C# - How to use Expression<Func> with parametrs in LINQ where

Question

Task: I need to give some expression with parameters into LINQ's where to get some data from database, but have an error above This example of working expression:

var shopExp = GetPersonForShop(PersonTypeIds.Director, new Guid("adda423f-8c38-40e0-9f39-6deceb787bc0")); // id
Where(shopExp)

But i need assign id dynamically, but got error above :

_repository.Persons
.Where(GetPersonForShop(PersonTypeIds.Director, person.PersonId)

And got error:

{"Unable to cast object of type 'System.Linq.Expressions.InstanceMethodCallExpression2' to type 'System.Linq.Expressions.LambdaExpression'."}

How does function for where(linq) look:

private Expression<Func<Person, bool>> GetPersonForShop(PersonTypeIds personTypeId, Guid personId)
        {
            return person => person .PeronTypeId== (int) personTypeId && person .PersonId == personId;
        }

This is approximate look like out production, just change names of parametrs code

How can I add expression with parameters to Where clause??

EF/EF Core already allow dynamic queries. There are multiple answers that show how to add Where clauses dynamically. I suspect the real problem is adding a "generic" repository on top of EF and losing the ability to perform LINQ queries — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Jan 21, 2020 at 13:59
I add some more info about this task. Yes, I know how lambda work, I need to move expression(the function that return expression) with parameters into 'where' linq method. — Denys Matsiura
– Denys Matsiura, Commented Jan 21, 2020 at 14:17
I'm confused. First you use GetPersonForShop but then you show us the GetKpisForEntity method with the same (apparent) signature. Are you not just confused between these two method names? There are also a lot of typos in your code, which makes it impossible for us to judge if you made these while writing the question or writing the code itself. — Flater
– Flater, Commented Jan 21, 2020 at 14:19
From what i can get, this should help you: [ref] stackoverflow.com/questions/14297633/… — venter
– venter, Commented Jan 21, 2020 at 14:23

Peter B · Accepted Answer · 2020-01-21 15:40:10Z

Lambda expressions use => notation. Try something like this:

var idToFind = new Guid("adda423f-8c38-40e0-9f39-6deceb787bc0");

var result = _repository.Persons
    .Where(p => p.TypeId == PersonTypeIds.Director && p.PersonId == idToFind);

In this expression, p represents each Person record in the Persons table, compared one-by-one using the boolean expression that follows it.

Depending on your datasource, the comparison for each p will either be done by .NET in memory, or it will happen inside your database using a SQL WHERE clause which is constructed from the boolean expression. The last would be optimal because it would mean that not the entire Persons table has to be transferred into .NET memory before comparison can take place.

Update - To apply the same condition multiple times without repeating it in your code, while still keeping the advantages of LINQ to SQL translation intact, you can put the condition in an Expression<Func<Person, bool>> object and then use that multiple times:

Expression<Func<Person, bool>> expression =
    p => p.TypeId == PersonTypeIds.Director && p.PersonId == idToFind;

var result1 = datasource1.Where(expression);
var result2 = datasource2.Where(expression);
var result3 = datasource3.Where(expression);

Or through a method that produces the Expression object:

var result1 = datasource1.Where(GetExpression(idToFind));
var result2 = datasource2.Where(GetExpression(idToFind));
var result3 = datasource3.Where(GetExpression(idToFind));

public Expression<Func<Person, bool>> GetExpression(Guid idToFind)
{
    return p => p.TypeId == PersonTypeIds.Director && p.PersonId == idToFind;
}

Or alternatively you can use a helper method:

var result1 = FilterByTypeAndId(datasource1, idToFind);
var result2 = FilterByTypeAndId(datasource2, idToFind);
var result3 = FilterByTypeAndId(datasource3, idToFind);

public IQueryable<Person> FilterByTypeAndId(IQueryable<Person> datasource, Guid idToFind)
{
    return datasource.Where(p => p.TypeId == PersonTypeIds.Director && p.PersonId == idToFind);
}

Problem that this where repeats in 11 places in code, thats why i want to use expression to simplify it. More, i have more complicated logic for select, but cant use becouse where want to use expression without paramets

Iria · Accepted Answer · 2020-01-21 15:29:23Z

0

based on the previous response, I am going to give you a few alternatives and suggestions.

var idToFind = new Guid("adda423f-8c38-40e0-9f39-6deceb787bc0");

var result = _repository
    .Persons
    .Where(p => p.TypeId == PersonTypeIds.Director)
    .Where(p => p.PersonId == idToFind)
    .ToList();

First is doing the where clause in 2 steps and then, adding the ToList(), with the ToList(), you will deal with collections and LINQ that is pretty useful. And by doing the where clause in 2 steps, is more for readable purposes.

answered Jan 21, 2020 at 15:29

Iria

4971 gold badge9 silver badges21 bronze badges

Collectives™ on Stack Overflow

C# - How to use Expression<Func> with parametrs in LINQ where

2 Answers 2

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related