asp classic - 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

Answer 1

If Request("FieldName").Count > 0 Then
    ...
End If

Or, for short:

If Request("FieldName").Count Then
    ...
End If

Background:

• The Request collection is magic, in so far as it does not throw an error when you try to access a key that was not part of the request - but the .Count will be 0 for non-existing keys.
• In a URL-encoded query string it's legal to send keys that don't have a value, like foo&bar&baz
• It's also legal to send the same key multiple times, i.e. multiple values per key, like foo=value1&foo=value2.

Therefore, the reliable way to determine if a key has been sent by the client is to count how many times the client has sent it.

A special case of this test is checking whether there was a non-empty value for that key (If Request("FieldName") > ""). This may or may not be what you want in the end; just be aware that the underlying behavior of query strings is broader than that.