An approximate C# implementation of a piece of Scala code that showed how to make builders that enforce (at compile-time) that certain properties must be set.

CheckedPersonBuilder.cs

using System;

public class Demo
{
    public static void Main()
    {
        Person p = CreateA.Person().WithAge(23).WithName("");
        Person q = CreateA.Person().WithName("").WithAge(23);
        Person r = CreateA.Person().WithName("");
        // The next two don't compile because the name is not set
        Person s = CreateA.Person().WithAge(23);
        Person t = CreateA.Person();
    }
}

public class Person
{
    public string Name { get; private set; }
    public int Age { get; private set; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    // Not necessary if you don't want any references to the builder assembly; just add .Build() at the end of each builder invocation
    public static implicit operator Person(CheckedPersonBuilder<NameIsSet> builder)
    {
        return builder.Build();
    }
}

public abstract class NameStatus
{
}

public abstract class NameIsSet : NameStatus
{
}

public abstract class NameNotSet : NameStatus
{
}

public class CheckedPersonBuilder<TNameStatus>
    where TNameStatus : NameStatus
{
    public String Name { get; private set; }
    public int? Age { get; private set; }

    public static CheckedPersonBuilder<NameNotSet> New()
    {
        return new CheckedPersonBuilder<NameNotSet>(null, null);
    }

    private CheckedPersonBuilder(string name, int? age)
    {
        Name = name;
        Age = age;
    }

    public CheckedPersonBuilder<NameIsSet> WithName(string name)
    {
        return new CheckedPersonBuilder<NameIsSet>(name, Age);
    }

    public CheckedPersonBuilder<TNameStatus> WithAge(int age)
    {
        return new CheckedPersonBuilder<TNameStatus>(Name, age);
    }
}

public static class CreateA
{
    public static CheckedPersonBuilder<NameNotSet> Person()
    {
        return CheckedPersonBuilder<NameNotSet>.New();
    }

    public static Person Build(this CheckedPersonBuilder<NameIsSet> builder)
    {
        return new Person(builder.Name, builder.Age ?? 18);
    }
}