Trying to use the await keyword in a LINQ query and I get this:
The 'await' operator may only be used in a query expression within the first collection expression of the initial 'from' clause or within the collection expression of a 'join' clause
Sample Code:
var data = (from id in ids
let d = await LoadDataAsync(id)
select d);
Is it not possible to await something in a LINQ query, or does it need to be structured a different way?
LINQ has very limited support for async/await. For LINQ-to-objects, the only really useful operation I know of is to do a Select with an async delegate (which results in a sequence of tasks).
List<T> data = new List<T>();
foreach (var id in ids)
data.Add(await LoadDataAsync(id));
If you can do LoadDataAsync in parallel safely, your example could be rewritten as:
T[] data = await Task.WhenAll(ids.Select(id => LoadDataAsync(id)));
You can define some async linq operations by yourself (for linq to objects):
for example: you can write your own WhereAsync extension method:
public static async Task<IEnumerable<T>> WhereAsync<T>(
this IEnumerable<T> target, Func<T, Task<bool>> predicateAsync)
{
var tasks = target.Select(async x => new { Predicate = await predicateAsync(x).ConfigureAwait(false), Value = x }).ToArray();
var results = await Task.WhenAll(tasks).ConfigureAwait(false);
return results.Where(x => x.Predicate).Select(x => x.Value);
}
And use it like that:
var ints = new List<int> { 1, 2, 3 };
var smallInts = await ints.WhereAsync(IsSmallIntAsync);
Using reactive extensions, it is possible to handle the results of a linq query asynchronously like this:
(from d in ids
select LoadDataAsync(d).ToObservable()).Merge()
This is gives you an observable stream you can respond to in various ways. For example, you can then .Buffer the results into a list with a timeout.
The above in essence says "for every d in ids, apply an asynchronous function to it, which yields a task for each d, and treat that as an observable of an individual result (ToObservable), and treat all those observables together as a single observable stream (Merge)
Related
I'd like to know it is it possible to create programmatically single LINQ query (for EntityFramework 6) with N .Where() clauses, but with OR between these .Where() clauses.
Imagine IQueryable object defined like:
var query = dbContext.MyTable.Where(mt => mt.TimeStamp >= DateBegin);
What I need else is add N (unknown number) of Where clauses, but with OR condition between them.
Image list of some object:
List<MyObject> myObj =
new List<MyObject>({new MyObject {val = "a" }, new MyObject { val = "b"}}); //In real code there is more than 1 property.
then I'd like to add Where() clauses to query like:
myObj.ForEach(mo =>{
// THIS CREATES -AND- BETWEEN WHERE CLAUSES, BUT I NEED -OR-
query.Where(q=>q.MyValue == mo.val); // In real code there is more than 1 property to compare
});
I was thinking about .Union() beteween queries, but It could generate union between separated queries and it's not optimal I think.
Thanks!
Here's the solution: linq-to-entities-combining-predicates
Or course is necessary to use "latest" answer:
Copy/Paste class ParameterRebinder
Copy/Paste static class Utility
Usage:
Expression<Func<Car, bool>> theCarIsRed = c => c.Color == "Red";
Expression<Func<Car, bool>> theCarIsCheap = c => c.Price < 10.0;
Expression<Func<Car, bool>> theCarIsRedOrCheap = theCarIsRed.Or(theCarIsCheap);
var query = carQuery.Where(theCarIsRedOrCheap);
Because in my solution is N of expressions, I take first expression and then append other expressions in ForEach cycle.
var firstExpression = expressionList.First();
expressionList.Skip(1).ToList().ForEach(ex => { firstExpression = firstExpression.Or(ex); });
I have a Lambda where expression which filters a list of customer based on a ID passed in. This works fine however I want to remove the timestamp from the CreationDate field when I return the records. Is there anyway to do this within the Lambda expression?
So this is my Lambda expression which returns my customer records:
customers = customers.Where(c => c.Business_Type == businessType);
However I'd like to do something like the following:
customers = customers.Where(c => c.Business_Type == businessType, c.CreationDate=c.CreationDate.Value.ToShortDateString());
LINQ is not meant to perform mutation of sequence elements. Just take the return value of Where and use foreach to perform the mutation, which is the idiomatic way to handle this:
var customers = customers.Where(c => c.Business_Type == businessType).ToArray();
foreach(var c in customers)
{
c.CreationDate = c.CreationDate.Value.ToShortDateString();
}
If I were to use IEnumerable instead of var in the code example below, will the SQL be generated only during the execution of the foreach statement? Or will it execute as an when the Linq statements are evaluated?
var query = db.Customers.Where (c => c.Age > 18);
query = query.Where (c => c.State == "CO");
var result = query.Select (c => c.Name);
foreach (string name in result) // Only now is the query executed!
