What is the use of static variable in C#? When to

问题:

I have searched about static variables in C#, but I still am not getting what its use is. Also, if I try to declare the variable inside the method it will not give me the permission to do this. Why?

I have seen some examples about the static variable. I\'ve seen that we don\'t need to create a instance of the class to access the variable, but that is not enough to understand what its use is and when to use it.

Second thing

class Book
{
    public static int myInt = 0;
}

public class Exercise
{
    static void Main()
    {
        Book book = new Book();

        Console.WriteLine(book.myInt); // Shows error why does it show me error?
                                       // Can\'t I access the static variable 
                                       // by making the instance of a class?

        Console.ReadKey();
    }
}

回答1:

A static variable shares the value of it among all instances of the class.

Example without declaring it static:

public class Variable
{
    public int i = 5;
    public void test()
    {
        i = i + 5;
        Console.WriteLine(i);
    }
}


public class Exercise
{
    static void Main()
    {
        Variable var = new Variable();
        var.test();
        Variable var1 = new Variable();
        var1.test();
        Console.ReadKey();
    }
}

Explanation: If you look at the above example, I just declare the int variable. When I run this code the output will be 10 and 10. Its simple.

Now let\'s look at the static variable here; I am declaring the variable as a static.

Example with static variable:

public class Variable
{
    public static int i = 5;
    public void test()
    {
        i = i + 5;
        Console.WriteLine(i);
    }
}


public class Exercise
{
    static void Main()
    {
        Variable var = new Variable();
        var.test();
        Variable var1 = new Variable();
        var1.test();
        Console.ReadKey();
    }
}

Now when I run above code, the output will be 10 and 15. So the static variable value is shared among all instances of that class.

回答2:

C# haven\'t static variables at all. You can declare static field in the particular type definition via C#. Static field is a state, shared with all instances of particular type. Hence, the scope of the static field is entire type. That\'s why you can\'t declare static field within a method - method is a scope itself, and items declared in a method must be inaccessible over the method\'s border.

回答3:

static variables are used when only one copy of the variable is required. so if you declare variable inside the method there is no use of such variable it\'s become local to function only..

example of static is

class myclass
{
    public static int a = 0;
}

Variables declared static are commonly shared across all instances of a class.

Variables declared static are commonly shared across all instances of a class. When you create multiple instances of VariableTest class This variable permanent is shared across all of them. Thus, at any given point of time, there will be only one string value contained in the permanent variable.

Since there is only one copy of the variable available for all instances, the code this.permament will result in compilation errors because it can be recalled that this.variablename refers to the instance variable name. Thus, static variables are to be accessed directly, as indicated in the code.

回答4:

Some \"real world\" examples for static variables:

building a class where you can reach hardcoded values for your application. Similar to an enumeration, but with more flexibility on the datatype.

public static class Enemies
{
    public readonly static Guid Orc = new Guid(\"{937C145C-D432-4DE2-A08D-6AC6E7F2732C}\");
}

The widely known singleton, this allows to control to have exactly one instance of a class. This is very useful if you want access to it in your whole application, but not pass it to every class just to allow this class to use it.

public sealed class TextureManager
    {
        private TextureManager() {}
        public string LoadTexture(string aPath);

        private static TextureManager sInstance = new TextureManager();

        public static TextureManager Instance
        {
            get { return sInstance; }
        }
    }

and this is how you would call the texturemanager

TextureManager.Instance.LoadTexture(\"myImage.png\");

About your last question: You are refering to compiler error CS0176. I tried to find more infor about that, but could only find what the msdn had to say about it:

A static method, field, property, or event is callable on a class even when no instance of the class has been created. If any instances of the class are created, they cannot be used to access the static member. Only one copy of static fields and events exists, and static methods and properties can only access static fields and static events.

回答5:

Static classes don\'t require you to create an object of that class/instantiate them, you can prefix the C# keyword static in front of the class name, to make it static.

Remember: we\'re not instantiating the Console class, String class, Array Class.

class Book
{
    public static int myInt = 0;
}

public class Exercise
{
    static void Main()
    {
        Book book = new Book();
       //Use the class name directly to call the property myInt, 
      //don\'t use the object to access the value of property myInt

        Console.WriteLine(Book.myInt);

        Console.ReadKey();

    }
}

回答6:

The data members and function members that operate on the instance of the type are called instance members. The int’s ToString method (for example) are examples of instance members. By default, members are instance members. Data members and function members that don’t operate on the instance of the type, but rather on the type itself, must be marked as static. The Test.Main and Console.WriteLine methods are static methods. The Console class is actually a static class, which means all its members are static. You never actually create instances of a Console—one console is shared across the whole application.

