Question

I have got following message:

Warning: Field is never assigned to, and will always have its default value null.

My code looks like (it is simplified, so useless):

public class MyEntity
{
    // ...
    public string MyProp { get; set; }
}

public class MyClass
{
    string dbMyProp;

    public string MyProp { get { return dbMyProp.Replace("a", "b"); } }

    public static readonly Expression<Func<MyEntity, MyClass>> FromMyEntity = e => new MyClass
    {
        dbMyProp = e.MyProp // ...
    };
}

I think that the message is not true.

Is it a bug in C# compiler or I have missed something?

UPDATE The field is dbMyProp. It is simplified but it still produces this warning.

UPDATE2 The following code does not produce such warning:

public class MyClass2
{
    string dbMyProp;

    public string MyProp { get { return dbMyProp.Replace("a", "b"); } }

    public static MyClass2 FromMyEntity(MyEntity e)
    {
        return new MyClass2 
        {
            dbMyProp = e.MyProp // ...
        };
    }
}

Answer 1

Yes, it seems a shortcoming of the compiler. It apparently cannot see through (does not include) the Expression.
The following prints “bbc” as expected:

var exp = MyClass.FromMyEntity.Compile();
var mc = exp(new MyEntity { MyProp = "abc"});
Console.WriteLine(mc.MyProp);

I tried a few tricks like a private constructor on MyClass but the warning remains.

It seems the compiler applies a more conservative algorithm, only checking if any (normal) member assigns to dbMyProp .

---

You are confusing things by having 2 different elements with the name dbMyProp but MyEntity.MyProp is never written to here.