Query string using LINQ

Question

For parsing SDP information from an RTSP, currently I use default c# string functions (like loop the string line by line) and foreach/switch methods. Default SDP information comes from a device in this form:

v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
[email protected] (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 3456 RTP/AVP 0
m=video 2232 RTP/AVP 31
m=whiteboard 32416 UDP WB
a=orient:portrait

Im wondering is this string is query-able using LINQ or something instead of foreaching each line, switching the first character and then storing the rest as a value, because this method is very sensitive to malfunction and error (like when 2 attributes/params are on 1 line or the first character isn't accidentally the right one (e.g. with a space before)). Before I begin to cover every damn exception than can occur, I'm wondering if there's a technique to query a string for values/keys using LINQ.

You have three items for m key. How you suppose to handle that? — Sergey Berezovskiy
– Sergey Berezovskiy, Commented Nov 1, 2012 at 9:50
@CuongLe For the unique keys like v,o,s, I have specific variables. For * keys like m, as you see in the example above, I'd like to use lists. There are another type of key/value not in this example: Attributes. They are written as following: 'a=fmpt:RTP/96' So I'd like to query for attributes (a's), then query for the key (string before :) and values (string after :) — user1450661
– user1450661, Commented Nov 1, 2012 at 9:52

Sergey Berezovskiy · Accepted Answer · 2012-11-01 10:01:08Z

3

var query = text.Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries)
                .Select(line => line.Split('='))
                .GroupBy(x => x[0], x => x[1])
                .Select(g => new { Key = g.Key, Values = g.ToList() });

This will return entries grouped by key with values as list.

Or this way (if Linq Lookup is OK for you):

var lookup = text.Split(new[] { '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries)
                 .Select(line => line.Split('='))
                 .ToLookup(x => x[0], x => x[1]);

Usage:

foreach (var value in lookup["m"])
    // use value

edited Nov 1, 2012 at 10:01

answered Nov 1, 2012 at 9:53

Sergey Berezovskiy

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1 Comment

user1450661 Over a year ago

This! A hundred-times! Thank you very much for this understandable example.

Marc Gravell · Accepted Answer · 2012-11-01 09:48:45Z

1

you undoubtedly can do it with linq, but a simple loop (perhaps using StringReader to get the lines) checking line.Length != 0 then looking and line[0] and line.Substring(1) is probably more appropriate:

static void Process(string input)
{
    using (var reader = new StringReader(input))
    {
        string line;
        while ((line = reader.ReadLine()) != null)
        {
            if(line.Length == 0) continue;

            char first = line[0];
            string rest = line.Substring(1);
            // ... process this line
        }
    }
}

answered Nov 1, 2012 at 9:48

Marc Gravell

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2 Comments

user1450661 Over a year ago

What about the case when 2 key/value pairs occur on one line? ReadLine would be useless in this case.

Marc Gravell Over a year ago

@user1450661 then you need to define what the correct behavior is in that case (which you have not yet done)... it sounds to me like that is an error scenario, and the user should not expect it to work.

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