C# accesing non static member in a static function

问题:

So I have a function:

List<string> names = new string();

private static void getName(string name)
{
    names.add(name);
}

When I attempt to compile I get a: 'object reference is required for the non-static field' notice. What do I have to do to make this member (names) compatible with getName?

I need it to be non static or converted because I want to put the results into other non static functions and forms.

回答1:

You need to tell the system which list of names you're interested in. It's part of the state of an object, an instance of the class... but which one? Maybe you've created several instances of the class - maybe you've created no instances of the class. The static method has no visibility of that - so which instance do you want it to fetch the names variable value from?

To put it in another example, suppose we had a class like this:

public class Person
{
    public double MassInGrams { get; set; }
    public double HeightInMetres { get; set; }

    public static double ComputeBodyMassIndex()
    {
        // Which person are we interested in?
    }
}

Person p1 = new Person { MassInGrams = 76203, HeightInMetres = 1.8 };
Person p2 = new Person { MassInGrams = 65000, HeightInMetres = 1.7 };

double bmi = Person.ComputeBodyMassIndex();

What would you expect the result to be? You've asked the Person class to compute "the BMI" but without telling it whose BMI to compute. You need to give it that information.

Some options for your situation:

• Change names to be static instead
• Change the method to be an instance method
• Pass in an instance of the class
• Create an instance of the class, possibly returning it
• Fetch an instance of the class some other way

By the way, that's a very strange method name for something which adds a name. It's also somewhat unconventional...

回答2:

You need to make names static if you want to use it from inside of a static method:

 // If this is static, you can use it from your static method
 static List<string> names = new List<string>();

The issue is that getName is defined on your type, not on an instance of the type. However, names is defined so each instance of your type gets its own value.

回答3:

names is an object that will exist in the instances of the class e.g. MyClass mc = new MyClass(); then you can access mc.names. A static field can be called without an instance of the class just with the classname, e.g. MyClass.getName(""); will work. So when you think logically, the class doesn't contain names, only 'the instances of that class' contain it. Therefore, you either make that list static too and it will be 'the same List instance' everywhere when you call MyClass.names or make the getName method non-static, and it will be only called from instances, therefore no MyClass.getName("") will be possible, but mc.getName(""); It's a matter of what you are exactly trying to do.

回答4:

Static methods can not access class fields. Either make names static, or make getName() non-static. What do you mean by "Compatible". Ask yourself... does the method need to be static? What is its purpose and how do you intend to use it?

回答5:

You cant access it this way, you need to instanciate the class containing a member.