double check locking in singleton pattern

Question

it may be basic question

to have a singleton in multi-threaded environment we can use a lock. Please refer the code snippet. But why do we need double-checked locki

Answer 1

The "best" way I know is this:

public class MySingleton {
    // object for synchronization
    private static readonly object syncRoot = new object();
    // the singleton instance
    private static MySingleton @default;

    public static MySingleton Default {
        get {
            // geting singleton instance without locking
            var result = @default;
            // if result is NOT null, no additional action is required
            if ( object.ReferenceEquals(result, null) ){
                // lock the synchronization object
                lock(syncRoot) {
                    // geting singleton instanc in lock - because
                    // the value of @default field could be changed
                    result = @default;

                    // checking for NULL
                    if ( object.ReferenceEquals(result, null) ) {
                        // if result is NULL, create new singleton instance
                        result = new MySingleton();
                        // set the default instance
                        @default = result;
                    }
                }
            }

            // return singleton instance
            return result;
        }
    }
}