vb中可选参数的问题

function multi(a as integer,b as integer,optional third)
  dim n as integer
  n = a * b
if not ismissing (third) then
    n = n * thid
end if
multi = n
end function
------------------------------------------------------------------------
private sub command1_click()
dim a as integer,b as integer
dim x as integer
a = 2 : b = 3
x = multi (a , b , 2)
print x
end sub

您好,我想问一下  not ismissing (third) 是什么意思.
如果下面的实参中没有第三个参数话  not ismissing (third)又是什么意思.我是新手,谢谢大家!!!!

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http://topic.csdn.net/t/20010323/13/87952.html

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引用

function multi(a as integer,b as integer,optional third)
  dim n as integer
  n = a * b
if not ismissing (third) then
    n = n * thid
end if
multi = n
end function
------------------------------------------------------------------------
private sub command1_click()
dim a as integer,b as integer
dim x as integer
a = 2 : b = 3
x = multi (a , b , 2)
print x
end sub

您好,我想问一下  not ismissing (third) 是什么意思.
如果下面的实参中没有第三个参数话  not ismissing (third)又是什么意思.我是新手,谢谢大家!!!!

Function 语句

声明 F