1. Metaprogramming in .NET
By Brandon D’Imperio
http://imaginarydevelopment.blogspot.com

2. What is it?
• Metaprogramming is most easily defined as
beside-programming or after-programming
– Late-binding
– Runtime compilation
– Code generation

3. Where is it?
• You’ve already seen and used it, most likely
– Microsoft
• System.Reflection
• CodeDom
• Emit
• T4
• Dynamic Language Runtime (DLR, dynamic keyword)
Generating Intermediate Language (IL) with Expression Trees
• Roslyn (compiler as a service)
• Enterprise Library’s Policy Injection block
• Generics
– Aspect Oriented Programming

4. Why?
• DRY – Don’t repeat yourself
– don’t duplicate authority
– Boiler-plate code
– Simplify manual coding hand work
• Update the entire system at a single point rather than
search and find by hand
• Lack of higher order functions
• Make parts of the application update or adjust
based on each other at the click of a button

5. Javascript

6. System.Reflection
Databinding

7. Emit

8. CodeDom

9. CodeDom usage

10. T4
<#@ template language="C#" #>
<#@ output extension=".cs" #>
<#@ assembly name="System.Core" #>
<#@ import namespace="System.Linq" #>
<# Type[] types_to_generate = new[] { typeof(object), typeof(bool), typeof(byte),typeof(char),
typeof(decimal), typeof(double),typeof(float),
typeof(int), typeof(long), typeof(sbyte),
typeof(short), typeof(string),typeof(uint),
typeof(ulong), typeof(ushort) }; #>
using System;
public static class greater {
<# foreach (var type in types_to_generate) { #>
public static <#= type.Name #> of(<#= type.Name #> left, <#= type.Name #> right) {
<# Type generic_icomparable =(from intf in type.GetInterfaces()
let args = intf.GetGenericArguments()
where intf.Name == "IComparable`1“ && args != null && args[0].Equals(type)
select intf).FirstOrDefault();
if (generic_icomparable != null || type is IComparable) { #>
return left.CompareTo(right) < 0 ? right : left;
<# } else { #>
throw new ApplicationException( "Type <#= type.Name #>
must implement one of the " + "IComparable or IComparable<<#= type.Name #>> interfaces.");
<# } #>
}
<# } #>
Many of my T4’s available at http://ideone.com/imaginarydevelopment/t4

11. Expressions

12. Manually built Expression

13. Dynamics

14. Roslyn – Code Issue

15. Final words
• “metaprogramming is all about making software simpler
and reusable. However, rather than depending strictly on
language features to reduce code complexity or to increase
reusability, metaprogramming achieves those goals
through a variety of libraries and coding techniques”
• “For the most part however, metaprogramming is a set of
language independent skills.”
• “As perceived complexity from the end user's standpoint
goes down, internal complexity of the design often goes
up. Complexity reduction when metaprogramming follows
the same rules. To achieve simplicity on the outside, the
code on the inside of a metaprogramming-enabled
component typically takes on some added Responsibilities”

16. More about me
• http://imaginarydevelopment.blogspot.com
• https://www.ohloh.net/accounts/Maslow
• http://stackoverflow.com/users/57883/maslo
w

17. Emit – vs 2012 dark theme

18. Appendix
• Roslyn Code source
– http://code.msdn.microsoft.com/Implementing-a-Code-
Action-28579fb1
• Many of my T4’s available
– http://ideone.com/imaginarydevelopment/t4
• Emit Full source
– http://breusable.codeplex.com/SourceControl
/changeset/view/92288#1863265
• AOP – was a little deeper than I thought
we would have time for
– http://www.sharpcrafters.com/
– http://www.mono-project.com/Cecil