Thank you to all the folks who answered and contributed to my last question. I have run into another interesting incarnation of a LINQ question... so like before...
I have the following
const String A_CONVERSATION = "AD Channels";
const String NOT_REPUTABLE = "AD Resolution";
const String DO_NOT_KNOW = "Capture Input";
private enum Properties
{ MyHow, Thats, It }
List<String> MyList = new List<string>
{
A_CONVERSATION,
NOT_REPUTABLE,
DO_NOT_KNOW
}
private Dictionary <Properties, String> PropertyToString;
private Dictionary <String, Properies> StringToProperty;
How can I use LINQ to populate each of the dictionaries so that I can use the following? Is there a one line LINQ statement that would populate each?
Properties MyResult1 = StringToProperty[A_CONVERSATION];
String MySResult2 = PropertyToString[Properties.It];
I specifically would like to use the actaull property to index in the second case.
Providing this is what you want...
In two statements (one could be merged I guess via some lookup somehow - but I didn't think it was that relevant - not everything has to be in one line:)
var properties = ((Properties[])Enum.GetValues(typeof(Properties))).ToList();
var propertyToString = properties.Zip(MyList, (p, s) => new { Prop = p, Text = s }).ToDictionary(x => x.Prop, x => x.Text);
var stringToProperty = properties.Zip(MyList, (p, s) => new { Prop = p, Text = s }).ToDictionary(x => x.Text, x => x.Prop);
I deliberately separated the makings of the enum list - for clarity - you can move that into one line if you'd like.
Related
im running this in asp.net core v3.1
my question is similar to this question:
How to use Linq to check if a list of strings contains any string in a list
with the specific question relating to the first answer such that
filterTags = ["abc", "cd", "efg"]
var results = db.People
.Where(p => filterTags.Any(tag => p.Tags.Contains(tag)));
so basically saying
give me results from the db of all People
who's Tags field contains any of the filterTags
where Tags = a big text field populated by a bunch of space-delimited tags
This seems straightforward (esp since this has been written before)
but i get an error back
System.InvalidOperationException: The LINQ expression 'DbSet
.Where(p => __filterTags_0
.Any(tag => p.Tags.Contains(tag)))' 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 either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()
does anyone know what this means or what im doing wrong?
This is not possible with pure EF LINQ. You have to create helper which transforms your search list in Expression Tree.
public static class QueryExtensions
{
private static MethodInfo _containsMethodInfo = typeof(string).GetMethod("Contains")!;
public static IQueryable<T> FilterUsingContains<T>(this IQueryable<T> query, Expression<Func<T, string>> prop, IList<string> items)
{
if (items.Count == 0)
return query.Where(e => 1 == 2);
var param = prop.Parameters[0];
var predicate = items.Select(i =>
(Expression)Expression.Call(prop.Body, _containsMethodInfo, Expression.Constant(i, typeof(string))))
.Aggregate(Expression.OrElse);
var lambda = Expression.Lambda<Func<T, bool>>(predicate, param);
return query.Where(lambda);
}
}
Then you can use this extension in your queries
filterTags = ["abc", "cd", "efg"]
var results = db.People
.Where(p => p.Tags.AsQueryable().FilterUsingContains(t => t, filterTags).Any());
Here is a workaround for you:
using System.Linq;
string[] filterTags = {"abc", "cd", "efg"};
var results = db.People.Where(p => filterTags.Contains(p.Tags)).ToList();
This should be simple but I could not wrap my head around it.. Here is how I am doing it now but it seems so wasteful.
There is a
List<string> committees
and
List<string> P.committees
I just want to see if one list has any strings that are contained in the other.
List<Person> listFilteredCommitteesPerson = new List<Person>();
foreach (Person p in listFilteredPerson)
{
foreach (string strCommittee in p.Committees)
{
if (committees.Contains(strCommittee))
{
listFilteredCommitteesPerson.Add(p);
}
}
}
listFilteredPerson = listFilteredCommitteesPerson;
For a boolean value:
var match =
committees.Intersect(listFilteredPerson.SelectMany(p => p.Committees)).Any();
If you want a collection of Person that have a match you can use:
var peopleThatMatch =
listFilteredPerson.Where(p => committees.Intersect(p.Committees).Any());
or:
var peopleThatMatch =
listFilteredPerson.Where(p => p.Committees.Any(s => committees.Contains(s)));
You might want to consider another collection type (e.g. HashSet<T>) for performance reasons if you have large collections.
I have a cluttery piece of code that I would like to shorten using Linq. It's about the part in the foreach() loop that performs an additional grouping on the result set and builds a nested Dictionary.
Is this possible using a shorter Linq syntax?
var q = from entity in this.Context.Entities
join text in this.Context.Texts on new { ObjectType = 1, ObjectId = entity.EntityId} equals new { ObjectType = text.ObjectType, ObjectId = text.ObjectId}
into texts
select new {entity, texts};
foreach (var result in q)
{
//Can this grouping be performed in the LINQ query above?
var grouped = from tx in result.texts
group tx by tx.Language
into langGroup
select new
{
langGroup.Key,
langGroup
};
//End grouping
var byLanguage = grouped.ToDictionary(x => x.Key, x => x.langGroup.ToDictionary(y => y.PropertyName, y => y.Text));
result.f.Apply(x => x.Texts = byLanguage);
}
return q.Select(x => x.entity);
Sideinfo:
What basically happens is that "texts" for every language and for every property for a certain objecttype (in this case hardcoded 1) are selected and grouped by language. A dictionary of dictionaries is created for every language and then for every property.
Entities have a property called Texts (the dictionary of dictionaries). Apply is a custom extension method which looks like this:
public static T Apply<T>(this T subject, Action<T> action)
{
action(subject);
return subject;
}
isn't this far simpler?
foreach(var entity in Context.Entities)
{
// Create the result dictionary.
entity.Texts = new Dictionary<Language,Dictionary<PropertyName,Text>>();
// loop through each text we want to classify
foreach(var text in Context.Texts.Where(t => t.ObjectType == 1
&& t.ObjectId == entity.ObjectId))
{
var language = text.Language;
var property = text.PropertyName;
// Create the sub-level dictionary, if required
if (!entity.Texts.ContainsKey(language))
entity.Texts[language] = new Dictionary<PropertyName,Text>();
entity.Texts[language][property] = text;
}
}
Sometimes good old foreach loops do the job much better.
Language, PropertyName and Text have no type in your code, so I named my types after the names...
Hi I have a query like this:
var queryGridData = from question in questions
select new {
i = question.Id,
cell = new List<string>() { question.Id.ToString(), question.Note, question.Topic }
};
The ToString() part needed to convert the int is causing:
LINQ to Entities does not recognize the method 'System.String.ToString()' method, and this method cannot be translated into a store expression.
Hmmmmmmmmmmm. I need it as a string to go into the collection. Any ideas?
I would personally perform just enough of the query in the database to provide the values you want, and do the rest in .NET:
var queryGridData = questions.Select(q => new { q.Id, q.Note, q.Topic })
.AsEnumerable() // Do the rest locally
.Select(q => new { i = q.Id,
cell = new List<string> {
q.Id.ToString(),
q.Note,
q.Topic
} });
(This formatting is horrible, but hopefully it'll be easier to do nicely in an IDE where you've got more space :)
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)