Raleigh Code Camp 2103 - .Net Metaprogramming Essentials

Nov 12, 20131 like333 views

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

About me...
•
•
•
•

Technology lead for a team at LexisNexis
Blog: http://seanmccarthy.me
Twitter: mccartsc
G+: scmccart

About this talk...
Slides and Code at, https://github.
com/scmccart/MetaprogrammingEssentialsDemo

Or,

http://bit.ly/1go2IsO

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

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.

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

Expression Trees
Expression Trees map very closely to an AST
An example, x => x.y * 2
As a tree, would be:
*
.
x

2
y

Expression Trees
Trees maps to the Expression builder’s use.
Expression.Multiply(
Expression.Property(
x,
"y"),
Expression.Constant(2))

*
.
x

2
y

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!

1. Declare Parameters
var value = Expression.Parameter(typeof(int), "value");

2. Build out body
var value = Expression.Parameter(typeof(int), "value");
var body = Expression.Multiply(value, Expression.Constant(2));

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);

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();

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);

Quick Tips
•
•
•
•

DebugView
ToString
Cache compiled trees
Limits
• Async/Await
• Yield Return

Object Routing Example
• Routing for objects to eliminate nested switch

statements.
• Uses Expressions for
•
•
•
•

Getting property values
Invoking methods
Ternary expressions
Constructing Arrays

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