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Use WhereIf for multiple condition in c#

Hi can someone help me how best we can use whereif in LINQ, here I have a code which works fine, but I want to convert this query with WhereIf.
public async Task LoadQuery(IEnumerable<string> codes)
{
var query = _dBContext.QueryTable.Where(x => !x.InActive).AsQueryable();
if (codes!= null && codes.Any())
query = query.Where(x => codes.Contains(x.FirstCode) || query.Contains(x.SecondCode));
else
query = query.Where(x => !x.HasException.HasValue);
var data = query.ToList();
}
I have tried it with WhereIF ienumerable but not succeed. Here is the link which I followed.
https://extensionmethod.net/csharp/ienumerable-t/whereif

WhereIf isn't really suitable for your case, for 2 reasons:
You're calling two different functions on your if-else, while WhereIf is built to accept a single function (predicate) to be executed if some condition is satisfied.
WhereIf is an extension method for IEnumerable<TSource>, while your'e trying to use it as an extension method for IQueryable<TSource>.
If you insist, you'd have to define an extension method for IQueryable<TSource>, and in doing so, just define it as WhereIfElse:
public static class ExtensionMethods
{
public static IQueryable<TSource> WhereIfElse<TSource>(this IQueryable<TSource> source, bool condition, Func<TSource, bool> predicateIf, Func<TSource, bool> predicateElse)
{
if (condition)
return source.Where(predicateIf).AsQueryable();
else
return source.Where(predicateElse).AsQueryable();
}
}
So, let's say that query's type is IQueryable<Item> (replace Item with your actual type):
public async Task<List<Item>> LoadQuery(IEnumerable<string> codes)
{
var query = _dBContext.QueryTable.Where(x => !x.InActive).AsQueryable();
query = query.WhereIfElse(
// condition
codes != null && codes.Any(),
// predicateIf
(Item x) => codes.Contains(x.FirstCode) || codes.Contains(x.SecondCode),
// predicateElse
(Item x) => !x.HasException.HasValue
);
var data = query.ToList();
return data;
}
P.S. note I changed your return value, though there still isn't an await.

bool condition = codes!= null && codes.Any();
var data = _dBContext.QueryTable
.WhereIf(condition, a=> codes.Contains(a.FirstCode) || codes.Contains(a.SecondCode))
.WhereIf(!condition, a=> !a.HasException.HasValue && !a.InActive).ToList();

Construct expression for where through lambdas

The situation
I have a method that takes in a POCO. This POCO is like the following
private class SearchCriteria
{
public string name get;set;
public string age get;set;
public string talent get;set;
..
....
}
The method basically has a query to the db , that uses the above criteria.
public void query(SearchCriteria crit)
{
if(crit.name!=null && crit.age!=null && crit.talent !=null)
{
dbContext.Students.Where(c=>c.name ==crit.name && c.age==crit.age...)
}
else if(crit.name !=null && crit.age!=null)
{
}
else if(....
{
}
As you can see there is a definite problem above , where in, in case of large number of criteria, I will have to write a lot of if-elses to drop out specific arguments from the where clause .
The possible solution ?
I am actually new to the lambda expressions world but I believe we must be having a facility which would allow us to do something like below.
dbContext.Students.Where(processCriteria(searchCriteriaPOCO)).
Can you folks lead me to the proper direction ?. Thanks

Get a queryable and then keep adding where clauses to it. That way you only need to test each possible criteria the once and also only generate the number of where clauses that are absolutely needed.
IQueryable<Student> q = dbContext.Students.AsQueryable();
if (crit.name != null)
q = q.Where(c => c.name == crit.name);
if (crit.age != null)
q = q.Where(c => c.age== crit.age);

