Dictionary extensions.cs

using System.Collections.Generic;
using System.Collections.ObjectModel;

internal static class Program
{
  private static TValue GetValueOrDefault<TKey, TValue>(this IDictionary<TKey, TValue> source, TKey key) { return default(TValue); }
  private static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> source, TKey key) { return default(TValue); }

  private static void Main() {
    var dictionary = new Dictionary<int, int>();
    var v1 = dictionary.GetValueOrDefault(0); // error CS0121: The call is ambiguous between the following methods or properties…

    var readOnly = new ReadOnlyDictionary<int, int>(dictionary);
    var v2 = readOnly.GetValueOrDefault(0); // error CS0121: The call is ambiguous between the following methods or properties…
  }
}