Default implementation of a method for C# interfaces?

Question

Is it possible to define an interface in C# which has a default implementation? (so that we can define a class implementing that interface without implementing that particul

Answer 1

Not directly, but you can define an extension method for an interface, and then implement it something like this

public interface ITestUser
{
    int id { get; set; }
    string firstName { get; set; }
    string lastName { get; set; }

    string FormattedName();
}

static class ITestUserHelpers
{
    public static string FormattedNameDefault(this ITestUser user)
    {
        return user.lastName + ", " + user.firstName;
    }
}

public class TestUser : ITestUser
{
    public int id { get; set; }
    public string firstName { get; set; }
    public string lastName { get; set; }

    public string FormattedName()
    {
        return this.FormattedNameDefault();
    }
}

Edit* It is important that the extension method and the method that you are implementing are named differently, otherwise you will likely get a stackoverflow.