I have the following LINQ statement that does on where on the date and a LabID.
I'm passing in a list of LABS and a date, however they are not required, and I could potentially only pass in a date, and no lab, in which case I'd like to get results for all labs for that particular lab.
here is what I have now:
List<dExp> lstDatExp = (from l in ctx.dExp.Include("datLab")
where values.Contains(l.datL.Lab_ID)
&& l.reportingPeriod == reportingPeriod
select l).ToList<dExp>();
But this breaks if the value getting passed in is not there. How do I change this to make sure both of my where statements are optional?
With IQueryable you can simply add conditions in steps:
int? reportingPeriod = ...;
IQueryable<dExp> resultsQuery = // don't use `var` here.
ctx.dExp.Include("datLab");
if (values != null)
resultsQuery = resultsQuery.Where(exp => values.Contains(exp.datL.Lab_ID));
if (reportingPeriod.Hasvalue)
resultsQuery = resultsQuery.Where(exp => exp.reportingPeriod == reportingPeriod.Value);
// additional .Where(), .OrderBy(), .Take(), .Skip() and .Select()
// The SQL query is made and executed on the line below
// inspect the string value in the debugger
List<dExp> results = resultsQuery.ToList();
Here are two ways to do that.
But first, please don't use a single lowercase l as an identifier. It is way too easy to confuse it with the number 1. More generally, stp using abbrevs in yr cde, it mks it hrdr to rd.
First technique:
var query = from lab in ctx.dExp.Include("datLab")
where values == null || values.Contains(lab.datL.Lab_ID)
where reportingPeriod == null || lab.reportingPeriod == reportingPeriod
select lab;
var list = query.ToList<dExp>();
Second technique:
IEnumerable<dExp> query = ctx.dExp.Include("datLab");
if (values != null)
query = query.Where(lab=>values.Contains(lab.datL.Lab_ID));
if (reportingPeriod != null)
query = query.Where(lab=>lab.reportingPeriod == reportingPeriod);
var list = query.ToList<dExp>();
What we do is something like (l.reportingPeriod == reportingPeriod || reportingPeriod == null) So you check to see if the parameter is its default meaning it hasnt been used or if there is something there check it against the database.
You need to check if your values are null before doing the query, and if they are, don't do the extra condition.
List<dExp> lstDatExp =
(from l in ctx.dExp.Include("datLab")
where
(values == null || values.Contains(l.datL.Lab_ID)) &&
(reportingPeriod == null || l.reportingPeriod == reportingPeriod)
select l).ToList<dExp>();
This way if values or reportingPeriod are null they are essentially optional.
Related
I am not sure if this is possible, but I have a linq to sql statement that takes in input filter parameter which could be null or missing a value like so
public List<MyViewModel> GetRecords(SearchDto? filter)
{
List<MyViewModel> results =
this.dbContext.MyTable
Where(s => s.IsActive == 'Y' &&
filter != null && !stringIsNullOrEmpty(s.deptId) && s.deptId == filter.deptId)
ToList();
}
When I add the conditional filter in the Where clause, my result is 0 count when the filter is null or empty.
I prefer not to do the old fashion way which is after the linq is call, add many lines of code like the following
if (!string.IsNullEmpty(filter.deptId)
{
results = result.Where(s => s.deptId == filter.deptId);
}
I would like to add the conditional Where filter in the linq statement instead of old fashion C# code.
Thanks for any advice
To correctly represent conditionally added filtering like in the "old fashion way" you should use something like this:
.Where(s.IsActive == 'Y'
&& (filter == null
|| (string.IsNullOrEmpty(s.deptId) || s.deptId == filter.deptId))`
So the "optional" condition evaluates to true for your AND clause when filter is null or empty.
I'm working on a LINQ statement. I have a table of cities where the records have either a countryId or a stateId. I'd like to just write the one statement and have the where clause check to see which of the two parameters is null and then select on the one that is not.
Here's what I'm working with:
public List<City> Cities(int? countryTypeId, int? stateTypeId)
{
if (countryTypeId == null && stateTypeId == null)
return null;
return _db.City
.Where(x => x.StateTypeId == stateTypeId
&& x.CountryTypeId == countryTypeId)
.OrderBy(x => x.Description)
.ToDTOs();
}
I'm pretty new to LINQ, and I know this code isn't right, just adding it for context.
If the State and Country ids are all >0 you simply can do this, no need to check for null
.Where(x => x.StateTypeId == stateTypeId.GetValueOrDefault()
&& x.CountryTypeId == countryTypeId.GetValueOrDefault())
Else you need to add the condition if those nullable inputs have value or not, as mentioned in the comment
Edit: after seeing some comments, if you are looking for list of cities based on either of the parameters, then you should be using || not && in your where condition
Where(x => (stateTypeId.HasValue && stateTypeId.Value == x.StateTypeId)
|| (countryTypeId.HasValue && countryTypeId.Value == x.CountryTypeId))
Note the order matters, this code will first check if stateTypeId has value and if it has it'll match only the cities with that stateTypeId
_db.City.Where(c => c.CountryTypeId?.Equals(countryTypeId) ?? false
| c.StateTypeId?.Equals(stateTypeId) ?? false);
Using null conditional operators - when a type Id is null use the null coalescing operator to return false and fail the match - otherwise check for equality and return matching.
Note you cannot short circuit the OR operator here!
