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c#linqpredicatebuilder
Abdul Ahmad
Abdul Ahmad

Reputation: 10021

predicate builder c# confusion

I'm trying to understand predicate builder so I can apply it to a web app I'm creating.

Basically I have 4 parameters that come in through a POST request, 'name', 'location', 'age', 'gender', and I have to filter out people from a database table based on these parameters.

problem is, every parameter has the possibility of being 'All' (meaning, if name = 'All', that means don't filter out people by name, if location = 'All' don't filter people by location etc...).

So one way I thought of doing this is to get all people into a list, and have 4 if statements:

if (name != 'All') {
 //filter list of people by name string
}
if (location != 'All') {
 //filter list of people by location
}

but I don't want to do this, I want to use predicate builder to build the linq expression and only get a list of people who match the parameters, but I don't understand what predicate builder is doing.

This is the site I'm looking at but it doesn't really explain what's going on, and I don't know how to apply it to my situation

Upvotes: 2

Views: 523

Answers (2)

Spencer Ruport
Spencer Ruport

Reputation: 35117

Perhaps I'm not understanding the issue but why can't you just do:

query = name == "All" ? query : query.Where(x => x.Name == name);
query = location == "All" ? query : query.Where(x => x.Location == location);

For your particular situation I think what you need to do is:

var query = db.People;
query = name == "All" ? query : query.Where(x => x.Name == name);
query = location == "All" ? query : query.Where(x => x.Location == location);
query = age == "All" ? query : query.Where(x => x.Age == age);
query = weight == "All" ? query : query.Where(x => x.Weight == weight);
var results = query.ToList();

Upvotes: 3

Michael Humelsine
Michael Humelsine

Reputation: 601

If there are only four parameters then I would just use default parameter values and a conditional Linq Where clause. I included the StartsWith(), EndsWith(), and Contains() to show other possibilities.

Updated to clarify where the database interaction is happening.

public class Example {
    private IRepository repos;

    //pass in your database context abstract here
    public Example(IRepository repos){
        this.repos = repos;
    }

    public IEnumerable<Person> PostMethod(string name = "All", string age = "All",
    string height = "All", string weight = "All") {
    //reference your database abstract here
    return repos.People.Where(x => name == "All" || x.Name == name)
        .Where(x => age == "All" || x.Age.Contains(age))
        .Where(x => height == "All" || x.Height.StartsWith(height))
        .Where(x => weight == "All" || x.Weight.EndsWith(weight));
    }
}

Upvotes: 2

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