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

2. CIS162AD2 Overview Of Topics  Review Top-Down Design  Calling Event Methods  Review pass-by-value Arguments  Introduce Void Methods  Introduce pass-by-reference Arguments

3. CIS162AD3 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. CIS162AD4 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. CIS162AD5 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. CIS162AD6 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. CIS162AD7 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. CIS162AD8 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 decimal calcExtendedPrice(int intQty, decimal decPrice) { decimal decExtended; decExtended = intQty * decPrice; return decExtended; }

9. CIS162AD9 Values are Passed between Methods ProcedureAddressVariableValue btnCalculate( ) 1010 1020 1030 intQty decPrice decExtendedPrice 2 35.50 71.00 calcExtendedPrice 1040 1050 1060 intQty decPrice decExtended 2 35.50 71.00

10. CIS162AD10 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. CIS162AD11 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. CIS162AD12 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. CIS162AD13 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. CIS162AD14 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. CIS162AD15 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. CIS162AD16 Declare Variables and Get Values ProcedureAddressVariableValue btnCalculate 1010 1020 1030 intQty decPrice decExtendedPrice 2 35.50 0 calcExtendedPrice 1040 1050 1060 intQty decPrice decExtended 0000000

17. CIS162AD17 calcExtendedPrice(intQty, decPrice, out decExtendedPrice) pass-by-reference: address of decExtendedPrice is sent ProcedureAddressVariableValue btnCalculate 1010 1020 1030 intQty decPrice decExtendedPrice 2 35.50 0 calcExtendedPrice 1040 1050 1060 intQty decPrice decExtended 2 35.50 1030

18. CIS162AD18 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. CIS162AD19 Perform Calculations decExtended = intQty * decPrice; ProcedureAddressVariableValue btnCalculate 1010 1020 1030 intQty decPrice decExtendedPrice 2 35.50 71.00 calcExtendedPrice 1040 1050 1060 intQty decPrice decExtended 2 35.50 1030 Value is stored at memory location provided through argument.

20. CIS162AD20 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. CIS162AD21 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. CIS162AD22 Summary  Pass-by-value Reviewed  Void Methods  Pass-by-reference