read userinput and pass to Get-EventLog function

Please suggest the way forward for this, similarly I have to do for enddate, username etc. sample:

$StartDate, $String = "", ""

$StartDate = Read-Host -Prompt 'Enter the start date of the logs, Ex: 17/07/2017 09:00:00 '

if ($StartDate -and ( $StartDate -ne " ") -and ($StartDate -ne "")) {
    $StartDate = $StartDate -replace "`t|`n|`r", ""
    $String += " -After '$StartDate'"
} else {
    'You did not enter a valid Start date!'
}
echo "Get-EventLog -LogName Application $String"

Get-EventLog -LogName Application $String

Output:

Get-EventLog -LogName Application  -After '19/07/2017'
Get-EventLog : Cannot bind parameter 'InstanceId'. Cannot convert value
" -After '19/07/2017'" to type "System.Int64". Error: "Input string was not
in a correct format."
At C:\Users\kumars2\Downloads\Santosh\Powershell scripts\Enhancements\View logs examples\small_test.ps1:17 char:13
+ Get-EventLog <<<<  -LogName Application $String
    + CategoryInfo          : InvalidArgument: (:) [Get-EventLog], ParameterBindingException
    + FullyQualifiedErrorId : CannotConvertArgumentNoMessage,Microsoft.PowerShell.Commands.GetEventLogCommand

Solution

If you want to construct a parameter list for a cmdlet you should use splatting instead of building (partial) string commandlines. You're getting the error you observed because PowerShell passes the entire string " -After '$StartDate'" as an argument to the parameter -InstanceId. Also, your date string has the format dd/MM/yyyy. PowerShell can't automagically convert this string to a DateTime value, so you need to do that yourself.

$culture = [Globalization.CultureInfo]::InvariantCulture
$pattern = 'dd\/MM\/yyyy'

$StartDate = $StartDate -replace '\s'  # remove all whitespace from date string
$EndDate   = $EndDate -replace '\s'    # remove all whitespace from date string

$params = @{
    'LogName' = 'Application'
}

if ($StartDate) {
    $params['After'] = [DateTime]::ParseExact($StartDate, $pattern, $culture)
}
if ($EndDate) {
    $params['Before'] = [DateTime]::ParseExact($EndDate, $pattern, $culture)
}

Get-EventLog @params