預設

對於類，介面，委託，陣列，可空（例如 int？）和指標型別，default(TheType) 返回 null：

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

對於結構和列舉，default(TheType) 返回與 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);

當 T 是一個通用引數時，default(T) 特別有用，因為沒有約束來判斷 T 是引用型別還是值型別，例如：

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