ReSharper.Multi-Dictionary-Initializer.cs

using System.Collections.Generic;

// ReSharper disable once CheckNamespace
class MultiDictionary<TKey, TValue> : Dictionary<TKey, IList<TValue>>
{
  public void Add(TKey key, params TValue[] values) { }
}

static class Program
{
  static void Main() {
    // ReSharper disable once UnusedVariable
    var dict = new MultiDictionary<int, string> {
      { 1, "1", "One", },
      //             ^ See trailing comma inside brackets.
    };
  }
}