an expression tree lambda may not contain a null propagating operator

问题

Question: The line price = co?.price ?? 0, in the following code gives me the above error. but if I remove ? from co.? it works fine. I was trying to follow this MSDN example where they are using ? on line select new { person.FirstName, PetName = subpet?.Name ?? String.Empty }; So, it seems I need to understand when to use ? with ?? and when not to.

Error:

an expression tree lambda may not contain a null propagating operator

public class CustomerOrdersModelView
{
    public string CustomerID { get; set; }
    public int FY { get; set; }
    public float? price { get; set; }
    ....
    ....
}
public async Task<IActionResult> ProductAnnualReport(string rpt)
{
    var qry = from c in _context.Customers
              join ord in _context.Orders
                on c.CustomerID equals ord.CustomerID into co
              from m in co.DefaultIfEmpty()
              select new CustomerOrdersModelView
              {
                  CustomerID = c.CustomerID,
                  FY = c.FY,
                  price = co?.price ?? 0,
                  ....
                  ....
              };
    ....
    ....
 }

回答1:

The example you were quoting from uses LINQ to Objects, where the implicit lambda expressions in the query are converted into delegates... whereas you're using EF or similar, with IQueryable<T> queryies, where the lambda expressions are converted into expression trees. Expression trees don't support the null conditional operator (or tuples).

Just do it the old way:

price = co == null ? 0 : (co.price ?? 0)

(I believe the null-coalescing operator is fine in an expression tree.)

回答2:

The code you link to uses List<T>. List<T> implements IEnumerable<T> but not IQueryable<T>. In that case, the projection is executed in memory and ?. works.

You're using some IQueryable<T>, which works very differently. For IQueryable<T>, a representation of the projection is created, and your LINQ provider decides what to do with it at runtime. For backwards compatibility reasons, ?. cannot be used here.

Depending on your LINQ provider, you may be able to use plain . and still not get any NullReferenceException.

来源：https://stackoverflow.com/questions/44681362/an-expression-tree-lambda-may-not-contain-a-null-propagating-operator

标签

linq

null-coalescing-operator