Let me start by saying that this answer uses the same basic idea as #PhilWright's answer. It just wraps it up in an extension method that applies this pattern for you, and allows you to have a syntax that reads nice.
public static class SearchExtensions
{
public static IQueryable<Student> Query(this SearchCriteria criteria, IQueryable<Student> studentQuery)
{
return studentQuery
.Match(criteria.name, (student) => student.name == criteria.name)
.Match(criteria.age, (student) => student.age == criteria.age)
.Match(criteria.talent, (student) => student.talent == criteria.talent);
// add expressions for other fields if needed.
}
private static IQueryable<Student> Match<T>(
this IQueryable<Student> studentQuery,
T criterionValue,
Expression<Func<Student, bool>> whereClause) where T : class
{
// only use the expression if the criterion value is non-null.
return criterionValue == null ? studentQuery : studentQuery.Where(whereClause);
}
}
You can then use it in your code like this:
var criteria = new SearchCriteria() {
name = "Alex",
talent = "Nosepicking"
};
var results = criteria.Query(dbContext.Students);

Maybe I'm missing something, as the code example is not the clearest I've seen, but for your specific example, I would think the following should be fine:
dbContext.Students.Where(c => (crit.name == null || crit.name == c.name) &&
(crit.age == null || crit.age == c.age) &&
(crit.talent == null || crit.talent == c.talent));
No need to chain a bunch of if statements.
For more complicated scenarios, you might prefer something like PredicateBuilder

You can use a pattern like this:
dbContext.Students.Where(c=>(crit.name == null || c.name ==crit.name) && ...)
A search criterion which is null will give a subexpression which is always true.

Reusing existing linq expressions in a compiled query

Consider the following code which provides two methods: One to return an IQueryable, and one which leverages a compiled query to very efficient return the location matching a specific ID:
public IQueryable<Location> GetAllLocations()
{
return from location in Context.Location
where location.DeletedDate == null
&& location.Field1 = false
&& location.Field2 = true
&& location.Field3 > 5
select new LocationDto
{
Id = location.Id,
Name = location.Name
}
}
private static Func<MyDataContext, int, Location> _getByIdCompiled;
public Location GetLocationById(int locationId)
{
if (_getByIdCompiled == null) // precompile the query if needed
{
_getByIdCompiled = CompiledQuery.Compile<MyDataContext, int, Location>((context, id) =>
(from location in Context.Location
where location.DeletedDate == null
&& location.Field1 = false
&& location.Field2 = true
&& location.Field3 > 5
&& location.Id == id
select new LocationDto {
Id = location.Id,
Name = location.Name
})).First());
}
// Context is a public property on the repository all of this lives in
return _getByIdCompiled(Context, locationId);
}
This is a pretty big simplification of the actual code, but I think it gets the idea accross, and it works fine. The next thing I want to do is refactor the code, so that the common bit of the expression can be reused, since it will be used in many other types of compiled queries. In other words, this expression:
from location in Context.Location
where location.DeletedDate == null
&& location.Field1 = false
&& location.Field2 = true
&& location.Field3 > 5
select new LocationDto
{
Id = location.Id,
Name = location.Name
};
How can I somehow capture this in a variable or function and reuse it in multiple compiled queries? My attempts so far have led to errors complaining about things not being translatable to SQL, Member access not allowed, etc.
Update: Another potentially better way I could have asked this question is as follows:
Consider the two compiled queries below:
_getByIdCompiled = CompiledQuery.Compile<MyDataContext, int, LocationDto>((context, id) =>
(from location in Context.Location // here
where location.DeletedDate == null // here
&& location.Field1 = false // here
&& location.Field2 = true // here
&& location.Field3 > 5 // here
&& location.Id == id
select new LocationDto { // here
Id = location.Id, // here
Name = location.Name
})).First()); // here
_getByNameCompiled = CompiledQuery.Compile<MyDataContext, int, LocationDto>((context, name) =>
(from location in Context.Location // here
where location.DeletedDate == null // here
&& location.Field1 = false // here
&& location.Field2 = true // here
&& location.Field3 > 5 // here
&& location.Name == name
select new LocationDto { // here
Id = location.Id, // here
Name = location.Name // here
})).First()); // here
All of the lines marked // here are duplicate very un-dry pieces of code. (In my code base, this actually 30+ lines of code.) How do I factor it out and make it reusable?

