Create a Tuple in a Linq Select

Question

I'm working with C# and .NET Framework 4.5.1 retrieving data from a SQL Server database with Entity Framework 6.1.3.

I have this:

codes = codesRepo.SearchFor(predicate)       .Select(c => new Tuple<string, byte>(c.Id, c.Flag))       .ToList(); 

And when I run it, I get this message:

Only parameterless constructors and initializers are supported in LINQ to Entities.

I don't know how I have to create the Tuple because all the examples that I have found are mostly like this one.

I have tried this:

codes = codesRepo.SearchFor(predicate)       .Select(c => Tuple.Create(c.Id, c.Flag))       .ToList(); 

And get this error:

LINQ to Entities does not recognize the method 'System.Tuple`2[System.String,System.Byte] Create[String,Byte](System.String, Byte)' method, and this method cannot be translated into a store expression.

Where is the problem?

Answer 1

While the answer by octavioccl works, it's better to first project the query result into anonymous type, and then switch to enumerable and convert it to tuple. This way your query will retrieve from the data base only the fields needed.

codes = codesRepo.SearchFor(predicate)     .Select(c => new { c.Id, c.Flag })     .AsEnumerable()     .Select(c => new Tuple<string, byte>(c.Id, c.Flag))     .ToList(); 

---

Note: The above rule applies to EF6. EF Core naturally supports tuples (in projection or as join/group keys) via tuple constructor, e.g. the original query simply works

codes = codesRepo.SearchFor(predicate)   .Select(c => new Tuple<string, byte>(c.Id, c.Flag))   .ToList(); 

but not the Tuple.Create method (EF Core 2.x).

Answer 2

Just an updated answer for C# 7, now you can use a simpler syntax to create ValueTuples.

codes = codesRepo.SearchFor(predicate) .Select(c => new { c.Id, c.Flag }) .AsEnumerable() .Select(c => (c.Id, c.Flag)) .ToList(); 

You can even name the properties of the tuple now:

codes = codesRepo.SearchFor(predicate) .Select(c => new { c.Id, c.Flag }) // anonymous type .AsEnumerable() .Select(c => (Id: c.Id, Flag: c.Flag)) // ValueTuple .ToList(); 

So instead of using it as Item1 or Item2 you can access it as Id or Flag.

More docs on choosing-between-anonymous-and-tuple