Question

Good morning.

Let me show you some code:

    //Db is a database of Tag objects, sort of
    Db db;   
    string code = "123";
    var selectedTag = db.Select(new Predicate<Tag>(tag => tag.Code == code));

    //...

    class Db
    {
        //A query method, accepting a Predicate
        public Tag Select(Predicate<Tag> predicate)
        {
            /* Here, using Intellisense, I see that Target property is a Tag object,
              but I can't cast it to Tag! Why??
            */
            var t = predicate.Target as Tag; //Always null!
        }
    }

I basically have a simple database of Tag object; when a caller invoke Select method, I would retirieve Tag object istance from predicate, but actually I can’t get it work.
Using Intellisens in debug, when I am in Select method I clearly see that predicate.Target is referenced to my Tag object, but I don’t know how to retrieve it.
Any ideas?
Thank you.

EDIT
If I try to unsafely cast predicate.Target to Tag, this way:

var t = (Tag)predicate.Target;

I get an InvalidCastException; ok, but Exception message says I cannot cast object of type DbTests to type Tag.
DbTests is the NUnit test class I’m using for my testing purpose (obviuosly…:).
Weird!
EDIT 2
Typo in line:

var selectedTag = db.Select(new Predicate<Tag>(tag => tag.Code == code));

Answer 1

Your Select method takes a delegate (the type of that delegate is a Predicate<T>, more specifically Predicate<Tag>). that delegate points to an instance and a method. What you’ve passed in for the delegate is a lambda tag => tag.Code == code. That lambda causes the compiler to generate a method for this code. That method is either contained on the instance that calls Select, or in a new class. If the generated method is static, there will be no instance and Target will be null. In any case that class that contains that method is not Tag (unless the call to Select is within the Tag class; but you didn’t detail that). So, Target cannot be a Tag object and can never be cast to Tag successfully.

The predicate that select receives should be given a Tag object. For example:

public Tag Select(Predicate<Tag> predicate)
{
    Tag tag = new Tag();
    bool result = predicate(tag);
    return tag;
}

it’s not clear what you really want to do within Select; but Predicate<Tag> just returns true or false depending on the Tag given to it.

Update:

In your example, it’s as if you wrote this:

public class DbTests
{
    public void Test1()
    {
        Db db = new Db();
        Tag selectedTag = db.Select((Predicate<Tag>) TestTag);
    }

    private bool TestTag(Tag tag)
    {
        return tag.Code == "123";
    }
}

There is no Tag instance anywhere in the above snippet.

Update:

If you used an instance method from the containing class, for example:

public class DbTests
{
    public string code = "123";
    public void Test1()
    {
        Db db = new Db();
        Tag selectedTag = db.Select(tag => tag.Code == code);
    }
}

Then the predicate sent to select will have an instance associated with it and thus Target will not be null. In the above code, Target would be of type DbTest But, if you put the call to Select within TagData (e.g. DbTests was renamed to TagData) then Target would be of type TagData and show you instance members from TagData and you’d get what you see in your screen capture.

Select doesn’t seem like the right term for a method that uses a Predicate<T>