How to make LINQ execute a (SQL) LIKE range search

Question

I am in big need of help, i have been trying to do this for some time now.

So I have this Query:

Select name from BlaBlaBla

order by 

case when nam         


        

Answer 1

The problem here is that expressions containing Regex can't be translated to SQL, so even when you'd succeed in building a correct expression, you can't use it in LINQ to a SQL backend. However, SQL's LIKE method also supports range wildcards like [0-9], so the trick is to make your LINQ translate to SQL containing a LIKE statement.

LINQ-to-SQL offers the possibility to use the SQL LIKE statement explicitly:

return namesList.OrderBy(r => SqlMethods.Like(r.Name, "[0-9]%")) ...

This SqlMethods class can only be used in LINQ-to-SQL though. In Entity Framework there are string functions that translate to LIKE implicitly, but none of them enable the range wildcard ([x-y]). In EF a statement like ...

return namesList.OrderBy(r => r.Name.StartsWith("[0-9]")) ...

... would translate to nonsense:

[Name] LIKE '~[0-9]%' ESCAPE '~'

I.e. it vainly looks for names starting with the literal string "[0-9]". So as long as you keep using LINQ-to-SQL SqlMethods.Like is the way to go.

In Entity Framework 6.1.3 (and lower) we have to use a slightly different way to obtain the same result ...

return namesList.OrderBy(r => SqlFunctions.PatIndex("[0-9]%", c.Name) == 1) ...

... because PatIndex in SqlFunctions also supports range pattern matching.

But in Entity Framwork 6.2 we're back on track with LINQ-to-SQL because of the new DbFunctions.Like function:

return namesList.OrderBy(r => DbFunctions.Like(r.Name, "[0-9]%")) ...

Finally, also Entity Framework core has a Like function:

return namesList.OrderBy(r => EF.Functions.Like(r.Name, "[0-9]%")) ...

Answer 2

Below you see a sample for this kind of way to handle cases for your orderings.

    static void Main(string[] args)
    {
        List<Obvious> list = new List<Obvious>();
        for (int i = 0; i < 100; i++)
        {
            list.Add(new Obvious(i.ToString(), i));
        }

        string name = list[30].name;
        switch (name)
        {
            case "9":
                list.OrderBy(o => o.perc)
                    .ThenByDescending(o => o.name);
                break;
            default:
                list.OrderByDescending(o => o.name)
                    .ThenBy(o => o.perc);
                break;
        }
    }

    public class Obvious
    {
        public string name { get; set; }
        public int perc { get; set; }
        public Obvious(string _name, int _perc)
        {
            this.name = _name;
            this.perc = _perc;
        }

    }

Answer 3

If I was you I wouldn't try using Expressions to solve this issue since it brings in a lot of complexity.

I see that you would like to have a generic method, so it can work with different domain entities, yet you are expecting that each entity has a Name property. You can solve this in a more simple way by defining interface that contains Name property. Like this:

    public static void Main()
    {
        var test = new List<YourDomainEntity>()
        {
            new YourDomainEntity() { Name = "1test", OtherProperty = "1"},
            new YourDomainEntity() { Name = "2test",  OtherProperty = "2" },
            new YourDomainEntity() { Name = "2test", OtherProperty = "1"   }
        };

        var k = Foo(test).ToList();
    }

    public interface INameOrderable
    {
        string Name { get; set; }
    }

    public interface IOtherPropertyOrderable
    {
        string OtherProperty { get; set; }
    }

    public static IEnumerable<T> Foo<T>(IEnumerable<T> list) where T : INameOrderable, IOtherPropertyOrderable
    {
        return list.OrderBy(a => a.Name, new NamesDescComparer()).ThenBy(b => b.OtherProperty);
    }

    public class NamesDescComparer : IComparer<string>
    {
        public int Compare(string x, string y) => -String.CompareOrdinal(x, y);
    }

    class YourDomainEntity : INameOrderable, IOtherPropertyOrderable
    {
        public string OtherProperty { get; set; }
        public string Name { get; set; }
    }

I believe the method Foo is what you are looking for.

Note the where T : INameOrderable part. It restricts usage of this method to entities that implement INameOrderable interface