Group List of Objects based on Property using Linq?

This

var sites = new List<SiteInfo>()
{
    new SiteInfo(){Title="1",Type="a",URL="http://aaaa"},
    new SiteInfo(){Title="2",Type="b",URL="http://bbbb"},
    new SiteInfo(){Title="3",Type="a",URL="http://aaaa"},
    new SiteInfo(){Title="4",Type="b",URL="http://bbb"},
};

var json = new JavaScriptSerializer().Serialize(sites.GroupBy(s => s.Type));

would produce

[
    [{"Title":"1","URL":"http://aaaa","Type":"a"},{"Title":"3","URL":"http://aaaa","Type":"a"}],
    [{"Title":"2","URL":"http://bbbb","Type":"b"},{"Title":"4","URL":"http://bbb","Type":"b"}]
]

or

var json = new JavaScriptSerializer().Serialize(sites.GroupBy(s => s.Type)
                                                      .ToDictionary(x=>x.Key,x=>x));

would produce

{
    "a":[{"Title":"1","URL":"http://aaaa","Type":"a"},{"Title":"3","URL":"http://aaaa","Type":"a"}],
    "b":[{"Title":"2","URL":"http://bbbb","Type":"b"},{"Title":"4","URL":"http://bbb","Type":"b"}]
}

It sounds like you want something like:

// No need to sort sites first
var grouped = sites.OrderBy(x => x.Type)
                   .GroupBy(x => x.Type);

Then just serialize grouped. However, I don't know quite what an IGrouping will look like in JSON... and the type will be present in each case. You may want something like:

var grouped = sites.OrderBy(x => x.Type)
                   .GroupBy(x => x.Type)
                   .Select(g => new { Type = g.Key,
                                      Sites = g.Select(site => new {
                                                           site.Title,
                                                           site.URL
                                                       } });

I think that would give you a nicer JSON structure.

Here's a straightforward console application that does what you want:

static void Main(string[] args)
{
    List<SiteInfo> sites = new List<SiteInfo>()
    {
        new SiteInfo() { Title = "Site A", Type = "Whatever 2" },
        new SiteInfo() { Title = "Site B", Type = "Whatever 1" },
        new SiteInfo() { Title = "Site C", Type = "Whatever 1" },
        new SiteInfo() { Title = "Site D", Type = "Whatever 3" },
        new SiteInfo() { Title = "Site E", Type = "Whatever 3" }
    };

    var sitesGroupedByType =
        sites.GroupBy(s => s.Type).Select(g => new { Type = g.Key,
                                    Sites = g.Select(site => new
                                    {
                                          site.Title,
                                          site.URL
                                    })});

    foreach (var siteTypeGroup in sitesGroupedByType.OrderBy(g => g.Type))
    {
        foreach(var site in siteTypeGroup.Sites)
        {
            Console.WriteLine(string.Format("Type => {0}, Title => {1}",
                              siteTypeGroup.Type, site.Title));
        }
    }

    Console.ReadKey();
}

Output:

Type => Whatever 1, Title => Site B
Type => Whatever 1, Title => Site C
Type => Whatever 2, Title => Site A
Type => Whatever 3, Title => Site D
Type => Whatever 3, Title => Site E