[SerializableAttribute] [ComVisibleAttribute(false)] public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, ICollection<KeyValuePair<TKey, TValue>>, IDictionary, ICollection, IReadOnlyDictionary<TKey, TValue>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable, ISerializable, IDeserializationCallback { ... }

And this is of course ignoring the fact you copied that from the documentation witch shows all interfaces the class implements. IDictionary also implements ICollection & IEnumerable so you wouldn't need to include them. So to actually implement this class it would more likely be

[SerializableAttribute] [ComVisibleAttribute(false)] public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue>, ISerializable, IDeserializationCallback { ... }

maybe less

Annoyingly F# brought them along for the ride too, although you rarely have to explicitly write them.

I wrote a library of monads for C# [1], and implementing the SelectMany method is a good example of how messy it can be:

    public static RWS<R, W, S, V> SelectMany<R, W, S, T, U, V>( this RWS<R, W, S, T> self, Func<T, RWS<R, W, S, U>> bind, Func<T, U, V> project)
    {
    }

    Option<int>

    int option

    public static SelectMany R W S T U V ( this self : RWS R W S T, bind : Func T (RWS R W S U), project : Func T U V) : RWS R W S V
    {
    }

    class Thing int
    {
    }

    class Thing T : BaseThing T
    {
    }

[1] https://github.com/louthy/csharp-monad

A talented C# dev will need to know co/contravariance as well as implicits considering they also exist in C#.

In Scala, being familiar with these concepts is practically a necessity, though.

True for co/contravariance. For implicits, that depends on which implicits you are talking about.

Scala has:

* implicit conversion operators (these have existed in C# for some time, under the same name)

* implicit parameters (C# doesn't have an analog to these that I can think of)

* implicit classes (C# has an analog in classes providing extension methods, though the classes themselves are distinguished by a keywords as they are in Scala; in Scala, these are essentially syntactic sugar for creating a normal class and an implicit conversion from the extended class.)

C# allows default values for parameters, but it's not quite the same.

Example of C# default value for parameter:

  static void Addition(int a, int b = 42)  
  {  
    Console.WriteLine(a + b);  
  }

  Addition(4); // Prints 46  
  Addition(4, 5); // Prints 9

Like C#, implicit parameters must come after non-implicit parameters (if any).

Unlike C#, the implicit value does not need to be defined in the signature. Also unlike C#, if you provide the value for one implicit parameter, you must provide the value for all implicit parameters.

Right. Scala has C#-style default parameters as well, implicit parameters are a different (though very loosely related) thing.

  using System;
  
  public class Program
  {
  
  public class Person
  {
    private string _name;
    private int _age;
    
    public Person(string name, int age)
    {
      _name = name;
      _age = age;
    }
		
    public static implicit operator int(Person p)
    {
      return 42;
    }
  }
	
  public static void Main()
  {
    var person = new Person("Jim", 51);
    var number = 100;
    AdditionPrinter(person, number); // Prints 142
  }
	
  public static void AdditionPrinter(int a, int b)
  {
    Console.WriteLine(a + b);
  }
  
  }

Though implicits I feel are so much simpler than many people fear.

From a Ruby perspective: Just think of them as operating similarly to Refinements, except not completely insane because there is no global scope at compilation. You only have to worry about your own package, imports and inheritance.

Tracking down an implicit generally takes all of 10 seconds, and never more than a few minutes. Even moderately brain-bendy ones like akka.patterns.ask (where does the implicit "?" come from? "ask" is actually an implicit conversion to a class that defines it: https://github.com/akka/akka/blob/master/akka-actor/src/main...).

A Scala dev will look at the Scala type signature and shrug.

A C# dev will look at the C# type signature and shrug.

As usual, unfamiliar things are difficult and familiar things are easy.