Password must have at least one non-alpha character

Use regex pattern ^(?=.{8})(?=.*[^a-zA-Z])

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Explanation:

^(?=.{8})(?=.*[^a-zA-Z])
│└──┬───┘└──────┬──────┘
│   │           │
│   │           └ string contains some non-letter character
│   │
│   └ string contains at least 8 characters
│
└ begining of line/string

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If you want to limit also maximum length (let's say 16), then use regex pattern:

^(?=.{8,16}$)(?=.*[^a-zA-Z])

Run it through a fairly simple regex: [^a-zA-Z]

And then check it's length separately:

if(string.Length > 7)