C # Summary of interview questions

Using System;
Class
{
Public ()
{
PrintFields ();
}
Public virtual void PrintFields (){}
}
Class B:
{
Int x = 1;
Int y;
Public B ()
{
Y =-1;
}
Public override void PrintFields ()
{
Console. WriteLine ("x = {0}, y = {1}", x, y );
}
What is the output when new B () is used to create B's instance?
Answer: X = 1, Y = 0

Answer:
Before constructing B, execute the variable first. y does not explicitly assign values. The default value is 0. Execute the constructor of B. Because B inherits A, execute the constructor of A first. The PrintFields method called by A constructor is A virtual function in Class A, and its implementation is in Class B. Therefore, the PrintFields method of Class B is executed and the result is output. Although the constructor B is executed, the value of y is-1, but the result has been output before.

1-8 represents the execution process of new B.
Class
{
Public A () // ④
{
PrintFields (); // ⑤, found to be overwritten → ⑥
}
Public virtual void PrintFields (){}
}
Class B:
{
Int x = 1; // ①
Int y; // ②
Public B () // ③ it is found that the constructor of A inherits from A and runs the constructor of A immediately.
{
Y =-1; // finished. At this point, the object has been created.
}
Public override void PrintFields () // ⑥
{
Console. WriteLine ("x = {0}, y = {1}", x, y); // 7, output, at this time: x = 1, y = 0
}
}

When the program instantiates B, because of the inheritance relationship, B will first call the constructor of its parent class, And A's constructor calls PrintFields. In A, printFields is A virtual method ,, so it will call rewrite in B. When PrintFields is called, the construction of B is not executed yet, so y = 0 instead of-1. so the output result is x = 1, y = 0 if B B = new B (); B. printFields (); then the output result is x = 1, y =-1