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    <p>Use this <a href="https://gist.github.com/1266453" rel="nofollow">T4 template</a> I put together to solve this problem. It should generally suit your needs for whatever kinds of immutable objects you need to create.</p> <p>There's no need to go with generics or use any interfaces. For my purposes, I do not want my immutable classes to be convertible to one another. Why would you? What common traits should they share that means they should be convertible to one another? Enforcing a code pattern should be the job of a code generator (or better yet, a nice-enough type system to allow you to do define general code patterns, which C# unfortunately does not have).</p> <p>Here's some example output from the template to illustrate the basic concept at play (nevermind the types used for the properties):</p> <pre><code>public sealed partial class CommitPartial { public CommitID ID { get; private set; } public TreeID TreeID { get; private set; } public string Committer { get; private set; } public DateTimeOffset DateCommitted { get; private set; } public string Message { get; private set; } public CommitPartial(Builder b) { this.ID = b.ID; this.TreeID = b.TreeID; this.Committer = b.Committer; this.DateCommitted = b.DateCommitted; this.Message = b.Message; } public sealed class Builder { public CommitID ID { get; set; } public TreeID TreeID { get; set; } public string Committer { get; set; } public DateTimeOffset DateCommitted { get; set; } public string Message { get; set; } public Builder() { } public Builder(CommitPartial imm) { this.ID = imm.ID; this.TreeID = imm.TreeID; this.Committer = imm.Committer; this.DateCommitted = imm.DateCommitted; this.Message = imm.Message; } public Builder( CommitID pID ,TreeID pTreeID ,string pCommitter ,DateTimeOffset pDateCommitted ,string pMessage ) { this.ID = pID; this.TreeID = pTreeID; this.Committer = pCommitter; this.DateCommitted = pDateCommitted; this.Message = pMessage; } } public static implicit operator CommitPartial(Builder b) { return new CommitPartial(b); } } </code></pre> <p>The basic pattern is to have an immutable class with a nested mutable <code>Builder</code> class that is used to construct instances of the immutable class in a mutable way. The only way to set the immutable class's properties is to construct a <code>ImmutableType.Builder</code> class and set that in the normal mutable way and convert that to its containing <code>ImmutableType</code> class with an implicit conversion operator.</p> <p>You can extend the T4 template to add a default public ctor to the <code>ImmutableType</code> class itself so you can avoid a double allocation if you can set all the properties up-front.</p> <p>Here's an example usage:</p> <pre><code>CommitPartial cp = new CommitPartial.Builder() { Message = "Hello", OtherFields = value, ... }; </code></pre> <p>or...</p> <pre><code>CommitPartial.Builder cpb = new CommitPartial.Builder(); cpb.Message = "Hello"; ... // using the implicit conversion operator: CommitPartial cp = cpb; // alternatively, using an explicit cast to invoke the conversion operator: CommitPartial cp = (CommitPartial)cpb; </code></pre> <p>Note that the implicit conversion operator from <code>CommitPartial.Builder</code> to <code>CommitPartial</code> is used in the assignment. That's the part that "freezes" the mutable <code>CommitPartial.Builder</code> by constructing a new immutable <code>CommitPartial</code> instance out of it with normal copy semantics.</p>
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    1. POWhat is a "mostly complete" (im)mutability approach for C#?
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    1. USJames Dunne
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    1. COPattern selection + codegen enforcement of it + a framework of immutable types and non-codegen patterns seems like the leading candidate. You've offered up 2 of the 3 nicely, thanks! Some cool points on your approach 1) it's an almost-language-feature in that `new X.Builder()` is like a factory method + initializer syntax. 2) It's a decent alternative to interface-based mutability enforcement. Now to find the framework piece...
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    2. COI use the `new X.Builder(...)` form with C# 4's explicitly-named parameters and it works out very nicely, e.g. `new X.Builder(pField1: value1, pField2: value2, ...)`. I'm not sure what "framework piece" you think is missing? I have my own framework and I removed what minor tendrils it had in this T4 template. It was just stuff for generating strongly-typed identifier structs, computing SHA-1 identifiers, and other such things.
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    3. COBasically immutable stacks, queues, and other structures.
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