Question

Does anyone know of a definitive list of LINQ to SQL query limitations that are not trapped at compile time, along with (where possible) workarounds for the limitations?

The list we have so far is:

• Calling methods such as .Date on DateTime
  • no workaround found
• string.IsNullOrEmpty
  • simple, just use == '' instead
• .Last()
  • we used .OrderByDescending(x => x.WhateverProperty).First()

Answer 1

Basically, that list is huge… it is everything outside of the relatively small set of things that are handled. Unfortunately, the Law Of Leaky Abstractions kicks in, and each provider has different answers…

LINQ-to-Objects will do anything (pretty much), since it is delegates; LINQ-to-SQL and Entity Framework have different sets of support.

In general, I’ve had a fair amount of success using the DateTime properties etc – but in reality, you’re going to have to ensure that your query expressions are covered by unit tests, so that if you ever change providers (or the provider gets updated) you know it all still works.

I guess one view is to think in terms of TSQL; there is no BOTTOM n, but there is a TOP 1 (re the OrderByDescending); In terms of string.IsNullOrEmpty, you could be quite literal: foo.Bar == null || foo.Bar == ''; and with DateTime.Date you can probably do quite a bit with DATEPART / the various components.

Another option with LINQ-to-SQL is to encapsulate the logic in a UDF – so you could write a UDF that takes a datetime and returns a datetime, and expose that via the dbml onto the data-context. You can then use that in your queries:

where ctx.Date(foo.SomeDate) == DateTime.Today 

This approach, however, doesn’t necessarily make good use of indexes.

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Update:

• The supported method translations etc are here.
• The supported query operations etc are here.

For the full gory details, you can look at System.Data.Linq.SqlClient.PostBindDotNetConverter+Visitor in reflector – in particular the Translate... methods; some string functions are handled separately. So not a huge selection – but this is an implementation detail.