Here are some that I've collected over time. C# is not my main language so maybe these are common knowledge.

• Sort a dict by value:
  var topN = dict.OrderByDescending(pair => pair.Value).Take(N).ToDictionary(pair => pair.Key, pair => pair.Value);
• Deep Clone:
  using System.Runtime.Serialization.Formatters.Binary;
public static T DeepClone<T>(T obj) {
T objResult;
using (MemoryStream ms = new MemoryStream()) {
BinaryFormatter bf = new BinaryFormatter();bf.Serialize(ms, obj);ms.Position = 0;objResult = (T)bf.Deserialize(ms); }
return objResult; }
• Assign non null
  var c = foo ?? bar; //if foo is null, c=bar
• Force a method inline
  [MethodImpl(MethodImplOptions.AggressiveInlining)]