Delimiter to use within a query string value

Having query parameters multiple times is legal, and the only way to guarantee no parsing problems in all cases:

http://someSite/someApp/myUtil.ashx?file=file1.txt&file=file2.bmp&file=file3.doc

The semicolon ; must be URI encoded if part of a filename (turned to %3B), yet not if it is separating query parameters which is its reserved use.

See section 2.2 of this rfc:

2.2. Reserved Characters

URIs include components and subcomponents that are delimited by characters in the "reserved" set. These characters are called "reserved" because they may (or may not) be defined as delimiters by the generic syntax, by each scheme-specific syntax, or by the implementation-specific syntax of a URI's dereferencing algorithm. If data for a URI component would conflict with a reserved character's purpose as a delimiter, then the conflicting data must be percent-encoded before the URI is formed.

 reserved    = gen-delims / sub-delims

 gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"

 sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
             / "*" / "+" / "," / ";" / "="

I would recommend making each file its own query parameter, i.e.

myUtil.ashx?file1=file1.txt&file2=file2.bmp&file3=file3.doc

This way you can just use standard query parsing and loop

If they're filenames, a good choice would be a character which is disallowed in filenames. Suggestions so far included , | & which are generally allowed in filenames and therefore might lead to ambiguities. / on the other hand is generally not allowed, not even on Windows. It is allowed in URIs, and it has no special meaning in query strings.

Example:

http://someSite/someApp/myUtil.ashx?files=file1.txt|file2.bmp|file3.doc is bad because it may refer to the valid file file1.txt|file2.bmp.

http://someSite/someApp/myUtil.ashx?files=file1.txt/file2.bmp/file3.doc unambiguously refers to 3 files.