c# 使用模式匹配以及 is 和 as 运算符安全地进行强制转换

class Animal
{
  public void Eat() { Console.WriteLine("Eating."); }
  public override string ToString()
  {
    return "I am an animal.";
  }
}
class Mammal : Animal { }
class Giraffe : Mammal { }

class SuperNova { }

class Program
{
  static void Main(string[] args)
  {
    var g = new Giraffe();
    var a = new Animal();
    FeedMammals(g);
    FeedMammals(a);
    // Output:
    // Eating.
    // Animal is not a Mammal

    SuperNova sn = new SuperNova();
    TestForMammals(g);
    TestForMammals(sn);
    // Output:
    // I am an animal.
    // SuperNova is not a Mammal
  }

  static void FeedMammals(Animal a)
  {
    if (a is Mammal m)
    {
      m.Eat();
    }
    else
    {
      // variable 'm' is not in scope here, and can't be used.
      Console.WriteLine($"{a.GetType().Name} is not a Mammal");
    }
  }

  static void TestForMammals(object o)
  {
    // You also can use the as operator and test for null
    // before referencing the variable.
    var m = o as Mammal;
    if (m != null)
    {
      Console.WriteLine(m.ToString());
    }
    else
    {
      Console.WriteLine($"{o.GetType().Name} is not a Mammal");
    }
  }
}

class Program
{
  static void Main(string[] args)
  {
    int i = 5;
    PatternMatchingNullable(i);

    int? j = null;
    PatternMatchingNullable(j);

    double d = 9.78654;
    PatternMatchingNullable(d);

    PatternMatchingSwitch(i);
    PatternMatchingSwitch(j);
    PatternMatchingSwitch(d);
  }

  static void PatternMatchingNullable(System.ValueType val)
  {
    if (val is int j) // Nullable types are not allowed in patterns
    {
      Console.WriteLine(j);
    }
    else if (val is null) // If val is a nullable type with no value, this expression is true
    {
      Console.WriteLine("val is a nullable type with the null value");
    }
    else
    {
      Console.WriteLine("Could not convert " + val.ToString());
    }
  }

  static void PatternMatchingSwitch(System.ValueType val)
  {
    switch (val)
    {
      case int number:
        Console.WriteLine(number);
        break;
      case long number:
        Console.WriteLine(number);
        break;
      case decimal number:
        Console.WriteLine(number);
        break;
      case float number:
        Console.WriteLine(number);
        break;
      case double number:
        Console.WriteLine(number);
        break;
      case null:
        Console.WriteLine("val is a nullable type with the null value");
        break;
      default:
        Console.WriteLine("Could not convert " + val.ToString());
        break;
    }
  }
}

class Animal
{
  public void Eat() { Console.WriteLine("Eating."); }
  public override string ToString()
  {
    return "I am an animal.";
  }
}
class Mammal : Animal { }
class Giraffe : Mammal { }

class SuperNova { }

class Program
{
  static void Main(string[] args)
  {
    // Use the is operator to verify the type.
    // before performing a cast.
    Giraffe g = new Giraffe();
    UseIsOperator(g);

    // Use the as operator and test for null
    // before referencing the variable.
    UseAsOperator(g);

    // Use the as operator to test
    // an incompatible type.
    SuperNova sn = new SuperNova();
    UseAsOperator(sn);

    // Use the as operator with a value type.
    // Note the implicit conversion to int? in
    // the method body.
    int i = 5;
    UseAsWithNullable(i);

    double d = 9.78654;
    UseAsWithNullable(d);
  }

  static void UseIsOperator(Animal a)
  {
    if (a is Mammal)
    {
      Mammal m = (Mammal)a;
      m.Eat();
    }
  }

  static void UsePatternMatchingIs(Animal a)
  {
    if (a is Mammal m)
    {
      m.Eat();
    }
  }

  static void UseAsOperator(object o)
  {
    Mammal m = o as Mammal;
    if (m != null)
    {
      Console.WriteLine(m.ToString());
    }
    else
    {
      Console.WriteLine($"{o.GetType().Name} is not a Mammal");
    }
  }

  static void UseAsWithNullable(System.ValueType val)
  {
    int? j = val as int?;
    if (j != null)
    {
      Console.WriteLine(j);
    }
    else
    {
      Console.WriteLine("Could not convert " + val.ToString());
    }
  }
}