If I have a where clause as follows:
Where item.field == "value"
How can I change the statement in LINQ to be:
Where item.field in ("value1","value2","value3")
Seems simple, not working.
Thanks in advance
Declare your "in" values collection in a variable first:
var collection = new[] { "value1","value2","value3" };
And then use it in query:
...
where collection.Contains(item.field)
...
Or in lambda syntax(say you have a collection you are searching through):
var lookup = new[] { "value1","value2","value3" };
var result = collection.Where(x=> lookup.Contains(x)).ToArray();
Related
I am just not understanding the LINQ non-query syntax for GroupBy.
I have a collection of objects that I want to group by a single property. In this case Name
{ Id="1", Name="Bob", Age="23" }
{ Id="2", Name="Sally", Age="41" }
{ Id="3", Name="Bob", Age="73" }
{ Id="4", Name="Bob", Age="34" }
I would like to end up with a collection of all the unique names
{ Name="Bob" }
{ Name="Sally" }
Based on some examples I looked at I thought this would be the way to do it
var uniqueNameCollection = Persons.GroupBy(x => x.Name).Select(y => y.Key).ToList();
But I ended up with a collection with one item. So I though maybe I was over complicating things with the projection. I tried this
var uniqueNameCollection = Persons.GroupBy(x => x.Name).ToList();
Same result. I ended up with a single item in the collection. What am I doing wrong here? I am just looking to GroupBy the Name property.
var names = Persons.Select(p => p.Name).Distinct().ToList()
If you just want names
LINQ's GroupBy doesn't work the same way that SQL's GROUP BY does.
GroupBy takes a sequence and a function to find the field to group by as parameters, and return a sequence of IGroupings that each have a Key that is the field value that was grouped by and sequence of elements in that group.
IEnumerable<IGrouping<TSource>> GroupBy<TSource, TKey>(
IEnumerable<TSource> sequence,
Func<TSource, TKey> keySelector)
{ ... }
So if you start with a list like this:
class Person
{
public string Name;
}
var people = new List<Person> {
new Person { Name = "Adam" },
new Person { Name = "Eve" }
}
Grouping by name will look like this
IEnumerable<IGrouping<Person>> groups = people.GroupBy(person => person.Name);
You could then select the key from each group like this:
IEnumerable<string> names = groups.Select(group => group.Key);
names will be distinct because if there were multiple people with the same name, they would have been in the same group and there would only be one group with that name.
For what you need, it would probably be more efficient to just select the names and then use Distinct
var names = people.Select(p => p.Name).Distinct();
var uniqueNameCollection = Persons.GroupBy(x => x.Name).Select(y => y.Key).ToList();
Appears valid to me. .net Fiddle showing proper expected outcome: https://dotnetfiddle.net/2hqOvt
Using your data I ran the following code statement
var uniqueNameCollection = people.GroupBy(x => x.Name).Select(y => y.Key).ToList();
The return results were List
Bob
Sally
With 2 items in the List
run the following statement and your count should be 2.
people.GroupBy(x => x.Name).Select(y => y.Key).ToList().Count();
Works for me, download a nugget MoreLinq
using MoreLinq
var distinctitems = list.DistinctBy( u => u.Name);
I have one SelectListItem for DropDownList. I have to filter based on some condition. If I try adding the condition then its gives me an error like this (LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression). I ll be adding that code here. Please guide me to solve this.
Code
IEnumerable<SelectListItem> IssueId = (from txt in Db.Issues where txt.BibId == BibId
select new SelectListItem()
{
Text = txt.Description,
Value = txt.Id.ToString(),
Selected = true,
});
SelectList IssueIds = new SelectList(IssueId, "Value", "Text");
ViewBag.IssueId = IssueIds;
Thanks
Try this:
LINQ2EF does not know ToString() but after AsEnumerable() you'll get a local collection when ToString() is implemented.
IEnumerable<SelectListItem> IssueId =
(from txt in Db.Issues.Where(e => e.BibId == BibId).AsEnumerable()
select new SelectListItem()
{
Text = txt.Description,
Value = txt.Id.ToString(),
Selected = true
});
Linq To Sql can't generate TSQL for txt.Id.ToString()
You will need to iterate the result instead after executing the query, or cast to Enumerable as xeondev suggests.
That extension does not seem to be sorted by linq to Entities but you could just do the mapping once you have the issues, e.g.
var issues = (from issue in Db.Issues
where issue .BibId == BibId
select issue ).ToList();
IEnumerable<SelectListItem> IssueId = (from txt in issues
where txt.BibId == BibId
select new SelectListItem()
{
Text = txt.Description,
Value = txt.Id.ToString(),
Selected = true,
});
I am trying to replace a string date value "01/01/1700" with an empty string in LINQ.
The date is of type string.
Something like this but I cant get it to work.
Query<Client>(sql).ToList().ForEach(x => x.DateOfBirth =
x.DateOfBirth.Replace("01/01/1700", ""));
This code works but its not LINQ.
var result = Query<Client>(sql).ToList();
foreach (var client in result)
{
if (client.DateOfBirth == "01/01/1700")
{
client.DateOfBirth = "n/a";
}
}
Thanks for your help.
The problem is the ToList(). The result is not visible in the variable you use afterwards.
