Null-conditional Operators

https://msdn.microsoft.com/en-us/library/dn986595.aspx

x?.y – null conditional member access. Returns null if the left hand operand is null.

a?[x] – null conditional indexing. Returns null if the left hand operand is null.

正式在使用VS2015了，安装了配套的Resharper

今天看到绿色的提示，Use null propagation

/// <summary>
        /// 传感器配置事件（通知DeviceInfo）
        /// </summary>
        public event EventHandler<SensorConfigedEventArgs> SensorConfiged;

        public void OnSensorConfiged(SensorConfigedEventArgs e)
        {
            var handler = SensorConfiged;
            if (handler != null)
            {
                handler(this, e);
            }
        }

Resharper的优化

  var handler = SensorConfiged;
            handler?.Invoke(this, e);

Used to test for null before performing a member access (?.) or index (?[) operation.

These operators help you write less code to handle null checks, especially for descending into data structures.

int? length = customers?.Length; // null if customers is null 
Customer first = customers?[0];  // null if customers is null
int? count = customers?[0]?.Orders?.Count();  // null if customers, the first customer, or Orders is null

The last example demonstrates that the null-condition operators are short-circuiting.

If one operation in a chain of conditional member access and index operation returns null, then the rest of the chain’s execution stops.

Other operations with lower precedence in the expression continue.

For example, E in the following always executes, and the ?? and == operations execute.

A?.B?.C?[0] ?? E
A?.B?.C?[0] == E

Another use for the null-condition member access is invoking delegates in a thread-safe way with much less code. The old way requires code like the following:

var handler = this.PropertyChanged;
if (handler != null)
    handler(…)

The new way is much simpler:

PropertyChanged?.Invoke(e)

The new way is thread-safe because the compiler generates code to evaluate PropertyChanged one time only, keeping the result in temporary variable.

You need to explicitly call the Invoke method because there is no null-conditional delegate invocation syntax PropertyChanged?(e).

There were too many ambiguous parsing situations to allow it.