So, this whole thing is somewhat odd in that the Compile method needs to not only see the Expression objects passed to each query operator (Where, Select, etc.) as something it can understand, but it needs to see the whole query, including the use of all of the operators, as something it can comprehend as Expression objects. This pretty much removes more traditional query composition as an option.
This is going to get a tad messy; more so than I would really like, but I don't see a whole lot of great alternatives.
What we're going to do is create a method to construct our queries. It's going to accept a filter as a parameter, and that filter will be an Expression that represents a filter for some object.
Next we're going to define a lambda that will look almost exactly like what you would pass to Compile, but with an extra parameter. That extra parameter will be of the same type as our filter, and it will represent that actual filter. We'll use that parameter, instead of the filter, throughout the lambda. We'll then use a UseIn method to replace all instances of that third parameter in our new lambda with the filter expression that we are providing to the method.
Here is the method to construct the query:
private static Expression<Func<MyDataContext, int, IQueryable<LocationDto>>>
ConstructQuery(Expression<Func<Location, bool>> filter)
{
return filter.UseIn((MyDataContext context, int id,
Expression<Func<Location, bool>> predicate) =>
from location in context.Location.Where(predicate)
where location.DeletedDate == null
&& location.Field1 == false
&& location.Field2 == true
&& location.Field3 > 5
&& location.Id == id
select new LocationDto
{
Id = location.Id,
Name = location.Name
});
}
Here is the UseIn method:
public static Expression<Func<T3, T4, T5>>
UseIn<T1, T2, T3, T4, T5>(
this Expression<Func<T1, T2>> first,
Expression<Func<T3, T4, Expression<Func<T1, T2>>, T5>> second)
{
return Expression.Lambda<Func<T3, T4, T5>>(
second.Body.Replace(second.Parameters[2], first),
second.Parameters[0],
second.Parameters[1]);
}
(The typing is a mess here, and I can't figure out how to give the generic types meaningful names.)
The following method is used to replace all instances of one expression with another:
public static Expression Replace(this Expression expression,
Expression searchEx, Expression replaceEx)
{
return new ReplaceVisitor(searchEx, replaceEx).Visit(expression);
}
internal class ReplaceVisitor : ExpressionVisitor
{
private readonly Expression from, to;
public ReplaceVisitor(Expression from, Expression to)
{
this.from = from;
this.to = to;
}
public override Expression Visit(Expression node)
{
return node == from ? to : base.Visit(node);
}
}
Now that we've gotten through this gory mess, comes the easy part. ConstructQuery should be able to be modified at this point to represent your real query, without much difficulty.
To call this method we simply need to provide whatever filter we want applied to this alteration of the query, such as:
var constructedQuery = ConstructQuery(location => location.Id == locationId);

Linq statements have to end in either select or group clause so you can't cut out part of the query and store it elsewhere, but if you will always be filtering by the same four conditions, you can use the lambda syntax instead, and then add any additional where clauses in new queries.
And as pointed out by #Servy, you need to call First() or FirstOrDefault() to get a single element from the query result.
IQueryable<Location> GetAllLocations()
{
return Context.Location.Where( x => x.DeletedDate == null
&& x.Field1 == false
&& x.Field2 == true
&& x.Field3 > 5
).Select( x => x );
}
Location GetLocationById( int id )
{
return ( from x in GetAllLocations()
where x.Id == id
select x ).FirstOrDefault();
}
//or with lambda syntax
Location GetLocationById( int id )
{
return GetAllLocations()
.Where( x => x.Id == id )
.Select( x => x )
.FirstOrDefault();
}

Custom function in Entity Framework query sometimes translates properly, sometimes doesn't

