使用System.Linq.Dynamic在或包含查询

Question

Background: I would like to use System.Linq.Dynamic library to post In/Contains query to MS SQL

Please note I am trying to use System.Linq.Dynamic in generic function so that I could apply customer filter to any class having CustomerId(integer) property. Also CustomerId property could be nullable

All SO posts redirects me to this solution . After implementing the same I keep getting this exception: "No generic method 'Contains' on type 'System.Linq.Enumerable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic."

And now this is how my code looks like :

public static IQueryable<T> ApplyCustomerFilter<T>(this IQueryable<T> collection, int[] customerIds)
{
    return collection.Where("@0.Contains(outerIt.CustomerId)", customerIds);
}


public class MyUser : IdentityUser
{    
    [Required]
    [MaxLength(50)]
    public string FirstName { get; set; }
    [Required]
    [MaxLength(50)]
    public string LastName { get; set; }
    public int? CustomerId { get; set; }
    [ForeignKey(nameof(CustomerId))]
    public virtual Customer Customer { get; set; }
}

Could you please guide me where I am going wrong?

Answer 1

Because your MyUser.CustomerId property is nullable - you should pass nullable int array as customerIds in this case. For example:

public static IQueryable<T> ApplyCustomerFilter<T>(
    this IQueryable<T> collection, 
    int?[] customerIds) { // < - note here
        return collection.Where("@0.Contains(outerIt.CustomerId)", customerIds);
} 

or convert passed array to array of nullable ints:

public static IQueryable<T> ApplyCustomerFilter<T>(
    this IQueryable<T> collection, 
    int[] customerIds) {
        return collection.Where("@0.Contains(outerIt.CustomerId)",
             customerIds.Cast<int?>()); // <- note here
}

Alternatives proposed by Ivan Stoev in comments (with them customerIds array can be regular int[] array, no need for it to be array of nullables):

"@0.Contains(outerIt.CustomerId.Value)" 

And this one will work in both cases (whether CustomerId is nullable or not):

"@0.Contains(Int32(outerIt.CustomerId))"