IEnumerableのCastを抹殺するためのT4 Templateの使い方

// わざとらしい例ですが
var q = Regex.Matches("aag0 hag5 zag2", @"(.)ag(\d)")
    .Cast<Match>()
    .SelectMany(m => m.Groups.Cast<Group>().Skip(1).Select(g => g.Value))
    .ToArray(); // a0h5z2

public static class MatchCollectionExtensions
{
    public static IEnumerable<TResult> Select<TResult>(this MatchCollection source, Func<Match, TResult> selector)
    {
        return source.Cast<Match>().Select(selector);
    }
 
    // Where, Aggregate, ....
}

var extMethods = typeof(Enumerable)
    .GetMethods()
    .Where(mi => Attribute.IsDefined(mi, typeof(ExtensionAttribute)));

var returnType = extMethods
    .Select(mi => mi.ReturnType)
    .Select(mi => Regex.Replace(mi.Name, "`.*$", "")
        + (mi.IsGenericType ? ("<" + string.Join(", ", mi.GetGenericArguments().Select(t => t.Name)) + ">") : ""));

<#@ template language="C#" #>
<#@ output extension="cs" #>
<#@ assembly Name="System.Core.dll" #>
<#@ import namespace="System" #>
<#@ import namespace="System.Collections.Generic" #>
<#@ import namespace="System.Linq" #>
<#@ import namespace="System.Text" #>
<#@ import namespace="System.Runtime.CompilerServices" #>
<#@ import namespace="System.Text.RegularExpressions" #>
<#@ import namespace="System.Reflection" #>
<#
    var target = new Dictionary<string, string>
    {
        {"MatchCollection", "Match"},
        {"GroupCollection", "Group"},
        {"CaptureCollection", "Capture"}
    };
#>
<#
    var ignoreMethods = new HashSet<string>
    {
        "Max", "Min", "Average", "Sum", "Zip", "OfType", "Cast",
        "Join", "GroupJoin", "ThenBy", "ThenByDescending", "LongCount"
    };
#>
using System;
using System.Collections.Generic;
using System.Linq;
 
namespace System.Text.RegularExpressions
{
<#
foreach (var kvp in target)
{
#>
    public static class <#= kvp.Key.Replace(".","") #>Extensions
    {
<#
foreach (var methodInfo in typeof(Enumerable).GetMethods().Where(mi => Attribute.IsDefined(mi, typeof(ExtensionAttribute))))
{
    if(ignoreMethods.Contains(methodInfo.Name)) continue;
#>
        public static <#= MakeReturnType(methodInfo, kvp.Value) #> <#= methodInfo.Name #><#= MakeGenericArguments(methodInfo) #>(this <#= kvp.Key #> source<#= MakeParameters(methodInfo, kvp.Value) #>)
        {
            return source.Cast<<#= kvp.Value #>>().<#= MakeMethodBody(methodInfo) #>;
        }
 
<#}#>
    }
<#}#>
}
<#+
    const string TSource = "TSource";
 
    static string ConstructTypeString(Type type, string castType)
    {
        var result = type.Name.Contains(TSource)
            ? type.Name.Replace(TSource, castType)
            : Regex.Replace(type.Name, "`.*$", "");
 
        if (type.IsGenericType)
        {
            result += string.Format("<{0}>", string.Join(", ", type.GetGenericArguments().Select(t => ConstructTypeString(t, castType))));
        }
        return result;
    }
 
    static string MakeReturnType(MethodInfo info, string castType)
    {
        return ConstructTypeString(info.ReturnType, castType);
    }
 
    static string MakeGenericArguments(MethodInfo info)
    {
        var types = info.GetGenericArguments().Select(t => t.Name).Where(s => s != TSource);
        return types.Any() ? string.Format("<{0}>", string.Join(", ", types)) : "";
    }
 
    static string MakeParameters(MethodInfo info, string castType)
    {
        var param = info.GetParameters()
            .Skip(1)
            .Select(pi => new { pi.Name, ParameterType = ConstructTypeString(pi.ParameterType, castType) });
 
        return param.Any()
            ? ", " + string.Join(", ", param.Select(a => a.ParameterType + " " + a.Name))
            : "";
    }
 
    static string MakeMethodBody(MethodInfo info)
    {
        var args = info.GetParameters().Skip(1).Select(pi => pi.Name);
        return string.Format("{0}({1})", info.Name, args.Any() ? string.Join(", ", args) : "");
    }
#>