Question

Which one do you choose and why:

public class User
{
    public long UserID { get; set; }
}

public class User
{
    public long UserId { get; set; }
}

And also as parameters:

public void DoSomething(long userId) { }
public void DoSomething(long userID) { }

Answer 1

The conventions make it very clear to choose UserId and userId respectively (emphasis mine):

The two abbreviations that can be used in identifiers are ID and OK. In Pascal-cased identifiers they should appear as Id, and Ok. If used as the first word in a camel-cased identifier, they should appear as id and ok, respectively.