回答7:

In response to the \"when to use it?\" question:

I often use a static (class) variable to assign a unique instance ID to every instance of a class. I use the same code in every class, it is very simple:

//Instance ID ----------------------------------------
    // Class variable holding the last assigned IID
    private static int xID = 0;
    // Lock to make threadsafe (can omit if single-threaded)
    private static object xIDLock = new object();
    // Private class method to return the next unique IID 
    //  - accessible only to instances of the class
    private static int NextIID()                    
    {
        lock (xIDLock) { return ++xID; }
    }
    // Public class method to report the last IID used 
    // (i.e. the number of instances created)
    public static int LastIID() { return xID; }
    // Instance readonly property containing the unique instance ID
    public readonly int IID = NextIID();
//-----------------------------------------------------

This illustrates a couple of points about static variables and methods:

1. Static variables and methods are associated with the class, not any specific instance of the class.
2. A static method can be called in the constructor of an instance - in this case, the static method NextIID is used to initialize the readonly property IID, which is the unique ID for this instance.

I find this useful because I develop applications in which swarms of objects are used and it is good to be able to track how many have been created, and to track/query individual instances.

I also use class variables to track things like totals and averages of properties of the instances which can be reported in real time. I think the class is a good place to keep summary information about all the instances of the class.

回答8:

Try calling it directly with class name Book.myInt

回答9:

On comparison with session variables, static variables will have same value for all users considering i am using an application that is deployed in server. If two users accessing the same page of an application then the static variable will hold the latest value and the same value will be supplied to both the users unlike session variables that is different for each user. So, if you want something common and same for all users including the values that are supposed to be used along the application code then only use static.

回答10:

You don\'t need to instantiate an object, because yau are going to use a static variable: Console.WriteLine(Book.myInt);

回答11:

Starting from this @Kartik Patel example , I have changed a little maybe now is more clear about static variable

 public class Variable
    {
        public static string StaticName = \"Sophia \";
        public string nonStName = \"Jenna \";
        public void test()
        {
            StaticName = StaticName + \" Lauren\"; 
            Console.WriteLine(\"  static ={0}\",StaticName);
            nonStName = nonStName + \"Bean \";
            Console.WriteLine(\"  NeStatic neSt={0}\", nonStName);

        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Variable var = new Variable();
            var.test();
            Variable var1 = new Variable();
            var1.test();
            Variable var2 = new Variable();
            var2.test();
            Console.ReadKey();

        }
    }

Output 
  static =Sophia  Lauren
  NeStatic neSt=Jenna Bean
  static =Sophia  Lauren Lauren
  NeStatic neSt=Jenna Bean
  static =Sophia  Lauren Lauren Lauren
  NeStatic neSt=Jenna Bean

1. Class Variable VS Instance Variable in C#

   Static Class Members C# OR Class Variable

   class A
   {
       // Class variable or \" static member variable\"  are declared with 
       //the \"static \" keyword
   
       public static int i=20;
       public int j=10;         //Instance variable 
       public static string s1=\"static class variable\"; //Class variable 
       public string s2=\"instance variable\";        // instance variable 
   }
   class Program
   {
       static void Main(string[] args)
       {
           A obj1 = new A();
   
           // obj1 instance variables 
           Console.WriteLine(\"obj1 instance variables \");
           Console.WriteLine(A.i);
           Console.WriteLine(obj1.j);
           Console.WriteLine(obj1.s2);
           Console.WriteLine(A.s1);
   
           A obj2 = new A();
   
           // obj2 instance variables 
           Console.WriteLine(\"obj2 instance variables \");
           Console.WriteLine(A.i);
           Console.WriteLine(obj2.j);
           Console.WriteLine(obj2.s2);
           Console.WriteLine(A.s1);
   
           Console.ReadKey();
   
       }
   
   
   }
   

   }

https://en.wikipedia.org/wiki/Class_variable

https://en.wikipedia.org/wiki/Instance_variable

https://en.wikipedia.org/wiki/Static_variable

https://javaconceptoftheday.com/class-variables-and-instance-variables-in-java/?fbclid=IwAR1_dtpHzg3bC5WlGQGdgewaTvuOI6cwVeFUtTV8IZuGTj1qH5PmKGwX0yM

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-classes-and-static-class-members

回答12:

Static variable retains it\'s previous value until the program exit. Static is used by calling directly class_Name.Method() or class_Name.Property. No object reference is needed. The most popular use of static is C#\'s Math class. Math.Sin(), Math.Cos(), Math.Sqrt().