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    <p>Is is a data <em>tree</em>, or a full <em>graph</em> - i.e. are there any circular references? If not, <a href="http://code.google.com/p/protobuf-net/" rel="nofollow noreferrer">protobuf-net</a> is a high-performance binary <em>tree</em> serializer. It supports streaming of enumerable items (so you can skip records, etc - rather than buffer everything), but to <em>efficiently</em> seek to a random element I expect you'd need some kind of index.</p> <p>Read/write is <strong>VERY</strong> hard for a single file; in particular, writing may require moving lots more of the disk than you expect... reading is also tricky and might require synchronization. It would be easier to use separate files...</p> <hr> <p>Example of skipping early items; I could probably add a helper method, but the <code>TryDeserializeWithLengthPrefix</code> method will work... the key point being to watch that between serialization and deserialization we only create one extra object.</p> <pre><code>using System; using System.IO; using System.Threading; using ProtoBuf; [ProtoContract] class Foo { static int count; public static int ObjectCount { get { return count; } } public Foo() { // track how many objects have been created... Interlocked.Increment(ref count); } [ProtoMember(1)] public int Id { get; set; } [ProtoMember(2)] public double Bar { get; set; } } static class Program { static void Main() { MemoryStream ms = new MemoryStream(); Random rand = new Random(); for (int i = 1; i &lt;= 5000; i++) { Foo foo = new Foo { Bar = rand.NextDouble(), Id = i }; Serializer.SerializeWithLengthPrefix(ms, foo,PrefixStyle.Base128, 1); } ms.Position = 0; // skip 1000 int index = 0; object obj; Console.WriteLine(Foo.ObjectCount); Serializer.NonGeneric.TryDeserializeWithLengthPrefix( ms, PrefixStyle.Base128, tag =&gt; ++index == 1000 ? typeof(Foo) : null, out obj); Console.WriteLine(Foo.ObjectCount); Console.WriteLine(((Foo)obj).Id); } } </code></pre>
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    1. CORight now it is a full graph (with circular references). But your suggestion is interesting. When you say streaming, do you mean, that while I enumerate over the collection it will page records from the serialized file on demand (i.e. not load everything into memory at once)?
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    2. COCorrect; primarily you can either pick items off as a sequence of top-level objects (assuming the data is essentially a sequence of homogeneous records) - but you can also do some fun things with cheeky collection classes that drop records - or process them in sequence and then drop them (rather than buffer them).
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    3. COI've also got an example of it running against "PushLINQ", which is a LINQ implementation (by Jon and myself) designed to efficiently process large data streams - i.e. to compute sums/averages (including group aggregates) etc without buffering the individual records.
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