How can I reuse expressions within LINQ statements?

Question

I like to reuse expressions for DRY reasons, but how do I reuse the expressions within a LINQ statement?

e.g.

I have

public static class MyExpressions {
    public static Expression<Func<Product,bool>> IsAGoodProduct() {
        return (p) => p.Quality>3;
    }
}

And would like to use that in LINQ statements, so

  var goodProds = from p in dataContext.Products
                  where ????? // how do I use IsAGoodProduct here?
                  select p;

Sure, I could use the IQueryableExtension.Where function, but that would make joins and other functions alot uglier for more complex queries.

Is this possible or is it a limitation of LINQ?

Answer 1

If you move from the LINQ syntactic sugar it is possible:

var goodProds = dataContext.Products.Where(MyExpressions.IsAGoodProduct());

Without it, it isn't possible.

There's nothing to stop you mixing the two styles to build a single query though.

Example:

  var goodProds = from p in dataContext.Products
                                       .Where(MyExpressions.IsAGoodProduct())
                  group p by p.Category into g 
                  select new {Category = g.Key, ProductCount = g.Group.Count()};

Answer 2

By the way, I've come across this useful article which explains how you can create dynamic LINQ Queries that reference functions wrapped as Expressions using a custom ToExpandable() extension method. The solution provided can be used within the various parts of a LINQ Query all while preserving the use of comprehension syntax instead of resorting to lambda syntax.