Linq .SingleOrDefault - how to setup a default for a custom class?

but how can i make it return something else ?

You cannot. You can make your own method — as shown by Oskar Kjellin — to return someting else, but SingleOrDefault will always behave as programmed, which means return the default value (null, 0) for an item.

This can be accomplished in a rather simple way. If you create your own extension method that is more specific than the generic SingleOrDefault, then the compiler will prefer the more type-specific version. Here's an example that shows how to do that with a simple Person class (you can copy-paste it into LINQPad to quickly see the result):

public class Person
{
    public string Name { get; set; }

    public override string ToString()
    {
        return Name ?? "";
    }
}

public static class PersonExtensionMethod
{
    public static Person SingleOrDefault(this IEnumerable<Person> source)
    {
        var person = Enumerable.SingleOrDefault(source);

        if (person == null)
            return new Person { Name = "Unnamed" };

        return person;
    }
}

public static void Main()
{
    var emptyCollection = new Person[0];
    var nonEmptyCollection = new Person[] { new Person { Name = "Jack" } };

    Debug.WriteLine("Empty collection: " + emptyCollection.SingleOrDefault());
    Debug.WriteLine("Non-empty collection: " + nonEmptyCollection.SingleOrDefault());
}

In the above example, SingleOrDefault(IEnumerable<Person>), takes precedence over SingleOrDefault<T>(IEnumerable<T>) which is less specific.

You will have to make an extension method:

    public static T SingleOr<T>(this IEnumerable<T> list, T defaultValue) where T : class
    {
        return list.SingleOrDefault() ?? defaultValue;
    }

There is no other way. All classes default to null.

Could you use DefaultIfEmpty() (psedo code follows) -

return myChannels.All.Where(_Channel => _Channel.Guid == this.ParentChannelGuid).DefaultIfEmpty(_SPECIFICCHANNEL).SingleOrDefault();