Convert anonymous type to new C# 7 tuple type

问题

The new version of C# is there, with the useful new feature Tuple Types:

public IQueryable<T> Query<T>();

public (int id, string name) GetSomeInfo() {
    var obj = Query<SomeType>()
        .Select(o => new {
            id = o.Id,
            name = o.Name,
        })
        .First();

    return (id: obj.id, name: obj.name);
}

Is there a way to convert my anonymous type object obj to the tuple that I want to return without mapping property by property (assuming that the names of the properties match)?

The context is in a ORM, my SomeType object has a lot of other properties and it is mapped to a table with lot of columns. I wanna do a query that brings just ID and NAME, so I need to convert the anonymous type into a tuple, or I need that an ORM Linq Provider know how to understand a tuple and put the properties related columns in the SQL select clause.

回答1:

The short answer is no, in the current form of C#7 there is no in-framework way to accomplish your goals verbatim, since you want to accomplish:

• Linq-to-entities
• Mapping to a subset of columns
• Avoiding property by property mapping from a custom or anonymous type to a C#7 tuple by mapping directly to a C#7 tuple.

Because Query<SomeType> exposes an IQueryable, any sort of projection must be made to an expression tree .Select(x => new {}).

There is an open roslyn issue for adding this support, but it doesn't exist yet.

As a result, until this support is added, you can either manually map from an anonymous type to a tuple, or return the entire record and map the result to a tuple directly to avoid two mappings, but this is obviously inefficient.

---

While this restriction is currently baked into Linq-to-Entities due to a lack of support and the inability to use parametered constructors in a .Select() projection, both Linq-to-NHibernate and Linq-to-Sql allow for a hack in the form of creating a new System.Tuple in the .Select() projection, and then returning a ValueTuple with the .ToValueTuple() extension method:

public IQueryable<T> Query<T>();

public (int id, string name) GetSomeInfo() {
    var obj = Query<SomeType>()
        .Select(o => new System.Tuple<int, string>(o.Id, o.Name))
        .First();

    return obj.ToValueTuple();
}

Since System.Tuple can be mapped to an expression, you can return a subset of data from your table and allow the framework to handle mapping to your C#7 tuple. You can then deconstruct the arguments with any naming convention you choose:

(int id, string customName) info = GetSomeInfo();
Console.Write(info.customName);

回答2:

Of course, by creating the tuple from your LINQ expression:

public (int id, string name) GetSomeInfo() {
    var obj = Query<SomeType>()
        .Select(o => (o.Id,o.Name))
        .First();

    return obj;
}

---

According to another answer regarding pre-C# 7 tuples, you can use AsEnumerable() to prevent EF to mix things up. (I have not much experience with EF, but this should do:)

public (int id, string name) GetSomeInfo() {
    var obj = Query<SomeType>()
        .AsEnumerable()
        .Select(o => (o.Id,o.Name))
        .First();

    return obj;
}

回答3:

While tuple literals are not currently supported in expression trees, it doesn't mean the ValueTuple type isn't. Just create it explicitly.

public (int id, string name) GetSomeInfo() =>
    Query<SomeType>()
        .Select(o => ValueTuple.Create(o.Id, o.Name))
        .First();

回答4:

Note for anyone on a lower version of .NET: If you are on a lower version of .NET than 4.7.2 or .NET Core, you should use Nuget Package Manager to install System.ValueTuple to your project.

Then, here's an example of getting a tuple from a Linq to SQL query:

var myListOfTuples = (from record1 in myTable.Query()
                     join record2 in myTable2.Query() on record1.Id = record2.someForeignKey
                     select new {record1, record2}).AsEnumerable()
                     .select(o => (o.record1, o.record2))
                     .ToList()

That ran for me, however, after check-in, I got a build failure...read on.

For even more fun and games, I unfortunately had an earlier version of C# on my build server for some reason. So I had to go back because it didn't recognize the new tuple format on the .select(o => (o.record1, o.record2)) line (specifically that it would be a tuple because of the parenthesis around o.record1 and o.record2). So, I had to go back and kind of finagle it a bit more:

var myListOfAnonymousObjects = (from record1 in myTable.Query()
                     join record2 in myTable2.Query() on record1.Id = record2.someForeignKey
                     select new {record1, record2}).ToList()

var myTuples = new List<Tuple<Record1sClass, Record2sClass>>();
foreach (var pair in myListOfAnonymousObjects)
{
    myTuples.Add(pair.record1, pair.record2);
}
return myTuples;

回答5:

    public IList<(int DictionaryId, string CompanyId)> All(string DictionaryType)
    {
        var result = testEntities.dictionary.Where(p => p.Catalog == DictionaryType).ToList();
        var resultTuple = result.Select(p => (DictionaryId: p.ID, CompanyId: p.CompanyId));
        return resultTuple.ToList();
    }

This method you can named your tuple item in a linq select

来源：https://stackoverflow.com/questions/43400071/convert-anonymous-type-to-new-c-sharp-7-tuple-type