What Does the Question Mark Mean in C# Value Types (e.g., `int?`)?

C# Value Type Question: Understand the empty type

Question mark "?" Play a variety of characters in programming, including the use in conditional sentences. However, it also appears in the value type, such as

. In this case, what is the role of this question mark? public int? myProperty

Can empty type

The question mark behind the value type indicates that it is a empty type. Can empty type is a special example of the structure, which can accommodate the entire effective range of the underlying type, and a special vacancy.

This function is particularly useful when processing database scenarios or other elements that may intentionally lack value. For example, a database list that allows Boolean values ​​may contain True, FALSE, or NULL. System.Nullable

sample code

Consider the following code fragment:

Here, Variables are defined as an air -available integer (

), which can save an integer or NULL value.

The attribute allows us to check whether the variable contains effective values. If is True, we can use

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

        // 判断 HasValue 是否为 true
        if (num.HasValue)
        {
            System.Console.WriteLine("num = " + num.Value);
        }
        else
        {
            System.Console.WriteLine("num = Null");
        }

        // 如果 num 为 null，则赋值默认值 0
        int y = num.GetValueOrDefault();

        // 如果 num.HasValue 为 false，则抛出 InvalidOperationException
        try
        {
            y = num.Value;
        }
        catch (System.InvalidOperationException e)
        {
            System.Console.WriteLine(e.Message);
        }
    }
}

to get the actual value. Otherwise, is considered NULL.

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