I have a list of a class that looks like this
public class MyClass
{
public ComplexType A {get;set;}
public ComplexTypeB B {get;set;}
}
var myList = new List<MyClass>();
I then have a target Dto which looks like this
public class MyTargetDto
{
public ComplexType A {get;set;}
public List<ComplexTypeB> ListOfB {get;set;}
}
It's very similar only that myTargetDto supports grouping by ComplexType
Given a flat list of MyClass, how can I (using Linq) convert it to a target list of MyTargetDto?
You should do something like this.
myList.GroupBy(x=>x.A)
.Select(x=> new MyTargetDto()
{
A= x.Key,
ListOfB = x.Select(s=>s.B).ToList()
});
If your ComplexType does not have an own implementation of Equals than at first you would need to implement an IEqualityComparer<ComplexType>:
public class Comparer : IEqualityComparer<ComplexType>
{
public bool Equals(ComplexType x, ComplexType y)
{
// code to check your complex type for equality
}
public int GetHashCode(ComplexType obj)
{
return obj.GetHashCode();
}
}
Then you can use this comparer to group your list using GroupBy:
List<MyClass> flatList = ...
List<MyTargetDto> result = flatList.GroupBy(e => e.A, e => e.B, new Comparer())
.Select(g => new MyTargetDto {
A = g.Key,
ListOfB = g.ToList()});
.ToList();
If ComplexType already has an own implementation of Equals that works appropriatly, than you can ommit that comparer:
List<MyTargetDto> result = flatList.GroupBy(e => e.A, e => e.B)
.Select(g => new MyTargetDto {
A = g.Key,
ListOfB = g.ToList()})
.ToList();
The first lambda of GroupBy selects the element by which the list is grouped. This will then be the Key property in the resulting IGrouping.
The second lambda selects the elements that should be contained in that group.
The final Select creates for each group a MyTargetDto, setting it's A property to the ComplexType and creating the ListOfB.
myList.GroupBy(item => item.A)
.Select(group => new MyTargetDto
{
A = group.Key,
ListOfB = group.Select(item => item.B).ToList()
});
As already posted by others, a solution would be
myList.GroupBy(x => x.A, x => x.B)
.Select(g => new MyTargetDto()
{
A = g.Key,
ListOfB = g.ToList()
});
Just wanted you to see an existing shortcut using a GroupBy overload
myList.GroupBy(x => x.A, x => x.B, (key, g) => new MyTargetDto()
{
A = key,
ListOfB = g.ToList()
});
Related
I have the following data model, and I need to group list of ResponseItem with these conditions:
First: Group by ResponseItem.Group
Second: Group by ResponseItem.SubGroup, but considering just the most recent one, which means considering the ResponseItem.CreationDate
Code:
public class ResponseItem
{
public string Group { get; set; }
public string SubGroup { get; set; }
public double Value { get; set; }
public DateTime CreationDate { get; set; }
}
public class GroupedResponseItem
{
public string Group { get; set; }
public List<ResponseItem> Items { get; set; }
}
The method is:
public List<GroupedResponseItem> GetGroupedData( IQueryable<ResponseItem> responseItems )
{
return responseItems
.OrderByDescending(i => i.CreationDate)
.GroupBy(i => i.Group)
.Select(grp => new GroupedResponseItem()
{
Group = grp.Key,
Items = grp
.GroupBy(i => new { i.SubGroup })
.Select(grp => grp.First())
.Select(i => new ResponseItem()
{
SubGroup = i.SubGroup,
CreationDate = i.CreationDate,
Value = i.Value
}).ToList()
})
.ToList();
}
But I get an error:
'The LINQ expression 'ProjectionBindingExpression: 0' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'
As I mentioned in the title, I'm using Entity Framework on .NET 6.
