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'List' does not contain a definition for 'Where'

Tags:

c#

linq

I am trying to create a method where I can pass a Linq expression as a parameter to return a new list of items.

Currently I am doing this (based off this answer):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

class Collection<T> {

    List<T> items = new List<T>();

    public Collection<T> Filter(Expression<Func<T, bool>> query) {
        Collection<T> c = new Collection<T>();
        c = items.Where(query);
        return c;
    }
}

'List' does not contain a definition for 'Where' and the best extension method overload 'Queryable.Where(IQueryable, Expression>)' requires a receiver of type 'IQueryable'

I am not exactly sure what to do here to fix this.

like image 971
Get Off My Lawn Avatar asked Nov 30 '16 18:11

Get Off My Lawn


2 Answers

List<T> is not IQueryable<T> but IEnumerable<T>. For the Where extension for IEnumerable<T> you can only pass a delegate not an expression.

I guess you want something like that:

public Collection<T> Filter(Func<T, bool> query) // only the lambda, NO expression
{
    Collection<T> c = new Collection<T>(items.Where(query).ToList());
    return c;
}

Note that Where only returns an IEnumerable<T> but not an Collection<T>. So you need to convert c to a List<T> and pass this to the constructor of Collection<T>.

like image 121
René Vogt Avatar answered Nov 02 '22 11:11

René Vogt


If you absolutely must use an expression, there are a couple of things you could do. You can do something like this:

public ICollection<T> Filter(Expression<Func<T, bool>> query) {     
    return items.AsQueryable().Where(query).ToList();
}

Converting you IList to a Queryable or you could do this:

public ICollection<T> Filter(Expression<Func<T, bool>> query) {     
    return items.Where(query.Compile()).ToList();
}

Compiling your expression to a Func<T, bool>

like image 29
Matt Burland Avatar answered Nov 02 '22 13:11

Matt Burland