Why are foreign keys in EF Code First marked as virtual?

Question

public virtual Student Student {get; set;}

Why does a foreign key constraint need to be marked as virtual? I've seen examples with both virtual and lacking virtual. Does it matter?

Answer 1

By looking at this : https://msdn.microsoft.com/en-us/library/jj574232(v=vs.113).aspx, and the link that has been provided in the comments by @Shoes.

I would say this :

1. if you declare your property virtual :

Your virtual property (by default) won't be loaded right away when querying the main object. It will be retreive from the database ONLY if you try to access it, or access one of it's components.

And this is called lazy loading.

2. if you declare it non-virtual :

Your property will (by default) be loaded right away along with all the other property in your main entity. This means your property will be ready to access : it has already been retreived. Entity won't have to query again the database because you access this property.

This is called eagerly loading.

My opinion :

More often i choose eagerly loading (non-virtual) because most of the time, i need every property of every entity to be used along without having to query back (faster in the case you really want everything quick) but if you access this property only once in a while (your not listing anything) and you want more often just the rest of the informations exept THIS one, then make it virtual so this property won't slow down the rest of the query just for a few access.

Hope this was clear...

Exemples :

Where I would NOT use virtual (Eagerly) :

foreach(var line in query)
{
    var v = line.NotVirtual; // I access the property for every line
}

Where I would use virtual or lazy loading :

foreach(var line in query)
{
    if(line.ID == 509)        // because of this condition
    var v = line.Virtual; // I access the property only once in a while
}

one last thing :

If you don't query over 1 000 lines of a database, then whatever you choose won't have a big effect. Also, you can declare these property virtual and if you want to test the other way around, you just have to do this (Entity 4.0) :

context.LazyLoadingEnabled = false;

It will cancel the virtual effect.

Edit

For newer versions of EF :

WhateverEntities db = new WhateverEntities() 
db.Configuration.LazyLoadingEnabled = false;