I'm not sure if this is the case, but if one of the input parameters was always null and the entries were guaranteed to always have one property null, the following would be a cool solution:
_db.City.Where(c => (c.CountryTypeId ?? c.StateTypeId) == (countryTypeId ?? stateTypeId))
My DBA has sufficiently beaten it into my head that ignoring parameters in a query (ex: WHERE Field = #PARAM or #PARAM IS NULL) can result in very bad things. As a result, I would encourage you to conditionally add only the parameters that you absolutely need. Fortunately, given that you are working with just two possible parameters, this is trivial.
Start with the base of your query, and then add to it.
var queryBase = _db.City.OrderBy(x => x.Description);
if (countryTypeId.HasValue)
{
queryBase = queryBase.Where(x => x.CountryTypeId == countryTypeId);
}
if (stateTypeId.HasValue)
{
queryBase = queryBase.Where(x => x.StateTypeId == stateTypeId);
}
return queryBase.ToDTOs(); // or .ToList() for a more universal outcome
Add whatever logic you may need if parameters are mutually exclusive, one supercedes the other, etc.
The following portion of code is from a larger linq query, but it is the one I need help with:
KnowledgeTypeText = Regex.Replace((from categoryVersion in _Context.ArticleCategoryVersions
join category in _Context.Categories
on categoryVersion.CategoryID equals category.CategoryID
where category.ParentID.HasValue == true
&& category.ParentID.Value == rootKnowledgeTypeID
&& categoryVersion.Version == articleLatestVersions.Version
&& categoryVersion.ArticleID == articleLatestVersions.ArticleID
select category).First().Name, #"(\d+[\\.]?\s*)", ""),
The short version: This part of the query will fetch a category for an article, but it doesn't necessary have to have a value. If it does have a value I need to strip out the numbering at the beginning of the text.
I'm getting exceptions when the category is null because it's obviously trying to do the Replace action on the property Name.
How do I add in code to deal with Nulls? It would be safe to return an empty string, but I'm not sure how to test for it and return an empty string.
If you fail on First() because Sequence contains no elements then replace with .FirstOrDefault().
If you get a value but it is null and then it fails on a null reference when accessing Name then use .FirstOrDefault()?.Name
If it is the Name property that is null then use the ??:
(/* query */).FirstOrDefault()?.Name ?? string.Empty
Read more on:
?. Null-Conditional Operator
?? Operator
In addition it is less readable putting it all inline like this (even more as it seems to be in an object initializer). First have the query and then do the replace:
var result = (from categoryVersion in _Context.ArticleCategoryVersions
join category in _Context.Categories
on categoryVersion.CategoryID equals category.CategoryID
where category.ParentID.HasValue &&
category.ParentID.Value == rootKnowledgeTypeID &&
categoryVersion.Version == articleLatestVersions.Version &&
categoryVersion.ArticleID == articleLatestVersions.ArticleID &&
select category).FirstOrDefault()?.Name ?? string.Empty;
KnowledgeTypeText = Regex.Replace(result, #"(\d+[\\.]?\s*)", "");
I'm writing a query that uses FirstOrDefault after an OrderBy query, which should check if it isn't null first then use some data in it. Is there a better way than writing it like this:
int count = db.Items.Count(i =>
i.Assignments.OrderByDescending(a =>
a.DateAssigned).FirstOrDefault() != null
&&
i.Assignments.OrderByDescending(a =>
a.DateAssigned).FirstOrDefault().DateReturned == null)
What this code does is there are items that has many assignments, I take the latest assignment by date, then check if it exist, then run a condition on a property (DateReturned). As you see, this query is long, and most of my queries seem to look like this where I check for null first then run a second query on it using their properties. Is there a better way of doing this?
Just call .Any(a => a.DateReturned == null) to check whether there are any items that meet the condition.
If you only want to check the latest assignment, add .Take(1) before the .Any().
My take:
int count =
itemsQuery.Select(i => i.Assignments.OrderByDescending(a => a.DateAssigned))
.Count(i => i.FirstOrDefault() != null &&
i.First().DateReturned == null);
You can put the result in a variable to avoid doing the same thing twice:
int count = itemsQuery.Count(i => {
var f = i.Assignments.OrderByDescending(a => a.DateAssigned).FirstOrDefault();
return f != null && f.DateReturned == null;
});
I have a this linq query:
var fling = (from b in flowering.FlowerViews
where ((!string.IsNullOrEmpty(flow_name)) && b.FLOWER_NAME == flow_name) || flow_name==""
where ((!string.IsNullOrEmpty(color_name)) && b.COLOR_NAME == color_name) || color_name == ""
where ((!string.IsNullOrEmpty(size)) && b.FLOWER_SIZE == size) || size==""
where ((low_price!=0) && low_price<= b.FLOWER_PRICE) || low_price==0
where ((high_price!=0) && high_price >= b.FLOWER_PRICE) || high_price==0
orderby b.COLOR_NAME
select new { b.FLOWER_NAME, b.COLOR_NAME, b.FLOWER_SIZE, b.FLOWER_PRICE, b.CHAR_DESC});
my where clauses work for me but when I run a for each loop over the returned values there is duplicate data because b.CHAR_DESC has 3 values to it where all the other return data only have one. I am wondering if there is a way to get the 3 values assigned to b.CHAR_DESC into a structure that does not cause duplicate b.Flower_name's to show up
Based on this post you should be able to call Distinct() for the anonymous type
var list = fling.Distinct().ToList();
And the compiler will take care of GetHashCode() and Equals() for the anonymous type based on attribute values.
Add .Distinct() at the end of your select clause, after the final parenthesis.