On the other hand, If I does not consider the second group by, query works fine:
public List<GroupedResponseItem> GetGroupedData(IQueryable<ResponseItem> responseItems)
{
return responseItems
.OrderByDescending(i => i.CreationDate)
.GroupBy(i => i.Group)
.Select(grp => new GroupedResponseItem()
{
Group = grp.Key,
Items = grp
.Select(i => new ResponseItem()
{
SubGroup = i.SubGroup,
CreationDate = i.CreationDate,
Value = i.Value
})
.ToList()
})
.ToList();
}
The culprit seems to be the secondary projection (Select) here
.GroupBy(i => new { i.SubGroup })
.Select(grp => grp.First()) // <-- (1)
.Select(i => new ResponseItem() // <-- (2)
{
SubGroup = i.SubGroup,
CreationDate = i.CreationDate,
Value = i.Value
})
.ToList()
While EF Core 6.0 has improved translation of GroupBy having additional operators on grouping result set (other than key/aggregates, which have natural SQL support), there are still limitations/defects preventing translation of some constructs. In particular, multiple projections.
Shortly, the Select after GroupBy must be the final LINQ operator. Which is kind of sad, since intermediate projection usually helps the translation and is often used to workaround EF Core limitations. But not in this case.
For this particular query, the projection looks redundant since the type of the elements of the group is the same as the projected type, so it could simply be removed
.GroupBy(i => new { i.SubGroup })
.Select(grp => grp.First()) // <-- final projection
.ToList()
So this is one of the solutions/workarounds. If you really need a projection, because you are selecting partial columns, or project to a different type, then move it inside the Select after GroupBy:
.GroupBy(i => new { i.SubGroup })
.Select(grp => grp
.Select(i => new ResponseItem()
{
SubGroup = i.SubGroup,
CreationDate = i.CreationDate,
Value = i.Value
})
.First()
) // <-- final projection
.ToList()
I want to group a table by a specific category.
The categories look like: "AAA", "BBB", "CCC", "DDD", "EEE", etc... Using the code below, I can easily get results for each group. However, there is a special case where two categories should be combined and a new Key generated (e.g. Category BBB and DDD should end up as one category).
My end result should be something like:
Key: "AAA", Items: (items under AAA)
Key: "BBB/DDD", Items: (items under BBB and DDD)
Key: "CCC", Items: (items under CCC)
I have been at this for a while and can't get anything that works.
var query= ds.Tables[0].AsEnumerable()
.GroupBy(g => g.Field<string>("category"))
.Select(a => new workType
{
Key = a.Key,
Item = a.ToList()
});
public class workType
{
public string Key { get; set; }
public List<DataRow> Item { get; set; }
}
Basically, all you need to do is to transform the key before grouping by it:
var sequence = ds.Tables[0].AsEnumerable();
var result = sequence
.Select(e => new { Key = GetCompoundKey(e.Field<string>("category")), Value = e })
.GroupBy(e => e.Key)
.Select(g => new { Key = g.Key, Items = g.SelectMany(e => e.Value).ToList() });
private static string GetCompoundKey(string originalKey)
{
if (originalKey.Equals("BBB") || originalKey.Equals("DDD"))
{
return "BBB/DDD";
}
return originalKey;
}
Besides that, GroupBy has an overload that takes IEqualityComparer<T> so you could write an IEqualityComparer<string> implementation and use it:
private class KeysEqualityComparer : IEqualityComparer<string>
{
public bool Equals(string x, string y)
{
if (x.Equals(y))
{
return true;
}
return (x.Equals("BBB") && y.Equals("DDD"))
|| (x.Equals("DDD") && y.Equals("BBB"));
}
public int GetHashCode(string str)
{
return str.GetHashCode();
}
}
// your original code using the comparer:
var query= ds.Tables[0].AsEnumerable()
.GroupBy(g => g.Field<string>("category"), new KeysEqualityComparer())
.Select(a => new workType
{
Key = a.Key,
Item = a.ToList()
});
I'm not sure if this is a good idea though as you will most probably get weird group keys that you'll need to modify anyway. And then the modification code lives in different place than the comparison code, even though the logic should be the same. So that's asking for some trouble in the future.
I have a simple model that I'm trying to group:
public class PracticeArea
{
[Key]
public int PracticeAreaID { get; set; }
public string Name { get; set; }
public string Type { get; set; }
}
I'd like to group by Type, how can I convert this:
var practiceAreas = (from c in db.PracticeAreas
select c).
to:
public Dictionary<string, string> groupedPracticeAreas { get; set; }
I'm not sure how grouping works within Linq - I can run .ToGroup(),. but that doesn't give me my dictionary.
