C # Summary of nullable types

A few days ago, the nullable type was suddenly used. Because it was rarely used, I forgot how to use it. I checked the relevant information and summarized it as follows (refer to msdn ). 1. The nullable type is a structure: system. nullable <t>. The writing format is t ?. T indicates any value type. 2. The nullable type adds an extra value for the original T: "null ". This is quite useful. For example, the checkbox status has three states: checked, unchecked, and indeterminate. Bool is not enough, so the nullable type can be used at this time: bool ?. 3. Two very useful read-only attributes of the nullable type: hasvalue and value. For example, if (X. hasvalue) {Y = x. value;} 4. OPERATOR :??. It means that if the value of the nullable type instance is null, it will be given a non-null default value. Let's look at the following example: Int? X = NULL; int y = x ?? -1; 5. The getvalueordefault function is also very useful. 6. Boxing and unboxing problems.