LINQ lambda expression append OR statement

LINQ lambda expression append OR statement - c#

If I want to append a AND statement to my query, I can do:
query = query.Where(obj=>obj.Id == id);
if(name.HasValue)
query = query.Where(obj=>obj.Name == name);
and it will give me:
query.Where(obj=>obj.Id == id && obj.Name == name)
How can I append a OR statement that will result in:
query.Where(obj=>obj.Id == id || obj.Name == name)

You can't do it natively. However, you can use PredicateBuilder to compose the query before you run it, and it supports ORs.
var predicate = PredicateBuilder.False<Product>();
predicate = predicate.Or (obj=>obj.Id == id);
if(name.HasValue) predicate = predicate.Or (obj=>obj.Name == name);
return query.Where(predicate);

Simply this if I'm not missing something:
query.Where(obj=>obj.Id == id || (obj.Name == name && name.HasValue))
You might want to read this question (my question...) and answer for more complicated scenarios:
How to filter IEnumerable based on an entity input parameter

I would just build this into a single condition:
if (name.HasValue)
query = query.Where(obj=>obj.Id == id && obj.Name == name);
else
query = query.Where(obj=>obj.Id == id);

I would use gdoron's solution, but if it seems unpractical for larger sets of queries, a slightly more complicated solution containing set operations might help you:
var queryById = query.Where(obj => obj.Id == id);
var queryByName = query.Where(obj => obj.Name == name);
query = queryById.Union(queryByName);
It gets much more difficult if your original query contains duplicate items.
Another way may be using Expression to formulate your queries. You can modify the expression tree before executing it, so you can add more conditions to the Where sub-tree. That is a whole lot of work and it's an overkill on 99.9% (rough estimate :) ) of cases.

Part of the problem is that you over write your original query. Your OR operation would be equivalent to:
subquery1 = query.Where(obj=>obj.Id == id);
subquery2 = query.Where(obj=>obj.Name == name);
query = subquery1.Concat(subquery2).Distinct();
As you can see, that becomes pretty unwieldy as well, however if You are going to do this form, then you must maintain the original sequence so that you can get both the right and left sides of the equation processed then you need to be sure duplicates get removed.
Instead of all that, I would try to figure a way to build up that conditional statement dynamically using lambdas e.g.
I haven't actually run this, but something like this should work.
var cond = obj=>obj.Id == id;
...
// need an or
cond = obj=>cond(obj) || obj.Name == name;
query = query.Where(obj=>cond(obj));
Hope this gives you an idea.

Related

Combine two Lambda expressions using result of first Lambda expression

I was wondering if it's possible to combine these two statements into one.
Tbl_OrderFeeItem orderFee = adj.Tbl_Order.Tbl_OrderFeeItem.Single(x =>
x.OrderId == adj.OrderId
&& x.FeeTypeId == adj.FeeTypeId);
int querySource = orderFee.Tbl_FeeCheck
.Single(x => x.OrderFeeItemId == formFee.OrderFeeItemId)
.Tbl_PostPaymentOrderQuery
.PostPaymentOrderQueryTypeId;
What I'm trying to do is something like this...
int querySource = adj.Tbl_Order.Tbl_OrderFeeItem.Single(x =>
x.OrderId == adj.OrderId && x.FeeTypeId == adj.FeeTypeId)
.Tbl_FeeCheck.Single('use the id from the result of Tbl_OrderFeeItem.Single() call')
.Tbl_PostPaymentOrderQuery.PostPaymentOrderQueryTypeId;
I hope that made sense, I'm trying to use the id from the first query in the second but have it as one complete statement instead of two separate ones.
Thanks in advance

It's difficult to provide an accurate answer without any of the constructs present in your program, but by surrounding the original query with brackets it should allow you to perform the second query on the result of the first without using two seperate statements:
((Tbl_OrderFeeItem)adj.Tbl_Order.Tbl_OrderFeeItem.Single(x =>
x.OrderId == adj.OrderId
&& x.FeeTypeId == adj.FeeTypeId))
.Tbl_FeeCheck
.Single(x => x.OrderFeeItemId == formFee.OrderFeeItemId)
.Tbl_PostPaymentOrderQuery
.PostPaymentOrderQueryTypeId;

LINQ and C# - Dealing with a potentially null parameter

I am relatively new to LINQ but looking for some "best practice" advice on how to deal with the following. I know there are many ways to deal with this, but looking to see how more experienced people would write the code.
My LINQ at present:
var company = (from c in db.RPTINQUIRies
where c.CONCOM == concom && c.LOPER == engineer
orderby c.CREATION_DATE descending
select c);
Now the ActionResult parameter that is being passed in here (engineer) may or may not be empty. Where it is empty, I essentially want to remove the && C.LOPER == engineer clause all together.
What's the best way to deal with this?

