Reusing Lambda Select Fragments in LINQ

Question

I would like to be able to reuse fragments of my select lambda expressions in my Entity Framework Core 2.0 queries.

For example:

var result = await ctx.Customers
  .Select(cust => new CustomerDto {
    CustomerId = cust.Id,
    CustomerName = cust.Name,
    CurrentValue = cust.Orders
      .Where(order => order.OrderDate >= DateTime.Now.AddDays(-30)
      .Sum(order => order.TotalValue)
    })
    .ToListAsync();

Since I might want to calculate the CurrentValue property in other queries (in practice the sub-query is more complex than this), I would ideally like to refactor the above code to something like:

var result = await ctx.Customers
  .Select(cust => new CustomerDto {
    CustomerId = cust.Id,
    CustomerName = cust.Name,
    CurrentValue = CalculateCustomerCurrentValueExpr(cust)
  })
  .ToListAsync();

I have created Linq predicates using a Linq.Expression, but I have been unable to find a way to use an Expression as an element of the select statement.

Any help would be much appreciated.

Update - Performance with .AsExpandable()/.Invoke()

For anyone interested, I ran some test code ten times which produced the following result:

Standard Inline Code: 17ms (58,609 ticks) With .AsExpandable() and inline code 16ms (58,029 ticks) With .AsExpandable() and .Invoke() 16ms (58,224 ticks)

I suspect that if more test cycles had been run, the average processing time for all three scenarios would have been the same - at least with the level of accuracy I could measure at (simple StopWatch()).

Thanks to all contributors, particularly SergeyA for the solution and Ivan Stoev for the simple explanation of .AsExpandable()

Answer 1

You can reuse expressions with AsExpandable extension from LinqKit liblary (http://www.albahari.com/nutshell/linqkit.aspx).

Example:

Expression<Func<Customer,long>> func = c => c.Orders
  .Where(order => order.OrderDate >= DateTime.Now.AddDays(-30)
  .Sum(order => order.TotalValue);

var result = await ctx.Customers
  .AsExpandable() // this allow to unwrap injected expression
  .Select(cust => new CustomerDto {
    CustomerId = cust.Id,
    CustomerName = cust.Name,
    CurrentValue = func.Invoke(cust) // this inject predefined expression
  })
  .ToListAsync();