Raleigh Code Camp 2103 - .Net Metaprogramming Essentials
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The document discusses metaprogramming and expression trees in .NET. Metaprogramming allows writing code that generates other code at runtime or compile-time. Expression trees in .NET represent code as data and can be compiled into delegates. The document provides an example of building an expression tree that multiplies a parameter by 2, compiling it into a function, and calling that function. It also briefly mentions other metaprogramming techniques in .NET like Reflection.Emit and Roslyn.
Raleigh Code Camp 2103 - .Net Metaprogramming Essentials
- 2. About me... • • • • Technology lead for a team at LexisNexis Blog: http://seanmccarthy.me Twitter: mccartsc G+: scmccart
- 3. About this talk... Slides and Code at, https://github. com/scmccart/MetaprogrammingEssentialsDemo Or, http://bit.ly/1go2IsO
- 4. Metaprogramming, huh? • Writing code that can write code. • Metaprogramming uses a Metalanguage • S-Expressions, Attributes, Conventions • Wide support, in languages such as ruby, python, javascript, C++, lisp, template haskell • Compile time vs Runtime
- 5. Metalanguages in .Net • S-Expressions? • Not available, unless you are using IronScheme or clojureclr. Kind of via Expression<T> • Attributes • Available everywhere, widely understood. • Conventions • Common, but custom. • Asp.Net’s routing. • Entity Framework’s code first method. • T4 kind of counts, kind of.
- 6. Metaprogramming methods • Reflection.Emit • Powerful, can build anything. • Very manual, hard to use, error prone • System.Linq.Expressions • AKA Expression Trees • Easyish API, type checks • Can only do lambdas, not as fine grained • Roslyn • Compiler as a service • Not released yet
- 7. Expression Trees Expression Trees map very closely to an AST An example, x => x.y * 2 As a tree, would be: * . x 2 y
- 8. Expression Trees Trees maps to the Expression builder’s use. Expression.Multiply( Expression.Property( x, "y"), Expression.Constant(2)) * . x 2 y
- 9. Using Expression Trees Repeatable flow with the Expression builder, 1. Declare parameters 2. Build out body 3. Pull together in lambda 4. Compile 5. Use!
- 10. 1. Declare Parameters var value = Expression.Parameter(typeof(int), "value");
- 11. 2. Build out body var value = Expression.Parameter(typeof(int), "value"); var body = Expression.Multiply(value, Expression.Constant(2));
- 12. 3. Pull together in Lambda var value = Expression.Parameter(typeof(int), "value"); var body = Expression.Multiply(value, Expression.Constant(2)); var lambda = Expression.Lambda<Func<int, int>>(body, value);
- 13. 4. Compile var value = Expression.Parameter(typeof(int), "value"); var body = Expression.Multiply(value, Expression.Constant(2)); var lambda = Expression.Lambda<Func<int, int>>(body, value); var compiled = lambda.Compile();
- 14. 5. Use! var value = Expression.Parameter(typeof(int), "value"); var body = Expression.Multiply(value, Expression.Constant(2)); var lambda = Expression.Lambda<Func<int, int>>(body, value); var compiled = lambda.Compile(); Console.Write("input: "); var input = int.Parse(Console.ReadLine()); var output = compiled(input); Console.WriteLine("output: " + output);
- 15. Let’s see that in action.
- 16. Speed
- 17. Quick Tips • • • • DebugView ToString Cache compiled trees Limits • Async/Await • Yield Return
- 18. Object Routing Example • Routing for objects to eliminate nested switch statements. • Uses Expressions for • • • • Getting property values Invoking methods Ternary expressions Constructing Arrays