It sounds like you just want:
where c.CONCOM == concom && (engineer == null || c.LOPER == engineer)
Alternatively, you could build up the query step by step:
var query = db.RPTINQUIRies.Where(c => c.CONCOM == concom);
if (engineer != null)
{
query = query.Where(c => c.LOPER == engineer);
}
query = query.OrderByDescending(c => c.CREATION_DATE);

changing linq to sql query

I have a question regarding a Linq to SQL query.
I have following situation:
I have a search with lots of options, like location, availability, name, language etc ...
For this options i have to execute a query to retrieve the results according to options selected, how can i best do it, i cannot write a linq query like for each possibility and combination of options, but i cannot write one for all of them as it will not work, for example:
from p in context.people where p.location==model.location && p.availability==model.availability .... select p
In this case imagine availability is not selected and should not be searched for, but in this case it will be passed as false, or if location is not set and is null so it will only search for empty locations, although i just need all.
So my question is how do people handle this kind of behaviour with queries?

As you long as you do not execute the linq query immediately you can just add where clauses to it. You can do this for example:
var query = from p in context.people;
if(searchOnLocation)
{
query = query.where(p => p.location == model.location);
}
if(otherSearch)
{
query = query.where(p => p.someOtherProperty == someotherValue);
}
var result = query.ToList();
As long you don't call ToList() on your IQueryable, the linq will not be translated into SQL. It's only in the last call, that the linq will be translated and executed against the database

IQueryable<Person> query = context.people;
if(model.location != null)
query = query.Where(x => x.location == model.location);
if(model.availability != null)
query = query.Where(x => x.availability == model.availability);
// etc
Basically, you can compose more and more restrictions as you go.

If you want to implement query without if condition than you can use following syntax:
var query = context.people.
where(p => p.location == (model.location ?? p.location)
&& p.availability == (model.availability ?? p.availability))
.ToList();

Linq: how to exclude condition if parameter is null

I have some table and the following condition of query: if parameter A is null take all, if not, use it in the query. I know how to do that in 2 steps:
List<O> list = null;
if (A = null)
{
list = context.Obj.Select(o => o).ToList();
}
else
{
list = context.Obj.Where(o.A == A).ToList();
}
Is it possible to have the same as one query?
Thanks

How about:
list = context.Obj.Where(o => A == null || o.A == A)
.ToList();
You can do it in one query but still using a condition:
IEnumerable<O> query = context.Obj;
if (A != null)
{
query = query.Where(o => o.A == A);
}
var list = query.ToList();
Or you could use a conditional operator to put the query in a single statement:
var query = A is null ? context.Obj : context.Obj.Where(o => o.A == A);
var list = query.ToList();
I would personally suggest either of the latter options, as they don't require that the LINQ provider is able to optimise away the filter in the case where A is null. (I'd expect most good LINQ providers / databases to be able to do that, but I'd generally avoid specifying a filter when it's not needed.)

I opted for
var list = context.Obj.Where(o => A.HasValue ? o.a == A : true);

I would probably write the query like this:
IQueryable<O> query = context.Obj;
if (A != null)
query = query.Where(o => o.A == A);
var list = query.ToList()
It's not one expression, but I think it's quite readable.
Also, this code assumes that context.Obj is IQueryable<O> (e.g. you are using LINQ to SQL). If that's not the case, just use IEnumerable<O>.

Dynamic WHERE clause in LINQ

What is the best way to assemble a dynamic WHERE clause to a LINQ statement?
I have several dozen checkboxes on a form and am passing them back as: Dictionary<string, List<string>> (Dictionary<fieldName,List<values>>) to my LINQ query.
public IOrderedQueryable<ProductDetail> GetProductList(string productGroupName, string productTypeName, Dictionary<string,List<string>> filterDictionary)
{
var q = from c in db.ProductDetail
where c.ProductGroupName == productGroupName && c.ProductTypeName == productTypeName
// insert dynamic filter here
orderby c.ProductTypeName
select c;
return q;
}

(source: scottgu.com)
You need something like this? Use the Linq Dynamic Query Library (download includes examples).
Check out ScottGu's blog for more examples.

I have similar scenario where I need to add filters based on the user input and I chain the where clause.
Here is the sample code.
var votes = db.Votes.Where(r => r.SurveyID == surveyId);
if (fromDate != null)
{
votes = votes.Where(r => r.VoteDate.Value >= fromDate);
}
if (toDate != null)
{
votes = votes.Where(r => r.VoteDate.Value <= toDate);
}
votes = votes.Take(LimitRows).OrderByDescending(r => r.VoteDate);

You can also use the PredicateBuilder from LinqKit to chain multiple typesafe lambda expressions using Or or And.
http://www.albahari.com/nutshell/predicatebuilder.aspx

A simple Approach can be if your Columns are of Simple Type like String
public static IEnumerable<MyObject> WhereQuery(IEnumerable<MyObject> source, string columnName, string propertyValue)
{
return source.Where(m => { return m.GetType().GetProperty(columnName).GetValue(m, null).ToString().StartsWith(propertyValue); });
}

