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();
}
Related
Trying to add a second where clause to a linq expression but it won't register.
var query = _dbSetBookedResource.AsQueryable<Resource>();
var resources = (from Resource in query where Resource.DateFrom == date select Resource)
if(true)
{
resources.Where(b => b.MemberId == currentUserId);
}
For some reason the second where clause won't register.
For some reason the second where clause won't register.
That's because you're not using the return value anywhere. That's just setting up a query, but then ignoring it. No LINQ methods change the value they're called on - instead they create a new query which has the appropriate filtering, projection etc.
You need:
resources = resources.Where(b => b.MemberId == currentUserId);
Also note that your initial query could be written more simply as:
var resources = query.Where(r => r.DateFrom == date);
Query expressions are overkill when all you want is a simple filter or projection.
I have the following linq-to-entities query with 2 joined tables that I would like to add pagination to:
IQueryable<ProductInventory> data = from inventory in objContext.ProductInventory
join variant in objContext.Variants
on inventory.VariantId equals variant.id
where inventory.ProductId == productId
where inventory.StoreId == storeId
orderby variant.SortOrder
select inventory;
I realize I need to use the .Join() extension method and then call .OrderBy().Skip().Take() to do this, I am just gettting tripped up on the syntax of Join() and can't seem to find any examples (either online or in books).
NOTE: The reason I am joining the tables is to do the sorting. If there is a better way to sort based on a value in a related table than join, please include it in your answer.
2 Possible Solutions
I guess this one is just a matter of readability, but both of these will work and are semantically identical.
1
IQueryable<ProductInventory> data = objContext.ProductInventory
.Where(y => y.ProductId == productId)
.Where(y => y.StoreId == storeId)
.Join(objContext.Variants,
pi => pi.VariantId,
v => v.id,
(pi, v) => new { Inventory = pi, Variant = v })
.OrderBy(y => y.Variant.SortOrder)
.Skip(skip)
.Take(take)
.Select(x => x.Inventory);
2
var query = from inventory in objContext.ProductInventory
where inventory.ProductId == productId
where inventory.StoreId == storeId
join variant in objContext.Variants
on inventory.VariantId equals variant.id
orderby variant.SortOrder
select inventory;
var paged = query.Skip(skip).Take(take);
Kudos to Khumesh and Pravin for helping with this. Thanks to the rest for contributing.
Define the join in your mapping, and then use it. You really don't get anything by using the Join method - instead, use the Include method. It's much nicer.
var data = objContext.ProductInventory.Include("Variant")
.Where(i => i.ProductId == productId && i.StoreId == storeId)
.OrderBy(j => j.Variant.SortOrder)
.Skip(x)
.Take(y);
Add following line to your query
var pagedQuery = data.Skip(PageIndex * PageSize).Take(PageSize);
The data variable is IQueryable, so you can put add skip & take method on it. And if you have relationship between Product & Variant, you donot really require to have join explicitly, you can refer the variant something like this
IQueryable<ProductInventory> data =
from inventory in objContext.ProductInventory
where inventory.ProductId == productId && inventory.StoreId == storeId
orderby inventory.variant.SortOrder
select new()
{
property1 = inventory.Variant.VariantId,
//rest of the properties go here
}
pagedQuery = data.Skip(PageIndex * PageSize).Take(PageSize);
My answer here based on the answer that is marked as true
but here I add a new best practice of the code above
var data= (from c in db.Categorie.AsQueryable().Join(db.CategoryMap,
cat=> cat.CategoryId, catmap => catmap.ChildCategoryId,
cat, catmap) => new { Category = cat, CategoryMap = catmap })
select (c => c.Category)
this is the best practice to use the Linq to entity because when you add AsQueryable() to your code; system will converts a generic System.Collections.Generic.IEnumerable to a generic System.Linq.IQueryable which is better for .Net engine to build this query at run time
thank you Mr. Khumesh Kumawat
You would simply use your Skip(itemsInPage * pageNo).Take(itemsInPage) to do paging.
I have a LINQ query where I want to select and read the p.Api value.
var api = DataAccessNew.Instance.dcServers.Where(p => p.Ip == IpAddress).Select(p => p.Api);
How do I read the p.Api value?
I have tried api.ToString() but I get SQL instead of actual column value.
You are getting an IEnumerable<> back (and your ToString call is showing you the value of that expression).
If you are expecting a single value, do this:
var api = DataAccessNew.Instance.dcServers
.Where(p => p.Ip == IpAddress)
.Select(p => p.Api)
.Single();
You might be interested to read about the other methods like Single(): SingleOrDefault, First, FirstOrDefault. Which one you used depends on whether you are expecting a single or multiple values returned (Single vs. First) and what you want to happen if there are no values (the *Default methods will return the type default instead of throwing an exception).
Or if you want to look at all the returned values:
var api = DataAccessNew.Instance.dcServers
.Where(p => p.Ip == IpAddress)
.Select(p => p.Api);
foreach (var apiValue in api)
{
// apiValue will have the value you're looking for.