I have this function:
public static IQueryable<Article> WhereArticleIsLive(this IQueryable<Article> q)
{
return q.Where(x =>
x != null
&& DateTime.UtcNow >= x.PublishTime
&& x.IsPublished
&& !x.IsDeleted);
}
And it works just fine in this query:
from a in Articles.WhereArticleIsLive()
where a.Id == 5
select new { a.Title }
But it doesn't work in this only slightly more complex query:
from s in Series
from a in Articles.WhereArticleIsLive()
where s.Id == a.SeriesId
select new { s, a }
I get this error message:
NotSupportedException: LINQ to Entities does not recognize the method 'System.Linq.IQueryable1[TheFraser.Data.Articles.Article] WhereArticleIsLive(System.Linq.IQueryable1[TheFraser.Data.Articles.Article])' method, and this method cannot be translated into a store expression.
Any idea why? Is there another way to consolidate query parameters like this?
Thanks in advance.

EDIT: corrections by Craig.
I'm leaving this here, because I think it's a valuable tool: Use linqkit! But not for solving this question though :-)
Instead of returning IQueryable, use Expression to factor out predicates. E.g. you could define the following static method on Article:
public static Expression<Func<Article,bool>> IsLive()
{
return x =>
x != null
&& DateTime.UtcNow >= x.PublishTime
&& x.IsPublished
&& !x.IsDeleted
}
Then, ensure to store a reference to this expression when building your query, something along the lines of (not tested):
var isLive = Article.IsLive();
from s in Series
from a in Articles.Where(isLive)
where s.Id == a.SeriesId
select new { s, a }

How can I conditionally apply a Linq operator?

We're working on a Log Viewer. The use will have the option to filter by user, severity, etc. In the Sql days I'd add to the query string, but I want to do it with Linq. How can I conditionally add where-clauses?

if you want to only filter if certain criteria is passed, do something like this
var logs = from log in context.Logs
select log;
if (filterBySeverity)
logs = logs.Where(p => p.Severity == severity);
if (filterByUser)
logs = logs.Where(p => p.User == user);
Doing so this way will allow your Expression tree to be exactly what you want. That way the SQL created will be exactly what you need and nothing less.

If you need to filter base on a List / Array use the following:
public List<Data> GetData(List<string> Numbers, List<string> Letters)
{
if (Numbers == null)
Numbers = new List<string>();
if (Letters == null)
Letters = new List<string>();
var q = from d in database.table
where (Numbers.Count == 0 || Numbers.Contains(d.Number))
where (Letters.Count == 0 || Letters.Contains(d.Letter))
select new Data
{
Number = d.Number,
Letter = d.Letter,
};
return q.ToList();
}

I ended using an answer similar to Daren's, but with an IQueryable interface:
IQueryable<Log> matches = m_Locator.Logs;
// Users filter
if (usersFilter)
matches = matches.Where(l => l.UserName == comboBoxUsers.Text);
// Severity filter
if (severityFilter)
matches = matches.Where(l => l.Severity == comboBoxSeverity.Text);
Logs = (from log in matches
orderby log.EventTime descending
select log).ToList();
That builds up the query before hitting the database. The command won't run until .ToList() at the end.

I solved this with an extension method to allow LINQ to be conditionally enabled in the middle of a fluent expression. This removes the need to break up the expression with if statements.
.If() extension method:
public static IQueryable<TSource> If<TSource>(
this IQueryable<TSource> source,
bool condition,
Func<IQueryable<TSource>, IQueryable<TSource>> branch)
{
return condition ? branch(source) : source;
}
This allows you to do this:
return context.Logs
.If(filterBySeverity, q => q.Where(p => p.Severity == severity))
.If(filterByUser, q => q.Where(p => p.User == user))
.ToList();
Here's also an IEnumerable<T> version which will handle most other LINQ expressions:
public static IEnumerable<TSource> If<TSource>(
this IEnumerable<TSource> source,
bool condition,
Func<IEnumerable<TSource>, IEnumerable<TSource>> branch)
{
return condition ? branch(source) : source;
}

When it comes to conditional linq, I am very fond of the filters and pipes pattern.
http://blog.wekeroad.com/mvc-storefront/mvcstore-part-3/
Basically you create an extension method for each filter case that takes in the IQueryable and a parameter.
public static IQueryable<Type> HasID(this IQueryable<Type> query, long? id)
{
return id.HasValue ? query.Where(o => i.ID.Equals(id.Value)) : query;
}

Doing this:
bool lastNameSearch = true/false; // depending if they want to search by last name,
having this in the where statement:
where (lastNameSearch && name.LastNameSearch == "smith")
means that when the final query is created, if lastNameSearch is false the query will completely omit any SQL for the last name search.

