Exploring Design Patterns: Thread Safe Singleton Pattern in C#

Singleton (Thread Safe)

private static readonly Lazy<DbConnectionThreadSafe> _instance = null!;

Using Lazy<T> Because

• Thread safe by default
• Provides lazy initialization

static DbConnectionThreadSafe()
{
    _instance = new Lazy<DbConnectionThreadSafe>(
        () => new DbConnectionThreadSafe(_connectionString),
        LazyThreadSafetyMode.ExecutionAndPublication
    );
}

Let's break down this static constructor and the Lazy initialization:

static DbConnection()  // This is a static constructor
{
    _lazyInstance = new Lazy<DbConnection>(
        () => new DbConnection(_connectionString),  // Lambda expression
        LazyThreadSafetyMode.ExecutionAndPublication  // Thread safety mode
    );
}

Let's examine each part:

1. Static Constructor (static DbConnection())

   • A static constructor is called automatically before any static members are referenced
   • It runs only once per app domain
   • It's thread-safe by default (CLR guarantees this)
   • Used to initialize static fields

2. Lazy

   • Lazy<T> is a class that provides lazy initialization
   • The actual object isn't created until it's first accessed
   • When you access Lazy<T>.Value for the first time, it creates the instance

3. Lambda Expression (() => new DbConnection(_connectionString))

   • This is a factory function that creates the DbConnection instance
   • It's only executed when the Lazy.Value is first accessed
   • The => syntax defines a lambda expression (anonymous function)

4. LazyThreadSafetyMode.ExecutionAndPublication

   • This enum value specifies how the Lazy instance handles thread safety
   • Three possible modes:

// No thread safety
LazyThreadSafetyMode.None

// Locks initialization
LazyThreadSafetyMode.ExecutionAndPublication

// Multiple threads can attempt initialization
LazyThreadSafetyMode.PublicationOnly

The execution flow:

1. When the class is first referenced, the static constructor runs
2. The _lazyInstance is created, but the DbConnection instance is not yet created
3. When Instance is first accessed:
   • The lambda expression runs
   • Creates new DbConnection with the connection string
   • Stores it in the Lazy instance
4. Subsequent accesses to Instance return the same stored instance