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Multiple "order by" in LINQ

I have two tables, movies and categories, and I get an ordered list by categoryID first and then by Name.

The movie table has three columns, ID, Name, and CategoryID. The category table two has columns, ID, and Name.

I tried something like the following, but it didn't work.

var movies = _db.Movies.OrderBy( m => { m.CategoryID, m.Name })

Linq .Any VS .Exists - Whats the difference?

Using Linq on collections, what is the difference between the following lines of code?

if(!coll.Any(i => i.Value))

and

if(!coll.Exists(i => i.Value))

Update 1

When I disassemble .Exists it looks like there is no code.

Update 2

Anyone know why there is not code there for this one?

What is the Java equivalent for LINQ? [on hold]

What is Java equivalent for LINQ?

Group By Multiple Columns

How can I do GroupBy Multiple Columns in LINQ

Something similar to this in SQL:

SELECT * FROM <TableName> GROUP BY <Column1>,<Column2>

How can I convert this to LINQ:

QuantityBreakdown
(
    MaterialID int,
    ProductID int,
    Quantity float
)

INSERT INTO @QuantityBreakdown (MaterialID, ProductID, Quantity)
SELECT MaterialID, ProductID, SUM(Quantity)
FROM @Transactions
GROUP BY MaterialID, ProductID

Dynamic LINQ OrderBy on IEnumerable

I found an example in the VS2008 Examples for Dynamic LINQ that allows you to use a sql-like string (e.g. OrderBy("Name, Age DESC")) for ordering. Unfortunately, the method included only works on IQueryable<T>;. Is there any way to get this functionality on IEnumerable<T>?

LINQ equivalent of foreach for IEnumerable

I'd like to do the equivalent of the following in LINQ, but I can't figure out how:

IEnumerable<Item> items = GetItems();
items.ForEach(i => i.DoStuff());

What is the real syntax?

OrderBy descending in Lambda expression?

I know in normal linq grammar, "orderby xxx descending" is very easy, but how do I do this in Lambda expression?

How would you do a "not in" query with LINQ?

I have two collections which have property Email in both collections. I need to get a list of the items in the first list where Email does not exist in the second list. With SQL I would just use "not in", but I do not know the equivalent in LINQ. How is that done?

So far I have a join, like...

var matches = from item1 in list1
join item2 in list2 on item1.Email equals item2.Email
select new { Email = list1.Email };

But I cannot join since I need the difference and the join would fail. I need some way of using Contains or Exists I believe. I just have not found an example to do that yet.

Group by in LINQ

Let's suppose if we have a class like

class Person { 
    internal int PersonID; 
    internal string car  ; 
}

Now I have a list of this class: List<Person> persons;

Now this list can have instances multiple same PersonIDs, for ex.

persons[0] = new Person { PersonID = 1, car = "Ferrari" }; 
persons[1] = new Person { PersonID = 1, car = "BMW"     }; 
persons[2] = new Person { PersonID = 2, car = "Audi"    }; 

Is there a way I can group by personID and get the list of all the cars he has? For ex. expected result would be

class Result { 
   int PersonID;
   List<string> cars; 
}

So after grouping by I would get:

results[0].PersonID = 1; 
List<string> cars = results[0].cars; 

result[1].PersonID = 2; 
List<string> cars = result[1].cars;

From what I have done so far:

var results = from p in persons
              group p by p.PersonID into g
              select new { PersonID = g.Key, // this is where I am not sure what to do

Could someone please point me in the right direction?

When to use .First and when to use .FirstOrDefault with LINQ?

I've searched around and haven't really found a clear answer as to when you'd want to use .First and when you'd want to use .FirstOrDefault with LINQ.

• When would you want to use .First? Only when you'd want to catch the exception if no results where returned?

  var result = List.Where(x => x == "foo").First();
  

• And when would you want to use .FirstOrDefault? When you'd always want the default type if no result?

  var result = List.Where(x => x == "foo").FirstOrDefault();
  

• And for that matter, what about Take?

  var result = List.Where(x => x == "foo").Take(1);
  

Using LINQ to remove elements from a List

Say that I have LINQ query such as:

var authors = from x in authorsList
              where x.firstname == "Bob"
              select x;

Given that authorsList is of type List<Author>, how can I delete the Author elements from authorsList that are returned by the query into authors?

Or, put another way, how can I delete all of the firstname's equalling Bob from authorsList?

Note: This is a simplified example for the purposes of the question.

LINQ Aggregate algorithm explained

This might sound lame, but I have not been able to find a really good explanation of Aggregate.

Good means short, descriptive, comprehensive with a small and clear example.

LINQ's Distinct() on a particular property

I am playing with LINQ to learn about it, but I can't figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy to do, this is not the question). What I if want to use Distinct on a list of an Object on one or more properties of the object?

Example: If an object is Person, with Property Id. How can I get all Person and use Distinct on them with the property Id of the object?

Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"

How can I get just Person1 and Person3? Is that possible?

If it's not possible with LINQ, what would be the best way to have a list of Person depending on some of its properties in .NET 3.5?

Which method performs better: .Any() vs .Count() > 0?

in the System.Linq namespace, we can now extend our IEnumerable's to have theAny() and Count() extension methods.

I was told recently that if i want to check that a collection contains 1 or more items inside it, I should use the .Any() extension method instead of the .Count() > 0 extension method because the .Count() extension method has to iterate through all the items.

Secondly, some collections have a property (not an extension method) that is Count or Length. Would it be better to use those, instead of .Any() or .Count() ?

yea / nae ?

Retrieving Property name from lambda expression

Is there a better way to get the Property name when passed in via a lambda expression? Here is what i currently have.

eg.

GetSortingInfo<User>(u => u.UserId);

It worked by casting it as a memberexpression only when the property was a string. because not all properties are strings i had to use object but then it would return a unaryexpression for those.

public static RouteValueDictionary GetInfo<T>(this HtmlHelper html, 
    Expression<Func<T, object>> action) where T : class
{
    var expression = GetMemberInfo(action);
    string name = expression.Member.Name;

    return GetInfo(html, name);
}

private static MemberExpression GetMemberInfo(Expression method)
{
    LambdaExpression lambda = method as LambdaExpression;
    if (lambda == null)
        throw new ArgumentNullException("method");

    MemberExpression memberExpr = null;

    if (lambda.Body.NodeType == ExpressionType.Convert)
    {
        memberExpr = 
            ((UnaryExpression)lambda.Body).Operand as MemberExpression;
    }
    else if (lambda.Body.NodeType == ExpressionType.MemberAccess)
    {
        memberExpr = lambda.Body as MemberExpression;
    }

    if (memberExpr == null)
        throw new ArgumentException("method");

    return memberExpr;
}