Why can't I call a public method in another class?

问题

I have got these two classes interacting and I am trying to call four different classes from class one for use in class two.

The methods are public and they do return values but for some reason there is not a connection being made. The error I get when I try is: "An object reference is required for the nonstatic field, method, or property 'GradeBook.[method I want called]'"

I have everything initialized. I don't want to create the methods as static. I read over the specifics of my assignment again and I'm not even supposed to but I can't seem to get this to work anyway I word it.

myGradeBook.[method] GraceBook.[method]

It all seems to create errors.

The current errors:

The best overloaded method match or 'System.Console.WriteLine(string, object)' has some invalid arguments.

Arugment '2': cannot convert from 'method group' to 'object'

I'm not even sur what those mean....

EDIT: I just fixed that problem thanks to the Step Into feature of Visual Studio. I don't know why it took me so long to use it.

回答1:

You're trying to call an instance method on the class. To call an instance method on a class you must create an instance on which to call the method. If you want to call the method on non-instances add the static keyword. For example

class Example {
  public static string NonInstanceMethod() {
    return "static";
  }
  public string InstanceMethod() { 
    return "non-static";
  }
}

static void SomeMethod() {
  Console.WriteLine(Example.NonInstanceMethod());
  Console.WriteLine(Example.InstanceMethod());  // Does not compile
  Example v1 = new Example();
  Console.WriteLine(v1.InstanceMethod());
}

回答2:

It sounds like you're not instantiating your class. That's the primary reason I get the "an object reference is required" error.

MyClass myClass = new MyClass();

once you've added that line you can then call your method

myClass.myMethod();

Also, are all of your classes in the same namespace? When I was first learning c# this was a common tripping point for me.

回答3:

You have to create a variable of the type of the class, and set it equal to a new instance of the object first.

GradeBook myGradeBook = new GradeBook();

Then call the method on the obect you just created.

myGradeBook.[method you want called]

回答4:

For example 1 and 2 you need to create static methods:

public static string InstanceMethod() {return "Hello World";}

Then for example 3 you need an instance of your object to invoke the method:

object o = new object();
string s = o.InstanceMethod();

来源：https://stackoverflow.com/questions/330138/why-cant-i-call-a-public-method-in-another-class

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methods

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