vb中输入一串字符串，字符串中每个字符字典顺序递增1如何实现。如输入abc，输出bcd

Private Sub Command1_Click()

dim a,b as string

a=inputbox("输入字符串")

For i = 1 To Len(a)

b=b & chr(asc(mid(a,i,1))+1)

next

msgbox b

next