ValidateSet tests whether the specified argument, or each of an array of arguments, exists in a set of possible values:

function Test-ValidateSet {
    [CmdletBinding()]
    param (
        [ValidateSet('One', 'Two', 'Three')]
        [String]$Value
    )
}

The set of values must be hardcoded in the attribute, it cannot be derived from a variable or another command. By default, the set is not case-sensitive. If it is desirable, the set can be made case-sensitive by using the IgnoreCase named parameter:

function Test-ValidateSet {
    [CmdletBInding()]
    param (
        [ValidateSet('One', 'Two', 'Three', IgnoreCase = $false)]
        [String]$Value
    )
}

Like ValidatePattern and ValidateSet, ValidateSet gains an optional ErrorMessage parameter in PowerShell Core.