Try out the following:
var list = Query<Client>(sql).ToList();
list.ForEach(l => l.DateOfBirth = l.DateOfBirth.Replace("01/01/1700", "n/a"));
Should work fine. Use the list variable afterwards.
var result = Query<Client>(sql).ToList();
result.ForEach(l => l.DateOfBirth = l.DateOfBirth.Replace("01/01/1700", "n/a"));
Your code assumes that changes made to an object in a List will be reflected in the Query<Client> that the object came from. Apparently this is not the case. One thing you could try is assigning the list before calling ForEach() and using the list from that point on:
var clients = Query<Client>(sql).ToList();
clients.ForEach(x => x.DateOfBirth = x.DateOfBirth.Replace("01/01/1700", ""));
Also, ForEach is not a LINQ operator. It is a method in the List class. Unlike LINQ operators, it will modify the list that called it and will not return anything. The way to "modify" data with LINQ is by using select:
var clients = (from client in Query<Client>(sql).ToList()
select new Client(client)
{
DateOfBirth = client.DateOfBirth.Replace("01/01/1700", "")
}).ToList();
OK, another LINQ question. How do I do an "IN" condition using LINQ. I have an IEnumerable list of myObject and want to do something like myObject.Description in('Help', 'Admin', 'Docs'). How can I accomplish this? Thanks
IN in sql is equivalent is Contains in LINQ
string[] countries = new string[] { "UK", "USA", "Australia" };
var customers =
from c in context.Customers
where countries.Contains(c.Country)
select c;
Use Contains on a collection:
string[] descriptions = { "Help", "Admin", "Docs" };
var query = from foo in list
where descriptions.Contains(foo.Description)
select ...;
(For larger collections, a HashSet<T> might be better.)
I have a variable size array of strings, and I am trying to programatically loop through the array and match all the rows in a table where the column "Tags" contains at least one of the strings in the array. Here is some pseudo code:
IQueryable<Songs> allSongMatches = musicDb.Songs; // all rows in the table
I can easily query this table filtering on a fixed set of strings, like this:
allSongMatches=allSongMatches.Where(SongsVar => SongsVar.Tags.Contains("foo1") || SongsVar.Tags.Contains("foo2") || SongsVar.Tags.Contains("foo3"));
However, this does not work (I get the following error: "A lambda expression with a statement body cannot be converted to an expression tree")
allSongMatches = allSongMatches.Where(SongsVar =>
{
bool retVal = false;
foreach(string str in strArray)
{
retVal = retVal || SongsVar.Tags.Contains(str);
}
return retVal;
});
Can anybody show me the correct strategy to accomplish this? I am still new to the world of LINQ :-)
You can use the PredicateBuilder class:
var searchPredicate = PredicateBuilder.False<Songs>();
foreach(string str in strArray)
{
var closureVariable = str; // See the link below for the reason
searchPredicate =
searchPredicate.Or(SongsVar => SongsVar.Tags.Contains(closureVariable));
}
var allSongMatches = db.Songs.Where(searchPredicate);
LinqToSql strange behaviour
I recently created an extension method for creating string searches that also allows for OR searches. Blogged about here
I also created it as a nuget package that you can install:
http://www.nuget.org/packages/NinjaNye.SearchExtensions/
Once installed you will be able to do the following
var result = db.Songs.Search(s => s.Tags, strArray);
If you want to create your own version to allow the above, you will need to do the following:
public static class QueryableExtensions
{
public static IQueryable<T> Search<T>(this IQueryable<T> source, Expression<Func<T, string>> stringProperty, params string[] searchTerms)
{
if (!searchTerms.Any())
{
return source;
}
Expression orExpression = null;
foreach (var searchTerm in searchTerms)
{
//Create expression to represent x.[property].Contains(searchTerm)
var searchTermExpression = Expression.Constant(searchTerm);
var containsExpression = BuildContainsExpression(stringProperty, searchTermExpression);
orExpression = BuildOrExpression(orExpression, containsExpression);
}
var completeExpression = Expression.Lambda<Func<T, bool>>(orExpression, stringProperty.Parameters);
return source.Where(completeExpression);
}
private static Expression BuildOrExpression(Expression existingExpression, Expression expressionToAdd)
{
if (existingExpression == null)
{
return expressionToAdd;
}
//Build 'OR' expression for each property
return Expression.OrElse(existingExpression, expressionToAdd);
}
}
Alternatively, take a look at the github project for NinjaNye.SearchExtensions as this has other options and has been refactored somewhat to allow other combinations
There is another, somewhat easier method that will accomplish this. ScottGu's blog details a dynamic linq library that I've found very helpful in the past. Essentially, it generates the query from a string you pass in. Here's a sample of the code you'd write:
Dim Northwind As New NorthwindDataContext
Dim query = Northwind.Products _
.Where("CategoryID=2 AND UnitPrice>3") _
.OrderBy("SupplierId")
Gridview1.DataSource = query
Gridview1.DataBind()
More info can be found at scottgu's blog here.
Either build an Expression<T> yourself, or look at a different route.
Assuming possibleTags is a collection of tags, you can make use of a closure and a join to find matches. This should find any songs with at least one tag in possibleTags:
allSongMatches = allSongMatches.Where(s => (select t from s.Tags
join tt from possibleTags
on t == tt
select t).Count() > 0)