private List<FooBar> _fooBars;

public List<FooBar> FooBars
{
    get { return _fooBars ?? (_fooBars = new List<FooBar>()); }
}

The first time the property .FooBars is accessed the _fooBars variable will evaluate as null, thus falling through to the assignment statement assigns and evaluates to the resulting value.

Thread safety

This is not thread-safe way of implementing lazy properties. For thread-safe laziness, use the Lazy<T> class built into the .NET Framework.

C# 6 Syntactic Sugar using expression bodies

Note that since C# 6, this syntax can be simplified using expression body for the property:

private List<FooBar> _fooBars;

public List<FooBar> FooBars => _fooBars ?? ( _fooBars = new List<FooBar>() );

Subsequent accesses to the property will yield the value stored in the _fooBars variable.

Example in the MVVM pattern

This is often used when implementing commands in the MVVM pattern. Instead of initializing the commands eagerly with the construction of a viewmodel, commands are lazily initialized using this pattern as follows:

private ICommand _actionCommand = null;
public ICommand ActionCommand =>
   _actionCommand ?? ( _actionCommand = new DelegateCommand( DoAction ) );