Always show a "dot" when converting double to string

I need to convert some doubles, and always show the dot separator (whatever the culture), even if there are no decimals.

If my double is 3.5, I need as output 3.5, If double is 3 I need as output 3.

Is there a way to do that only using DecimalFormat, or ToString("something") or need to make a function just for that?

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