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Compiler Error CS0120

An object reference is required for the nonstatic field, method, or property 'member'

In order to use a non-static field, method, or property, you must first create an object instance. For more information about static methods, see Static Classes and Static Class Members (C# Programming Guide). For more information about creating instances of classes, see Instance Constructors (C# Programming Guide).

The following sample generates CS0120:

// CS0120_1.cs
public class MyClass
{
   // Non-static field
   public int i; 
   // Non-static method
   public void f(){}
   // Non-static property
   int Prop
   {
      get 
      {
         return 1; 
      }
   }

   public static void Main()
   {
      i = 10;   // CS0120
      f();   // CS0120
      int p = Prop;   // CS0120
      // try the following lines instead
      // MyClass mc = new MyClass();
      // mc.i = 10;
      // mc.f();
      // int p = mc.Prop;
   }
}

CS0120 will also be generated if there is a call to a non-static method from a static method, as follows:

// CS0120_2.cs
// CS0120 expected
using System;

public class MyClass
{
   public static void Main()
   {
      TestCall();   // CS0120
      // To call a non-static method from Main,
      // first create an instance of the class.
      // Use the following two lines instead:
      // MyClass anInstanceofMyClass = new MyClass();
      // anInstanceofMyClass.TestCall();
   }

   public void TestCall()
   {
   }
}

Similarly, a static method cannot call an instance method unless you explicitly give it an instance of the class, as follows:

// CS0120_3.cs
using System;

public class MyClass
{
   public static void Main()
   {
      do_it("Hello There");   // CS0120
   }

   private void do_it(string sText)
   // You could also add the keyword static to the method definition:
   // private static void do_it(string sText)
   {
      Console.WriteLine(sText);
   }
}
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