How can I reuse expressions within LINQ statements?

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()};

I had the same problem and wanted to preserve the ability to use extension methods within the query syntax (as with ordinary supported functions...). A solution might be this library (spoiler: I‘m the author).

You just implement the method to reuse twice, once for general use and once for queries.

public static class MyFunctions {
    [InjectLambda]
    public static bool IsAGoodProduct(Product product) {
        return product.Quality>3;
    }
    public static Expression<Func<Product,bool>> IsAGoodProduct() {
        return (p) => p.Quality>3;
    }
}

The actual query can then look like expected.

var goodProds = from p in dataContext.Products.ToInjectable()
                where p.IsAGoodProduct()
                select p;

The ToInjectable call creates a lightweight proxy, which replaces the IsAGoodProduct method call (if marked accordingly) with the desired lambda expression. Thus, you can use extension methods wherever within the query -- parameterized methods work as well.