Another option would be to use something like the PredicateBuilder discussed here.
It allows you to write code like the following:
var newKids = Product.ContainsInDescription ("BlackBerry", "iPhone");
var classics = Product.ContainsInDescription ("Nokia", "Ericsson")
.And (Product.IsSelling());
var query = from p in Data.Products.Where (newKids.Or (classics))
select p;
Note that I've only got this to work with Linq 2 SQL. EntityFramework does not implement Expression.Invoke, which is required for this method to work. I have a question regarding this issue here.

It isn't the prettiest thing but you can use a lambda expression and pass your conditions optionally. In TSQL I do a lot of the following to make parameters optional:
WHERE Field = #FieldVar OR #FieldVar IS NULL
You could duplicate the same style with a the following lambda (an example of checking authentication):
MyDataContext db = new MyDataContext();
void RunQuery(string param1, string param2, int? param3){
Func checkUser = user =>
((param1.Length > 0)? user.Param1 == param1 : 1 == 1) &&
((param2.Length > 0)? user.Param2 == param2 : 1 == 1) &&
((param3 != null)? user.Param3 == param3 : 1 == 1);
User foundUser = db.Users.SingleOrDefault(checkUser);
}

I had a similar requirement recently and eventually found this in he MSDN.
CSharp Samples for Visual Studio 2008
The classes included in the DynamicQuery sample of the download allow you to create dynamic queries at runtime in the following format:
var query =
db.Customers.
Where("City = #0 and Orders.Count >= #1", "London", 10).
OrderBy("CompanyName").
Select("new(CompanyName as Name, Phone)");
Using this you can build a query string dynamically at runtime and pass it into the Where() method:
string dynamicQueryString = "City = \"London\" and Order.Count >= 10";
var q = from c in db.Customers.Where(queryString, null)
orderby c.CompanyName
select c;

You can create and use this extension method
public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool isToExecute, Expression<Func<TSource, bool>> predicate)
{
return isToExecute ? source.Where(predicate) : source;
}

Just use C#'s && operator:
var items = dc.Users.Where(l => l.Date == DateTime.Today && l.Severity == "Critical")
Edit: Ah, need to read more carefully. You wanted to know how to conditionally add additional clauses. In that case, I have no idea. :) What I'd probably do is just prepare several queries, and execute the right one, depending on what I ended up needing.

You could use an external method:
var results =
from rec in GetSomeRecs()
where ConditionalCheck(rec)
select rec;
...
bool ConditionalCheck( typeofRec input ) {
...
}
This would work, but can't be broken down into expression trees, which means Linq to SQL would run the check code against every record.
Alternatively:
var results =
from rec in GetSomeRecs()
where
(!filterBySeverity || rec.Severity == severity) &&
(!filterByUser|| rec.User == user)
select rec;
That might work in expression trees, meaning Linq to SQL would be optimised.

Well, what I thought was you could put the filter conditions into a generic list of Predicates:
var list = new List<string> { "me", "you", "meyou", "mow" };
var predicates = new List<Predicate<string>>();
predicates.Add(i => i.Contains("me"));
predicates.Add(i => i.EndsWith("w"));
var results = new List<string>();
foreach (var p in predicates)
results.AddRange(from i in list where p.Invoke(i) select i);
That results in a list containing "me", "meyou", and "mow".
You could optimize that by doing the foreach with the predicates in a totally different function that ORs all the predicates.