CIS162AD – C#

1. CIS162AD – C# Call-by-Reference Methods 06_methods_by_ref.ppt

2. Overview Of Topics • Review Top-Down Design • Calling Event Methods • Review pass-by-value Arguments • Introduce Void Methods • Introduce pass-by-reference Arguments

3. Top-Down Design • A design method where the major task to be accomplished is divided into subtasks. • Each subtask should perform a single well-defined task. • Each subtask may produce some result. • Treat them as small programs.Input -> Process -> Output

4. Top-Down Design Implementation • Top-Down design is implemented using methods. • A method is a collection of statements that are grouped together to perform a specific operation. • Methods have inputs called arguments. • Methods can only return one value through the return statement. • A method that will not return a value should have it’s return type defined as void. • Methods can “return” more than one value through call-by-reference arguments.

5. Calling Event Methods • One of the reasons to breakup programs into small methods is so that code does not duplicated. If there is a method that does what we need to do, we just call it. • For example, we can add a menu option to perform the same calculations defined for a button (CS7). • Instead of duplicating the code, we can call the button method which has the code defined and tested from the menu method. Be sure to pass the required arguments. private void mnuEditCalculate_Click (object sender, System.EventArgs e){ btnCalculate_Click(sender, e);}

6. Local Variables • Variables are local to the method in which they are defined. • Variables defined in a particular method are assigned their own memory and can only be referenced in that method. • Variables defined in calcExtendedPrice( ) are assigned their own memory and can only be referenced in calcExtendedPrice( ). • Different methods cannot see or reference each others variables. • They have separate memory allocations even though the variable names may be the same.

7. Pass-by-value Reviewed • Methods with pass-by-value arguments will be passing the values of the arguments into the methods local variables. • The method cannot change the values stored in the calling method. • Two different memory locations are used for each variable. • One in the calling method and one in the called method.

8. Pass-by-value - Example private void btnCalculate_Click(…){ int intQty; decimal decPrice, decExtendedPrice; intQty = int.Parse(txtQuantity.Text); decPrice = decimal.Parse(txtPrice.Text); decExtendedPrice = calcExtendedPrice(intQty, decPrice);} private decimalcalcExtendedPrice(int intQty, decimal decPrice){ decimal decExtended; decExtended = intQty * decPrice; return decExtended;}

9. Values are Passed between Methods

10. Methods and Pass-by-Value • Values of the arguments are passed to the method’s local variables. • Many values can be passed to methods, but only one value can be returned. • Enter void and pass-by-reference methods…

11. Void Methods • No value returned through return statement. • Return statement is optional. • Defined the same way as call-by-value, but return type is void.private void btnExit_Click (object sender, System.EventArgs e){ this.Close();}

12. Calling Void Methods • A method that returns a value needs a variable to the left of the equal sign.decExtendedPrice = calcExtendedPrice(intQty, decPrice); • A call to a void method does not need a variable and equal sign on the left.calcExtendedPrice(intQty, decPrice, out decExtendedPrice);

13. Pass-by-Reference • Addresses of the arguments are passed to the method’s local variables. • Many values can be passed to the methods, and many values can be “returned”. • Use the keyword ref or out to make the argument pass-by-reference. • If you leave ref or out off, the default is pass-by-value.

14. ref or out • ref is used for a variable that will be initialized before the method call. • out is used for a variable that might not be initialized before the method call, but the variable will initialized within the called method.

15. Pass-by-reference Example private void btnCalculate_Click(…){ int intQty; decimal decPrice, decExtendedPrice; intQty = int.Parse(txtQuantity.Text); decPrice = decimal.Parse(txtPrice.Text); calcExtendedPrice(intQty, decPrice, out decExtendedPrice);} private void calcExtendedPrice(int intQty, decimal decPrice, out decimal decExtended){ decExtended = intQty * decPrice; return;}

16. Declare Variables and Get Values

17. calcExtendedPrice(intQty, decPrice, out decExtendedPrice) pass-by-reference: address of decExtendedPrice is sent

18. Method Uses Local Names of Arguments private void btnCalculate_Click(…){ int intQty; decimal decPrice, decExtendedPrice; intQty = int.Parse(txtQuantity.Text); decPrice = decimal.Parse(txtPrice.Text); calcExtendedPrice(intQty, decPrice, out decExtendedPrice);} private void calcExtendedPrice(int intQty, decimal decPrice, out decimal decExtended){decExtended = intQty * decPrice; return;}

19. Perform CalculationsdecExtended = intQty * decPrice; Value is stored at memory location provided through argument.

20. Returning Multiple Values • Addresses of the arguments are passed to the method’s local variables instead of the values stored in the arguments. • The called method can then modify memory locations assigned to the calling method. • When it does modify its memory location, it can be considered to have “returned” a value. • Code many pass-by-reference (ref or out) arguments to “return” more than one value.

21. Summary of Arguments • Arguments can be a mixture of pass-by-value and pass-by reference. • If a method should NOT change the value, send it as a pass-by-value. • If values are being returned through the arguments, then make the method a void method.

22. Summary • Pass-by-value Reviewed • Void Methods • Pass-by-reference