}
Try this snippet of code:
string apiValue = api.FirstOrDefault().ToString();
your syntex seems ok..
By the way try this
string api =DataAccessNew.Instance.dcServers.Where(p => p.Ip == IpAddress).Select(p => p.Api).FirstOrDefault();
if p.Ip is a unique key in your table you could try to add .FirstOrDefault() after your Linq query.
public string getselectedvalue(ListBox l)
{
string vtext="",vval="";
var selectedQueryText = l.Items.Cast<ListItem>().Where(item => item.Selected);
var selectedQueryVal = l.Items.Cast<ListItem>().Where(item => item.Selected).Select(item => item.Value);
vtext= String.Join("','", selectedQueryText ).TrimEnd();
vval= String.Join("','", selectedQueryVal ).TrimEnd();
return v;
}
If I want to iterate through a collection, and call a function on each element in the collection, I could go with :
foreach (var obj in objColl)
{
MyFunction(obj);
}
Should I want to do this with linq, I can use either of those :
//#1
var unused = (from var obj in objColl select MyFunction(obj)).ToList();
//#2
var unused = objColl.Select(obj => MyFunction(obj)).ToList();
I know this works, but it doesn't seem right. Of course, my actual cases are more complex queries that that, but it comes down to this since I could build my IQueryable with Linq and iterate through it and call the function.
Edit:
Here is one example of what I did. (Item# are things I can't disclose)
var dummyValue = (from
Item7 in dal.GetAgencyConvertions().Where(age => age.SourceName == "Item1" && age.TargetName == "Item2")
join Item6 in dal.GetAgencyConvertions().Where(age => age.SourceName == "Item2" && age.TargetName == "Item3") on Item6.TargetValue equals Item7.SourceValue
join agency in dal.GetAgencies() on Item7.SourceValue equals agency.Agency
orderby Item7.TargetValue
select vl.ValueListItems.Add(agency.ID, Item7.TargetValue)).ToList();
Go with the simple foreach, as you are clearly wanting to perform an action on (and/or using) the objects in your collection as opposed to wishing to project/filter/group/etc. the items in the sequence. LINQ is about the latter set of operations.
Edit: In the case of your update, I would simply create a query, and then iterate over the query in the foreach to perform the action.
var query = from Item7 in dal.GetAgencyConvertions().Where(age => age.SourceName == "Item1" && age.TargetName == "Item2")
join Item6 in dal.GetAgencyConvertions().Where(age => age.SourceName == "Item2" && age.TargetName == "Item3") on Item6.TargetValue equals Item7.SourceValue
join agency in dal.GetAgencies() on Item7.SourceValue equals agency.Agency
orderby Item7.TargetValue
select new { ID = agency.ID, Value = Item7.TargetValue };
foreach (var item in query)
vl.ValueListItems.Add(item.ID, item.Value);
To be frank, you have the same loop happening in your code, you merely mask it by using the ToList() extension method. As a byproduct, you are creating a list of values that you have no intention of using, while somewhat obfuscating the true intention of the code, all to save maybe a few characters.
Typically, a query shouldn't have any side effects (i.e. it shouldn't modify the state of the data or other data in your application) which raises the question, does MyFunction modify the state of your application? If it does, then you should use a foreach loop.
How about an Each() extension method?
public static void Each<T>(this IEnumerable<T> target, Action<T> action)
{
if (target == null) return;
foreach (T obj in target)
action(obj);
}
Let's say I have a table in a database that has three columns: Agency ID, Name, and Value.
I want to get a collection of <Name, Value> pairs grouped by Agency ID.
How can I do this? I tried something like below, which works, but makes a DB call for each agency!
from div in db.AgencyDivisionsENT
group div by div.AgencyId into NamePairCollection
select new KeyValuePair<int, IEnumerable<DivisionResults>>(NamePairCollection.Key,
NamePairCollection.Select(k => new DivisionResults
{
Name = k.Name,
Value = k.Value
));
I want to end up with something like this: IEnumerable<KeyValuePair<int, IEnumerable<NameValuePair>>>
Using chain syntax it would be:
db.AgencyDivisionsENT
.GroupBy(x=>x.AgencyId)
.ToDictionary(x=>x.Key, g=>g.Select(x=>new { k.Name, k.Value }).ToArray());
The easiest way to avoid round-tripping with this type of query is to group on the client side - by calling .AsEnumerable():
db.AgencyDivisionsENT
.Select (x => new { x.AgencyId, x.Name, x.Value } )
.AsEnumerable()
.GroupBy(...) // AsEnumerable() forces grouping to happen on the client
.ToDictionary(...)
This is in no way inefficient - as long as:
you select only the data you need from the server with the initial .Select statement
if you need .Where statement to filter the data, it is placed before the .AsEnumerable
you're selecting detail rows (as in this case) rather than just aggregates.