Distinct() with lambda?

Question

Right, so I have an enumerable and wish to get distinct values from it.

Using System.Linq, there\'s of course an extension method called Distinct<

Answer 1

Here's how you can do it:

public static class Extensions
{
    public static IEnumerable MyDistinct(this IEnumerable query,
                                                    Func f, 
                                                    Func,T> h=null)
    {
        if (h==null) h=(x => x.First());
        return query.GroupBy(f).Select(h);
    }
}

This method allows you to use it by specifying one parameter like .MyDistinct(d => d.Name), but it also allows you to specify a having condition as a second parameter like so:

var myQuery = (from x in _myObject select x).MyDistinct(d => d.Name,
        x => x.FirstOrDefault(y=>y.Name.Contains("1") || y.Name.Contains("2"))
        );

N.B. This would also allow you to specify other functions like for example .LastOrDefault(...) as well.

---

If you want to expose just the condition, you can have it even simpler by implementing it as:

public static IEnumerable MyDistinct2(this IEnumerable query,
                                                Func f,
                                                Func h=null
                                                )
{
    if (h == null) h = (y => true);
    return query.GroupBy(f).Select(x=>x.FirstOrDefault(h));
}

In this case, the query would just look like:

var myQuery2 = (from x in _myObject select x).MyDistinct2(d => d.Name,
                    y => y.Name.Contains("1") || y.Name.Contains("2")
                    );

N.B. Here, the expression is simpler, but note .MyDistinct2 uses .FirstOrDefault(...) implicitly.

---

Note: The examples above are using the following demo class

class MyObject
{
    public string Name;
    public string Code;
}

private MyObject[] _myObject = {
    new MyObject() { Name = "Test1", Code = "T"},
    new MyObject() { Name = "Test2", Code = "Q"},
    new MyObject() { Name = "Test2", Code = "T"},
    new MyObject() { Name = "Test5", Code = "Q"}
};