Basic LinqToSql question: Why won't this compi

问题:

I've been introducing myself to LinqToSQL lately through a poorly-made project at work. I'm curious as to why this works:

var territories = db.Territories.Where(t => t.PendingUserCount > 0);

But this results in a compilation error:

var territories = db.Territories;
if (someCondition)
    territories = territories.Where(t => t.PendingUserCount > 0);
// Cannot implicitly convert 'System.Linq.IQueryable<Territory> to System.Data.Linq.Table<Territory>

I've also tried to call db.Territories.ToList(), but to no avail.

I'm sure it's just a misunderstanding of how Linq works, but I'd be appreciative if someone could help me out.

回答1:

Alternative:

var territories = db.Territories.AsQueryable();
if (someCondition)
    territories = territories.Where(t => t.PendingUserCount > 0);

回答2:

db.Territories returns a table object. Hence the 'var' will be of type System.Data.Linq.Table. Later you try (based on some condition) to assign something of type System.Linq.IQueryable to the variable. As .NET is strongly typed, the compiler throws an error.

Variables of type var will be assigned a type when they get assigned first. That's how I try to remember myself.

回答3:

For this type of cumulative Where, you need to tell the compiler to use IQueryable<T>:

IQueryable<Territory> territories = db.Territories;
if (someCondition)
    territories = territories.Where(t => t.PendingUserCount > 0);
... etc

回答4:

change to this

var territories = db.Territories;

to

IQueryable territories = db.Territories.Where(t => t.PendingUserCount > 0);

The reasoning is that by calling db.Territories, you are getting all the data back, returning it in a linq.table object. Db.Territores.where(... will return an IQueryable object instead.

回答5:

One of the potentially confusing things about "var" is that its type is determined at compile time, so you can't assign a range of different types to it. People with experience of dynamic languages, like Python, sometimes get confused by this, at first.

回答6:

Your var territories is typed as a System.Data.Linq.Table<Territory> initially and then you are trying to assign the results of a Where clause (which is of type System.Linq.IQueryable<Territory>) to it.

Remember that the compiler infers the type of a var at assignment so once it is assigned the type cannot be changed.

Try something like this:

System.Linq.IQueryable<Territory> territories;
if (someCondition)
    territories = db.Territories.Where(t => t.PendingUserCount > 0);

回答7:

Because you've typed "var" as a Table<Territory> and then try to reassign it as a IQueryable<Territory>.

This is equivalent to saying

  var i = 0

  i = "a string";

EDIT: To clarify, var is implicitly strong typed at compile time not run time, unlike dynamically typed scripting language.

回答8:

You can't re-assign a var to a different type after you've declared it. So your var declaration has already typed it to System.Data.Linq.Table.

It's a fire once thing.

回答9:

You need to use the IQueryable tyupe as others have suggested:

also this linq query maybe also work:

var query = from territories in db.Territories
            where territories.SomeProperty == SomeCondition
            where territories.PendingUserCount > 0
            select territories;