Question

I’m a little confused about how ValueType objects are managed memory-wise in .NET. I get that value types are put on the stack for each instance/declaration. I’m not quite sure about what constitutes a new instance/declaration. I think my main question is:

If I pass a ValueType parameter to a method, but do not assign it a value, then is a new instance created on the stack?

I’m not sure if I am phrasing this correctly, so I wrote a simple sample program to make it easier to understand my confusion.

void Main()
{
    //-------------------------|  Line 1:
    int MichaelJordan = 23;      
    //-------------------------|  Line 2:   
    int HoursInADay = 24;           
    //-------------------------|  Line 3:
    Console.WriteLine("Michael Jordan is "+(NumberHelper.Is23(MichaelJordan) ? string.Empty : "not ")+"23");
    //-------------------------|  Line 4:
    Console.WriteLine("The last number that wasn't 23 was "+NumberHelper.LastNumberThatWasnt23);
    //-------------------------|  Line 5:
    Console.WriteLine("Hours in a Day is "+(NumberHelper.Is23(HoursInADay) ? string.Empty : "not ")+"23");
    //-------------------------|  Line 6:  
    Console.WriteLine("The last number that wasn't 23 was "+NumberHelper.LastNumberThatWasnt23);
}

public static class NumberHelper {  
    public static int LastNumberThatWasnt23;
    public static bool Is23(int candidate){
        if(candidate != 23){
            LastNumberThatWasnt23 = candidate;
            return false;
        }
        return true;
    }
}

So, here are my questions with respect to the program.

1. How many integer instances are created on the stack in this program?

2. Where in the code, exactly, is each value type instance created on the stack?

Although there is a good chance I am wrong, this is what I think happens, and what I’m not sure about.

• Line 1: I know that a new instance created on the stack for variable MichaelJordan

• Line 2: I know that a new instance created on the stack for variable HoursInADay

• Line 3: Is a copy of the value instance from the variable MichaelJordan created in
  order to be a method parameter? Is an instance created of the value 23 for the comparison?

• Line 4: The value for the LastNumberThatWasnt23 field is not initialized, so it returns the default value. Is a new instance with the default value created for this field?

• Line 5: Same questions as Line 3, but also, I know that a new instance is created on the stack for the field LastNumberThatWasnt23 because I am setting it.

• Line 6: Now, the field has a value. But, in referencing that value and passing it into the Console.WriteLine method, am I creating a new value instance on the stack?

I have one more part to this question, which is How would I find out stuff like this? Can I do it programmatically? Are there any tools that can be used to reveal this kind of information?

I know this is a long question, so I thank you for taking the time to read it.

Answer 1

This depends on the jitter your machine uses to run this program. But since you use only two local variables, it is very likely the correct answer is zero. The jit optimizer stores the variable values in CPU registers. That’s one of the standard optimizations.

You’ll find more about the kind of optimizations that the jitter performs in this answer.