It seems much simpler and simpler to use the ternary operator to decide dynamically if a condition is included
List productList = new List();
productList =
db.ProductDetail.Where(p => p.ProductDetailID > 0 //Example prop
&& (String.IsNullOrEmpty(iproductGroupName) ? (true):(p.iproductGroupName.Equals(iproductGroupName)) ) //use ternary operator to make the condition dynamic
&& (ID == 0 ? (true) : (p.ID == IDParam))
).ToList();

I came up with a solution that even I can understand... by using the 'Contains' method you can chain as many WHERE's as you like. If the WHERE is an empty string, it's ignored (or evaluated as a select all). Here is my example of joining 2 tables in LINQ, applying multiple where clauses and populating a model class to be returned to the view. (this is a select all).
public ActionResult Index()
{
string AssetGroupCode = "";
string StatusCode = "";
string SearchString = "";
var mdl = from a in _db.Assets
join t in _db.Tags on a.ASSETID equals t.ASSETID
where a.ASSETGROUPCODE.Contains(AssetGroupCode)
&& a.STATUSCODE.Contains(StatusCode)
&& (
a.PO.Contains(SearchString)
|| a.MODEL.Contains(SearchString)
|| a.USERNAME.Contains(SearchString)
|| a.LOCATION.Contains(SearchString)
|| t.TAGNUMBER.Contains(SearchString)
|| t.SERIALNUMBER.Contains(SearchString)
)
select new AssetListView
{
AssetId = a.ASSETID,
TagId = t.TAGID,
PO = a.PO,
Model = a.MODEL,
UserName = a.USERNAME,
Location = a.LOCATION,
Tag = t.TAGNUMBER,
SerialNum = t.SERIALNUMBER
};
return View(mdl);
}

Just to share my idea for this case.
Another approach by solution is:
public IOrderedQueryable GetProductList(string productGroupName, string productTypeName, Dictionary> filterDictionary)
{
return db.ProductDetail
.where
(
p =>
(
(String.IsNullOrEmpty(productGroupName) || c.ProductGroupName.Contains(productGroupName))
&& (String.IsNullOrEmpty(productTypeName) || c.ProductTypeName.Contains(productTypeName))
// Apply similar logic to filterDictionary parameter here !!!
)
);
}
This approach is very flexible and allow with any parameter to be nullable.

You could use the Any() extension method. The following seems to work for me.
XStreamingElement root = new XStreamingElement("Results",
from el in StreamProductItem(file)
where fieldsToSearch.Any(s => el.Element(s) != null && el.Element(s).Value.Contains(searchTerm))
select fieldsToReturn.Select(r => (r == "product") ? el : el.Element(r))
);
Console.WriteLine(root.ToString());
Where 'fieldsToSearch' and 'fieldsToReturn' are both List objects.

This is the solution I came up with if anyone is interested.
https://kellyschronicles.wordpress.com/2017/12/16/dynamic-predicate-for-a-linq-query/
First we identify the single element type we need to use ( Of TRow As DataRow) and then identify the “source” we are using and tie the identifier to that source ((source As TypedTableBase(Of TRow)). Then we must specify the predicate, or the WHERE clause that is going to be passed (predicate As Func(Of TRow, Boolean)) which will either be returned as true or false. Then we identify how we want the returned information ordered (OrderByField As String). Our function will then return a EnumerableRowCollection(Of TRow), our collection of datarows that have met the conditions of our predicate(EnumerableRowCollection(Of TRow)). This is a basic example. Of course you must make sure your order field doesn’t contain nulls, or have handled that situation properly and make sure your column names (if you are using a strongly typed datasource never mind this, it will rename the columns for you) are standard.

System.Linq.Dynamic might help you build LINQ expressions at runtime.
The dynamic query library relies on a simple expression language for formulating expressions and queries in strings.
It provides you with string-based extension methods that you can pass any string expression into instead of using language operators or type-safe lambda extension methods.
It is simple and easy to use and is particularly useful in scenarios where queries are entirely dynamic, and you want to provide an end-user UI to help build them.
Source: Overview in Dynamic LINQ
The library lets you create LINQ expressions from plain strings, therefore, giving you the possibility to dynamically build a LINQ expression concatenating strings as you require.
Here's an example of what can be achieved:
var resultDynamic = context.Customers
.Where("City == #0 and Age > #1", "Paris", 50)
.ToList();

Develop Reference

C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.

LINQ lambda expression append OR statement - c#

Simply this if I'm not missing something: query.Where(obj=>obj.Id == id || (obj.Name == name && name.HasValue)) You might want to read this question (my question...) and answer for more complicated scenarios: How to filter IEnumerable based on an entity input parameter

I would just build this into a single condition: if (name.HasValue) query = query.Where(obj=>obj.Id == id && obj.Name == name); else query = query.Where(obj=>obj.Id == id);

Related

Combine two Lambda expressions using result of first Lambda expression

LINQ and C# - Dealing with a potentially null parameter

changing linq to sql query

Linq: how to exclude condition if parameter is null

Dynamic WHERE clause in LINQ

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