Empty QueryString parameter

You get null because the foo parameter doesn't have a value with it.

...What's the problem exactly?

If you still want to check for its existence (although it lacks a value), try something like this:

bool doesFooExist = Request.Url.AbsoluteUri.IndexOf("foo=") >= 0 ? true : false;

Request.ServerVariables["QUERY_STRING"] will return the query string, complete, as a string. Then search it using a Regex or IndexOf

You can use null as the key for the NameValueCollection and it will give you a comma-delimited list of parameter names that don't have values.

For http://example.com?bar=3&foo you would use Request.QueryString[null] and it would retrieve foo.

If you have more than one parameter name without a value, it will give you a value that is comma-delimited.

For http://example.com?bar=3&foo&test you would get foo,test as a value back.

Update:

You can actually use Request.QueryString.GetValues(null) to get the parameter names that don't have values.