默认

对于类,接口,委托,数组,可空(例如 int?)和指针类型,default(TheType) 返回 null

class MyClass {}
Debug.Assert(default(MyClass) == null);
Debug.Assert(default(string) == null);

对于结构和枚举,default(TheType) 返回与 new TheType() 相同:

struct Coordinates
{
    public int X { get; set; }
    public int Y { get; set; }
}

struct MyStruct
{
    public string Name { get; set; }
    public Coordinates Location { get; set; }
    public Coordinates? SecondLocation { get; set; }
    public TimeSpan Duration { get; set; }
}

var defaultStruct = default(MyStruct);
Debug.Assert(defaultStruct.Equals(new MyStruct()));
Debug.Assert(defaultStruct.Location.Equals(new Coordinates()));
Debug.Assert(defaultStruct.Location.X == 0);
Debug.Assert(defaultStruct.Location.Y == 0);
Debug.Assert(defaultStruct.SecondLocation == null);
Debug.Assert(defaultStruct.Name == null);
Debug.Assert(defaultStruct.Duration == TimeSpan.Zero);

T 是一个通用参数时,default(T) 特别有用,因为没有约束来判断 T 是引用类型还是值类型,例如:

public T GetResourceOrDefault<T>(string resourceName)
{
   if (ResourceExists(resourceName))
   {
      return (T)GetResource(resourceName);
   }
   else
   {
      return default(T);
   }
}