Console.WriteLine (name);
Another example:
IEnumerable<Order> query = db.Orders.Where(o => o.Amount > 1000);
int orderCount = query.Count();
Would it be better to use var (or IQueryable) as it would be executed a select count(*)... when .Count() is executed or would it be exactly same with the IEnumerable code shown above?
It would make no difference. var is just syntactic sugar. If you hover over var, you will see what type C# thinks your query is.
From http://msdn.microsoft.com/en-us/library/bb383973.aspx
Beginning in Visual C# 3.0, variables that are declared at method scope can have an implicit type var. An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type. The following two declarations of i are functionally equivalent:
var i = 10; // implicitly typed
int i = 10; //explicitly typed
If you want to perform actions on your query that SQL wouldn't know what to do with, such as a method defined in your class, then you could use AsEnumerable().
For example:
var query = db.Customers
.Where(c => c.Age > 18)
.AsEnumerable()
.Select(c => GetCustomerViewModel());
//put definition of GetCustomerViewModel() somewhere in the same class
Update
As Omar mentioned, if you want your query to be executed immediately (rather than deferred), use ToList(). This will immediately enumerate the IQueryable/IEnumerable and fill the list with the actual data from the database.
In general, the SQL is generated when GetEnumerator is called on the IQueryable.
In your example, there is a subtle difference that you may want to consider. Let's take your original example:
var query = db.Customers.Where (c => c.Age > 18);
query = query.Where (c => c.State == "CO");
var result = query.Select (c => c.Name);
In this case if you change your first query to IEnumerable query = ..., then the second line would use the IEnumerable version of the Where extension (LINQ to Objects) rather than the IQueryable one (LINQ to SQL/EF). As a result, when we start iterating, the first where clause is passed to the database, but the second where clause is performed on the client side (because it has already been converted to an IEnumerable).
Another subtle item you want to be aware of is the following type of code:
var query = db.OrderBy(c => c.State);
query = query.Customers.Where(c => c.Age > 18); // Fails: Widening
In this case, since your original query returns IOrderedQueryable rather than IQueryable. If you try to then assign query to the result of the .Where operation, you're trying to widen the scope of the return type and the compiler will refuse to perform that widening. As a result, you have to explicitly specify the baseline type rather than using var:
IQueryable<Customer> query = db.OrderBy(c => c.State); // Is narrowing rather than widening.
query = query.Customers.Where(c => c.Age > 18);
Linq queries return IQueryable<> or IEnumerable<>, the execution of both is deferred.
As DanM stated, whether or not you use var or IEnumerable<> it all depends on the return value of the method you're calling: if it's an IEnumerable<> or IQuerable<> it'll be deferred, if you use .ToList(), it'll be executed right away.
When to use var comes down to personal choice/style. I generally use var when the return value is understood from the line of code and variable name or if I'm instantiating a generic with a long declartion, e.g. Dictionary<string, Func<Order, object>>.
From your code, it's clear that a collection of Customers/Orders is returned, so I would use the var keyword. Again, this is a matter of personal preference.
I'm using Enumerable.ToDictionary to create a Dictionary off of a linq call:
return (from term in dataContext.Terms
where term.Name.StartsWith(text)
select term).ToDictionary(t => t.TermID, t => t.Name);
Will that call fetch the entirety of each term, or will it only retrieve the TermID and the Name fields from my data provider? In other words, would I be saving myself database traffic if I instead wrote it like this:
return (from term in dataContext.Terms
where term.Name.StartsWith(text)
select new { term.TermID, term.Name }).ToDictionary(t => t.TermID, t => t.Name);
Enumerable.ToDictionary works on IEnumerable objects. The first part of your statement "(from ... select term") is an IQueryable object. Queryable is going to look at the expression and build the SQL statement. It will then convert that to an IEnumerable to pass to ToDictionary().
In other words, yes, your second version would be more efficient.
The generated SQL will return the entire term, so your second statement will bring down just what you need.
You can set dataContext.Log = Console.Out and look at the different results of the query.
Using my sample LINQPad database, here's an example:
var dc = (TypedDataContext)this;
// 1st approach
var query = Orders.Select(o => o);
dc.GetCommand(query).CommandText.Dump();
query.ToDictionary(o => o.OrderID, o => o.OrderDate).Dump();
// 2nd approach
var query2 = Orders.Select(o => new { o.OrderID, o.OrderDate});
dc.GetCommand(query2).CommandText.Dump();
query2.ToDictionary(o => o.OrderID, o => o.OrderDate).Dump();
The generated SQL is (or just peek at LINQPad's SQL tab):
// 1st approach
SELECT [t0].[OrderID], [t0].[OrderDate], [t0].[ShipCountry]
FROM [Orders] AS [t0]
// 2nd approach
SELECT [t0].[OrderID], [t0].[OrderDate]
FROM [Orders] AS [t0]
No. ToDictionary is an extension method for IEnumerable<T> not IQueryable<T>. It doesn't take an Expression<Func<T, TKey>> but simply a Func<T, TKey> that it'll blindly call for each item. It doesn't care (and doesn't know) about LINQ and the underlying expression trees and stuff like that. It just iterates the sequence and builds up a dictionary. As a consequence, in your first query, all columns are fetched.
