CIS162AD – C#
Call-by-Reference Methods
06_methods_by_ref.ppt
Overview Of Topics
 Review Top-Down Design
 Calling Event Methods
 Review pass-by-value Arguments
 Introduce Void Methods
 Introduce pass-by-reference Arguments
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Top-Down Design
 A design method where the major task to
be accomplished is divided into subtasks.
 Each subtask should perform a single welldefined task.
 Each subtask may produce some result.
 Treat them as small programs.
Input -> Process -> Output
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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.
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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);
}
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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.
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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.
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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;
}
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Values are Passed between Methods
Procedure
Address
Variable
Value
btnCalculate( )
1010
1020
1030
intQty
decPrice
2
35.50
71.00
1040
1050
1060
intQty
2
decPrice
35.50
decExtended 71.00
calcExtendedPrice
CIS162AD
decExtendedPrice
9
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…
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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();
}
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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);
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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 passby-value.
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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.
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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;
}
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Declare Variables and Get Values
Procedure
Address
Variable
Value
btnCalculate
1010
1020
1030
1040
1050
1060
intQty
decPrice
2
35.50
0
calcExtendedPrice
CIS162AD
decExtendedPrice
intQty
0
decPrice
0
decExtended 0
16
calcExtendedPrice(intQty, decPrice, out decExtendedPrice)
pass-by-reference: address of decExtendedPrice is sent
Procedure
Address
Variable
Value
btnCalculate
1010
1020
1030
1040
1050
1060
intQty
decPrice
2
35.50
0
calcExtendedPrice
CIS162AD
decExtendedPrice
intQty
2
decPrice
35.50
decExtended 1030
17
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;
}
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Perform Calculations
decExtended = intQty * decPrice;
Procedure
Address
Variable
Value
btnCalculate
1010
1020
1030
1040
1050
1060
intQty
decPrice
2
35.50
71.00
calcExtendedPrice
decExtendedPrice
intQty
2
decPrice
35.50
decExtended 1030
Value is stored at memory location provided through argument.
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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.
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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.
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Summary
 Pass-by-value Reviewed
 Void Methods
 Pass-by-reference
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