,LotSize,Ask,UseSlippage,0,0,
"Buy Order",MagicNumber,0,Green);
We use a while loop to evaluate IsTradeContextBusy(). If the function returns true, indicating that
the trade execution thread is occupied, the expert advisor will Sleep for 10 milliseconds. The while
loop will continue to execute as long as IsTradeContextBusy() returns true. Once the trade thread
is freed up, trading will commence.
If the expert advisor attempts to trade while the trade execution thread is occupied, an error 147 :
"trade context busy" will result. Although this method is fairly reliable at avoiding the "trade context
busy" error, it is not foolproof, especially when multiple expert advisors are attempting to trade at the
same time. Later in the book, we will explore ways to retry trade operations after certain error
conditions.
Refreshing Predefined Variables
The values of predefined variables such as Bid and Ask are set when the expert advisor begins its
execution. The amount of time required to execute our expert advisor code is very short, and can be
measured in milliseconds. But when you figure in delays for trade server response, and the fact that
prices can change very rapidly, it's very important that you always use the most current prices.
The RefreshRates() function updates the contents of predefined variables with the latest prices
from the server. It is recommended that you call this function every time you use the Bid or Ask
variables, especially after a prior trade execution.
Note that if you retrieve the price using the MarketInfo() function, it is not necessary to use
RefreshRates(). We covered MarketInfo() on page 29. When we get to the chapter on creating
functions, we will use MarketInfo() to retrieve prices instead of using predefined variables.
However, you may still want to use Bid and Ask in your start() function to reference the current
chart prices.
Error Handling
When placing, modifying or closing orders, errors can occur due to invalid trade parameters,
requotes, or server issues. We've done our best to make sure that the trade parameters we use are
valid and have been checked to prevent common, preventable errors. But when errors do occur, we
need to alert the user of the error and log any relevant information for troubleshooting.
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