Convert anonymous type to new C# 7 tuple type

Question

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.

Answer 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);