Question

In general, If I create an extension method that acts on an entity:

public static MyEntity Foo(this MyEntity entity)
{
  // do something to the entity
}

One cannot directly use this in a projection from Linq-To-Entities such as follows:

var result = myContext.MyEntities.Select(x=> x.Foo());

Doing so yields an error such as:

System.NotSupportedException: LINQ to Entities does not recognize the
method ‘Foo(MyEntity)’ method, and this method cannot be translated
into a store expression.

I fully understand why this error occurs. My question is this: If I can provide an implementation of Foo that uses an expression tree, is there some way that I can add Foo to the operations that LINQ-to-entities understands? And if so – how?

---

Note: I can certainly convert it to a list like this:

var result = myContext.MyEntities.ToList().Select(x=> x.Foo());

And it doesn’t error. But I no longer have an IQueryable. I have an IEnumerable. If I were to use it like this:

var result = myContext.MyEntities.ToList().Select(x=> x.Foo()).First();

I would end up fetching ALL entities before taking the top one and discarding the rest – which would be horrible for performance.

Answer 1

Use LINQKit project, but instead of your method foo, You will be obliged to write expression tree which is equivalent.