This ProBuilder code snippet demonstrates how to set a buy limit order with a specific validity period measured in bars. The order is placed at the close price of a bar where a moving average crossover occurs and remains valid for a predefined number of bars. If the order is not executed within this period, it is automatically cancelled, and optionally, a market order can be placed instead.

// Definition of the validity length of the order
ONCE NbBarLimit = 10
MM20 = Average[20](close)
MM50 = Average[50](close)

// If MM20 crosses over MM50, we define 2 variables "MyLimitBuy" and "MyIndex" containing the close price at that time and the index of the bar of the cross.
IF MM20 CROSSES OVER MM50 THEN
    MyLimitBuy = close
    MyIndex = Barindex
ENDIF

IF BarIndex >= MyIndex + NbBarLimit THEN
    MyLimitBuy = 0
ENDIF

// Place an order at the price MyLimitBuy valid as long as this variable is greater than 0 and we are not in a long position.
// Remember: MyLimitBuy is greater than 0 for the 10 bars after the bar of the crossing.
IF MyLimitBuy > 0 AND NOT LongOnMarket THEN
    BUY 1 SHARES AT MyLimitBuy LIMIT
ENDIF

// In case the order was not executed, it is possible to replace the expired buy limit order with a buy at market price order.
IF MyIndex + NbBarLimit AND MyLimitBuy > 0 AND NOT LongOnMarket THEN
    BUY 1 SHARES AT MARKET
ENDIF

Explanation of the code:

• Setting the Order Validity: The variable NbBarLimit is set to 10, defining the number of bars for which the limit order will remain valid.
• Detecting the Crossover: Two moving averages (MM20 and MM50) are calculated. When MM20 crosses over MM50, the closing price and the bar index at the time of the crossover are stored in MyLimitBuy and MyIndex respectively.
• Order Expiry: If the current bar index is greater than or equal to the sum of MyIndex and NbBarLimit, MyLimitBuy is set to 0, effectively cancelling the order if it has not been executed.
• Placing the Limit Order: A buy limit order is placed if MyLimitBuy is greater than 0 and there is no existing long position. The order is placed at the price stored in MyLimitBuy.
• Handling Unexecuted Orders: If the limit order has not been executed by the end of its validity period and there is still no long position, a market order is placed to buy 1 share.