When to use statics in C#. I'm sure there are other examples, but these are always valid.

static_methods.cs

// There are three times when I will openly use static methods in C#.
// 1. Providers
// 2. State Machines (variation on provider, really)
// 3. Any time where there *really* is *one* of something.


/// <summary>
/// 1. Providers give you a way to create new objects without using a constructor. Ideally, you would always
/// use Dependency Injection and Inversion of Control. However, these techniques do not work in all situations,
/// so we need a testable fallback. Providers fill this need.
/// </summary>
public class ConnectionProvider
{
  private static IDbConnection testConnection;

  public static IDbConnection GetConnection(/* there might be params here */)
  {
    if (HasTestConnection)
      return testConnection;
    
    // You might have logic here in how you build a DB connection instance,
    // especially if you have multiple databases, etc.
    // Snip
    return connection;
  }

  public static void ResetTestConnection()
  {
    testConnection = null;
  }

  public static void ConfigureTestConnection(IDbConnection dbConnection)
  {
    if (dbConnection == null)
      throw new ArgumentNullException("dbConnection");
      
    testConnection = dbConnection;
  }
  
  private static bool HasTestConnection
  {
    get { return testConnection != null; }
  }
}

// In your runtime, instead of using
//
//   var connection = new SqlConnection("connection string");
// or
//   var connection = new OracleConnection("connection string");
//
// you will instead use
//
//   var connection = ConnectionProvider.GetConnection();
//
// In your test, you will need a setup. This could point to a SQLite in-memory database, a 
// shared test DB, a Mock/Stub, whatever.
//
//   [TestSetup]
//   public void before_each_test()
//   {
//     IDbConnection fakeConnection = ...;
//     ConnectionProvider.ConfigureTestConnection(fakeConnection);
//   }
//
//   [TestTearDown]
//   public void after_each_test()
//   {
//     ConnectionProvider.ResetTestConnection();
//   }