I've tried:
practiceAreas = practiceAreas.ToDictionary(x => x.Type, x => x.Name);
But that gave me a cast error
This should not throw cast exception if both type and name are strings:
practiceAreas.ToDictionary(x => x.Type, x => x.Name)
But this would throw if there is more than one practice area exist for some type. You can use one of following options:
1) Use lookup instead of dictionary. It will create lookup for names by area type
practiceAreas.ToLookup(pa => pa.Type, pa => pa.Name)
2) Use dictionary with collection to hold all names for each type, e.g. Dictionary<string, List<string>> :
practiceAreas.GroupBy(pa => pa.Type)
.ToDictionary(g => g.Key, g => g.Select(pa => pa.Name).ToList())
3) Join all names in single string
practiceAreas.GroupBy(pa => pa.Type)
.ToDictionary(g => g.Key, g => String.Join("," g.Select(pa => pa.Name)))
I have a nested list of objects. That I need to group by identifierA and Sum its numeric properties, nested list shall group respectively:
public class TypeA
{
public String identifierA{ get; set; }
public Int32 number { get; set; }
public List<TypeB> nestedList { get; set; }
}
public class TypeB
{
public String identifierB { get; set; }
public Int32 otherNumber { get; set; }
}
So I'm expecting something like this:
var List<TypeA> groupedList = (from a in TypeAList
group a by a.identifierA
into groupedData
select new TypeA
{
identifierA = groupedData.Key,
number = groupedData.Sum(g => g.number ),
nestedList = //HOW TO GROUP NESTED PART?
}).ToList();
I think that this will resolve your issue.
List<TypeA> list = TypeAList
.GroupBy(a => a.identifierA)
.Select(
g =>
new TypeA
{
identifierA = g.Key,
number = g.Sum(n => n.number),
nestedList =
g.SelectMany(l => l.nestedList)
.GroupBy(b => b.identifierB)
.Select(
gg =>
new TypeB
{
identifierB = gg.Key,
otherNumber = gg.Sum(b => b.otherNumber)
}).ToList()
}).ToList();
SelectMany takes an IEnumerable<SomethingWithAnIEnumerable> and flattens all the SomethingWithAnIEnumerable's selected IEnumerables into a single IEnumerable:
nestedList = groupedData.SelectMany(pa => pa.nestedList).ToList()
use SelectMany
if you want to group the list into one use
nestedList = groupedData.SelectMany(d=>d.nestedList)
and if you want Sum of that list, use
nestedList = groupedData.SelectMany(d=>d.nestedList).Sum(o=>o.otherNumber)
I think your IdentfierB might be a key of some kind, and your result should reflect Grouped and Summed TypeBs.
List<TypeA> groupedList = TypeAList
.GroupBy(a => a.identifierA)
.Select(g => new TypeA()
{
identierA = g.Key,
number = g.Sum(a => a.number)
nestedList = g.SelectMany(a => a.nestedList)
.GroupBy(b => b.identifierB)
.Select(g2 => new TypeB()
{
identifierB = g2.Key,
otherNumber = g2.Sum(b => b.otherNumber)
}
}
I have two classes which share two common attributes, Id and Information.
public class Foo
{
public Guid Id { get; set; }
public string Information { get; set; }
...
}
public class Bar
{
public Guid Id { get; set; }
public string Information { get; set; }
...
}
Using LINQ, how can I take a populated list of Foo objects and a populated list of Bar objects:
var list1 = new List<Foo>();
var list2 = new List<Bar>();
and merge the Id and Information of each into a single dictionary:
var finalList = new Dictionary<Guid, string>();
Thank you in advance.
Sounds like you could do:
// Project both lists (lazily) to a common anonymous type
var anon1 = list1.Select(foo => new { foo.Id, foo.Information });
var anon2 = list2.Select(bar => new { bar.Id, bar.Information });
var map = anon1.Concat(anon2).ToDictionary(x => x.Id, x => x.Information);
(You could do all of this in one statement, but I think it's clearer this way.)
var finalList = list1.ToDictionary(x => x.Id, y => y.Information)
.Union(list2.ToDictionary(x => x.Id, y => y.Information))
.ToDictionary(x => x.Key, y => y.Value);
Make sure the ID's are unique. If not they will be overwritten by the first dictionary.
EDIT: added .ToDictionary(x => x.Key, y => y.Value);