Techie Head

This post follows on from  Reusing predicates in Entity Framework . In my previous post I introduced the "and" extension method which allowed me to create a new entity framework safe expression from two existing expressions. Here's how we make that happen: The code is from a blogpost from Colin Meek from 2008:  InvocationExpression and LINQ to Entities . I'll just dump my version of the code here. The idea is exactly the same (even using the visitor as an immutable stack): public class InvocationExpander : ExpressionVisitor { private readonly ParameterExpression _parameter; private readonly Expression _expansion; private readonly InvocationExpander _previous; public static T Expand<T>(T expr) where T : LambdaExpression { return (T) new InvocationExpander().Visit(expr); } public InvocationExpander() { } private InvocationExpander(ParameterExpression parameter, Expression expansion, Invocati...

Warning: some knowledge of Expressions  is required. Entity Framework simplifies a great number of things; for starters, I don't have to write a lot of SQL which makes development a lot faster - I'm a lot more productive in C# than SQL. One thing that works really well in EF, is the ability to reuse predicates - how often do you find yourself typing: posts.Where(p => !p.IsDeleted); In progamming we often get taught to DRY out the code - i.e. Don't Repeat Yourself. I think that extends to the expressions we're pass in to EF. The expressions are the specification language of EF (ignore that part if you don't do DDD). I'll look at a couple of ways of storing that expression - first up: extension methods. Set up Let's a assume we have a blog post entity like: public class BlogPost { public int Id { get ; set ; } public string Title { get ; set ; } public string Body { get ; set ; } public string AuthorName { get ; set ; } ...