What is the purpose of a question mark after a typ

问题:

Typically the main use of the question mark is for the conditional, x ? \"yes\" : \"no\".

But I have seen another use for it but can\'t find an explanation of this use of the ? operator, for example.

public int? myProperty
{
   get;
   set;
}

回答1:

It means that the value type in question is a nullable type

Nullable types are instances of the System.Nullable struct. A nullable type can represent the correct range of values for its underlying value type, plus an additional null value. For example, a Nullable, pronounced \"Nullable of Int32,\" can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value. A Nullable can be assigned the values true, false, or null. The ability to assign null to numeric and Boolean types is especially useful when you are dealing with databases and other data types that contain elements that may not be assigned a value. For example, a Boolean field in a database can store the values true or false, or it may be undefined.

class NullableExample
{
  static void Main()
  {
      int? num = null;

      // Is the HasValue property true?
      if (num.HasValue)
      {
          System.Console.WriteLine(\"num = \" + num.Value);
      }
      else
      {
          System.Console.WriteLine(\"num = Null\");
      }

      // y is set to zero
      int y = num.GetValueOrDefault();

      // num.Value throws an InvalidOperationException if num.HasValue is false
      try
      {
          y = num.Value;
      }
      catch (System.InvalidOperationException e)
      {
          System.Console.WriteLine(e.Message);
      }
  }
}

回答2:

It is a shorthand for Nullable<int>. Nullable<T> is used with value types that cannot be null.

回答3:

In

x ? \"yes\" : \"no\"

the ? declares an if sentence. Here: x represents the boolean condition; The part before the : is the then sentence and the part after is the else sentence.

In, for example,

int?

the ? declares a nullable type, and means that the type before it may have a null value.

回答4:

Nullable Types

Nullable types are instances of the System.Nullable struct. A nullable type can represent the normal range of values for its underlying value type, plus an additional null value. For example, a [Nullable<Int32>], pronounced \"Nullable of Int32,\" can be assigned any value from -2147483648 to 2147483647, or it can be assigned the null value. A [Nullable<bool>] can be assigned the values true or false, or null. The ability to assign null to numeric and Boolean types is particularly useful when dealing with databases and other data types containing elements that may not be assigned a value. For example, a Boolean field in a database can store the values true or false, or it may be undefined.

回答5:

it declares that the type is nullable.

回答6:

practical usage:

public string someFunctionThatMayBeCalledWithNullAndReturnsString(int? value)
{
  if (value == null)
  {
    return \"bad value\";
  }

  return someFunctionThatHandlesIntAndReturnsString(value);
}

回答7:

To add on to the answers above, here is a code sample

struct Test
{
    int something;
}
struct NullableTest
{
    int something;
}
class Example
{
    public void Demo()
    {
        Test t = new Test();
        t = null;

        NullableTest? t2 = new NullableTest();
        t2 = null;
    }
}

This would give a compilation error:

Error 12 Cannot convert null to \'Test\' because it is a non-nullable value type

Notice that there is no compilation error for NullableTest. (note the ? in the declaration of t2)

回答8:

int? it\'s shorthand for Nullable<int>, the two forms are interchangeable.

Nullable<T> is an operator that you can use with a value type T to make it accept null.

In case you don\'t know it: value types are types that accepts values as int, bool, char etc...

They can\'t accept references to values: they would generate a compile-time error if you assign them a null, as opposed to reference types, which can obviously accept it.

Why would you need that? Because sometimes your value type variables could receive null references returned by something that didn\'t work very well, like a missing or undefined variable returned from a database.

I suggest you to read the Microsoft Documentation because it covers the subject quite well.