怎样解决这个问题

程序在运行后结果为0
反复测试没有结果，为什么？请求高手指点




Public Class Form1
    Sub f(ByVal n As Integer, ByVal t As Double)
        Dim i As Integer
        t = 1
        For i = 1 To n
            t = t * i

        Next

 

    End Sub

    Private Sub Form1_click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim a As Double, b As Double, c As Double
        Dim sum As Double
        Call f(3, a)
        Call f(5, b)
        Call f(8, c)
        sum = a + b + c
        MsgBox(sum)
    End Sub

  
End Class