How to check if a POST submitted field exists in VBScript?

Question

After a form is submitted, how does one check server-side if a particular field exists? For example:

If [Exists] Request(\"FieldName\") Then
    ...
End If
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Answer 1

Check if it's not empty. There are a few different ways, but the one I've seen more frequently used is along the lines of:

If Request("FieldName") <> "" Then
 'etc.
End If

I usually explicitly check the Form and QueryString collections with some variation of one of the code below if I may be getting the variable from one or the other depending on context:

Select Case True
    Case Request.Form("FieldName") <> ""
        'Run if the Form isn't empty
    Case Request.QueryString("FieldName") <> ""
        'Run if the QueryString isn't empty
    Case Else
        'Set a predefined default if they're both empty
End Select

Or a nested If ... Then:

If Request.Form("FieldName") <> "" Then
    'Run if the Form isn't empty
ElseIf Request.QueryString("FieldName") <> "" Then
    'Run if the QueryString isn't empty
Else
    'Set a predefined default if they're both empty
End If

If I know exactly which collection it's coming from, I'll check that collection specifically. The reason is that I want to make sure it is pulling what I expect from where I expect it to come from. I don't want someone overriding a Form value by sending something in the QueryString when I'm not expecting it.

From MSDN:

If the specified variable is not in one of the preceding five collections, the Request object returns EMPTY.

All variables can be accessed directly by calling Request(variable) without the collection name. In this case, the Web server searches the collections in the following order:

• QueryString
• Form
• Cookies
• ClientCertificate
• ServerVariables

If a variable with the same name exists in more than one collection, the Request object returns the first instance that the object encounters.

It is strongly recommended that when referring to members of a collection the full name be used. For example, rather than Request.("AUTH_USER") use Request.ServerVariables("AUTH_USER"). This allows the server to locate the item more quickly.