A Visual Studio T4 template for generating immutable object models in C#

ImmutableObjectGenerator.tt

<#@ template debug="false" hostspecific="false" language="C#" #>
<#@ assembly name="System.Core.dll" #>
<#@ import namespace="System.Linq" #>
<#@ output extension=".generated.cs" #><#
    // Describe the immutable model types to generate:
    var types = new[] {
        new {
            name = "ImmutableSample",
            comment = "A complete commit object.",
            idType = "int",
            members = new[] {
                new { name = "Items",        type = "string[]",       bType = "List<string>", convert = "({0}).ToArray()", revert = "({0}).ToList()" },
                new { name = "ForeignKeyID", type = "int",            bType = (string)null,   convert = (string)null,                 revert = (string)null                 },
                new { name = "ModifiedBy",   type = "string",         bType = (string)null,   convert = (string)null,                 revert = (string)null                 },
                new { name = "ModifiedDate", type = "DateTimeOffset", bType = (string)null,   convert = (string)null,                 revert = (string)null                 },
                new { name = "Message",      type = "string",         bType = (string)null,   convert = (string)null,                 revert = (string)null                 },
            }
        },
    };
#>
using System;
using System.Collections.Generic;
using System.Linq;

namespace ImmutableModels
{
<#
    for (int i = 0; i < types.Length; ++i)
    {
        var ty = types[i];
        if (i != 0)
        {
#>

<#
        } // if (i != 0)

        if (ty.comment != (string)null)
        {
#>
    /// <summary>
    /// <#= ty.comment #>
    /// </summary>
<#
        } // if (ty.comment != (string)null)
#>
    public sealed partial class <#= ty.name #>
    {
<#
        if (ty.idType != (string)null)
        {
#>
        public <#= ty.idType #> ID { get; private set; }
<#
        } // if (ty.idType != (string)null)

        // Public read-only properties:
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
        public <#= mbr.type #> <#= mbr.name #> { get; private set; }
<#
        } // for (int j = 0; j < ty.members.Length; ++j)
#>

        public <#= ty.name #>(Builder b)
        {
<#
        // Property assignment statements:
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
            this.<#= mbr.name #> = <#= mbr.convert == null ? "b." + mbr.name : String.Format(mbr.convert, "b." + mbr.name, mbr.type, mbr.bType) #>;
<#
        } // for (int j = 0; j < ty.members.Length; ++j)
#>
        }

        public sealed class Builder
        {
<#
        // Public read-write properties:
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
            public <#= mbr.bType ?? mbr.type #> <#= mbr.name #> { get; set; }
<#
        } // for (int j = 0; j < ty.members.Length; ++j)
#>

            public Builder() { }

            public Builder(<#= ty.name #> imm)
            {
<#
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
                this.<#= mbr.name #> = <#= mbr.revert == null ? "imm." + mbr.name : String.Format(mbr.revert, "imm." + mbr.name, mbr.type, mbr.bType) #>;
<#
        } // for (int j = 0; j < ty.members.Length; ++j)
#>
            }

            public Builder(
<#
        bool doComma = false;
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
               <#= doComma ? "," : " " #><#= mbr.bType ?? mbr.type #> p<#= mbr.name #>
<#
            doComma = true;
        } // for (int j = 0; j < ty.members.Length; ++j)
#>
            )
            {
<#
        for (int j = 0; j < ty.members.Length; ++j)
        {
            var mbr = ty.members[j];
#>
                this.<#= mbr.name #> = p<#= mbr.name #>;
<#
        } // for (int j = 0; j < ty.members.Length; ++j)
#>
            }
        }

        public static implicit operator <#= ty.name #>(Builder b)
        {
            return new <#= ty.name #>(b);
        }
    }
<#
    } // for (int i = 0; i < types.Length; ++i)
#>
}