Let's say I have an array, and I want to do a LINQ query against a varchar that returns any records that have an element of the array anywhere in the varchar.
Something like this would be sweet.
string[] industries = { "airline", "railroad" }
var query = from c in contacts where c.industry.LikeAnyElement(industries) select c
Any ideas?
This is actually an example I use in my "Express Yourself" presentation, for something that is hard to do in regular LINQ; As far as I know, the easiest way to do this is by writing the predicate manually. I use the example below (note it would work equally for StartsWith etc):
using (var ctx = new NorthwindDataContext())
{
ctx.Log = Console.Out;
var data = ctx.Customers.WhereTrueForAny(
s => cust => cust.CompanyName.Contains(s),
"a", "de", "s").ToArray();
}
// ...
public static class QueryableExt
{
public static IQueryable<TSource> WhereTrueForAny<TSource, TValue>(
this IQueryable<TSource> source,
Func<TValue, Expression<Func<TSource, bool>>> selector,
params TValue[] values)
{
return source.Where(BuildTrueForAny(selector, values));
}
public static Expression<Func<TSource, bool>> BuildTrueForAny<TSource, TValue>(
Func<TValue, Expression<Func<TSource, bool>>> selector,
params TValue[] values)
{
if (selector == null) throw new ArgumentNullException("selector");
if (values == null) throw new ArgumentNullException("values");
if (values.Length == 0) return x => true;
if (values.Length == 1) return selector(values[0]);
var param = Expression.Parameter(typeof(TSource), "x");
Expression body = Expression.Invoke(selector(values[0]), param);
for (int i = 1; i < values.Length; i++)
{
body = Expression.OrElse(body,
Expression.Invoke(selector(values[i]), param));
}
return Expression.Lambda<Func<TSource, bool>>(body, param);
}
}
from c in contracts
where industries.Any(i => i == c.industry)
select c;
something like that. use the any method on the collection.
IEnumerable.Contains() translates to SQL IN as in:
WHERE 'american airlines' IN ('airline', 'railroad') -- FALSE
String.Contains() which translates to SQL LIKE %...% as in:
WHERE 'american airlines' LIKE '%airline%' -- TRUE
If you want the contacts where the contact's industry is LIKE (contains) any of the given industries, you want to combine both Any() and String.Contains() into something like this:
string[] industries = { "airline", "railroad" };
var query = from c in contacts
where industries.Any(i => c.Industry.Contains(i))
select c;
However, combining both Any() and String.Contains() like this is NOT supported in LINQ to SQL. If the set of given industries is small, you can try something like:
where c.Industry.Contains("airline") ||
c.Industry.Contains("railroad") || ...
Or (although normally not recommended) if the set of contacts is small enough, you could bring them all from the DB and apply the filter with LINQ to Objects by using contacts.AsEnumerable() or contacts.ToList() as the source of the query above:
var query = from c in contacts.AsEnumerable()
where industries.Any(i => c.Industry.Contains(i))
select c;
it will work if you build up the query as follows:
var query = from c in contacts.AsEnumerable()
select c;
query = query.Where(c=> (c.Industry.Contains("airline")) || (c.Industry.Contains("railroad")));
you just need to programmatically generate the string above if the parameters airline and railroad are user inputs. This was in fact a little more complicated than I was expecting. See article - http://www.albahari.com/nutshell/predicatebuilder.aspx
Unfortunately, LIKE is not supported in LINQ to SQL as per here:
http://msdn.microsoft.com/en-us/library/bb882677.aspx
To get around this, you will have to write a stored procedure which will accept the parameters you want to use in the like statement(s) and then call that from LINQ to SQL.
It should be noted that a few of the answers suggest using Contains. This won't work because it looks to see that the entire string matches the array element. What is being looked for is for the array element to be contained in the field itself, something like:
industry LIKE '%<element>%'
As Clark has mentioned in a comment, you could use a call to IndexOf on each element (which should translate to a SQL call):
string[] industries = { "airline", "railroad" }
var query =
from c in contacts
where
c.industry.IndexOf(industries[0]) != -1 ||
c.industry.IndexOf(industries[1]) != -1
If you know the length of the array and the number of elements, then you could hard-code this. If you don't, then you will have to create the Expression instance based on the array and the field you are looking at.