Check out example codes for "Assets/Scripts/Snake.cs(177,25): error CS1061: Type `Snake.SnakeBodyPart' does not contain a definition for `SetGridPostion' and no extension method `SetGridPostion' of type `Snake.SnakeBodyPart' could be found. Are you missing an assembly reference?". It will help you in understanding the concepts better.

Code Example 1

// cs1061.cs  
public class TestClass1  
{  
    // TestClass1 has one method, called WriteSomething.  
    public void WriteSomething(string s)  
    {  
        System.Console.WriteLine(s);  
    }  
}  
  
public class TestClass2  
{  
    // TestClass2 has one method, called DisplaySomething.  
    public void DisplaySomething(string s)  
    {  
        System.Console.WriteLine(s);  
    }  
}  
  
public class TestTheClasses  
{  
    public static void Main()  
    {  
        TestClass1 tc1 = new TestClass1();  
        TestClass2 tc2 = new TestClass2();  
        // The following call fails because TestClass1 does not have
        // a method called DisplaySomething.  
        tc1.DisplaySomething("Hello");      // CS1061  
  
        // To correct the error, change the method call to either
        // tc1.WriteSomething or tc2.DisplaySomething.  
        tc1.WriteSomething("Hello from TestClass1");  
        tc2.DisplaySomething("Hello from TestClass2");  
    }  
}

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