Error: inaccessible due to its protection level

The code you show will not compile - not because of a protection level, but becasue you must declare constructors with an appropriate protection declaration: public or private for example:

public class ClassA
    {
    public ClassA(int var)
    { }
    public static void tm()
    { }
    }

class Program
    {
    public static void Main(string[] args)
        {
        ClassA obj = new ClassA(1);
        //obj.tm();
        }
    }

If you then uncomment your tm method access, you will get a different error:

cannot be accessed with an instance reference; qualify it with a type name instead

because it is a static method - and you need to access those via the Class name instead:

public static void Main(string[] args)
    {
    ClassA obj = new ClassA(1);
    ClassA.tm();
    }

[edit]Typos - twice - I hate IE, I hate IE... - OriginalGriff[/edit]

This compile-time error means that the property you are trying to access is not public and the only way to access it is by either modifying its access modifier or using reflection.

When it's not visible enough to reach: If, for example, the class is in another project and the visibility is interal or lower (protected or private), you won't be able to use it. You'll have to change it to public in such a case:

public class ClassA
    {
    public ClassA(int var)
    { }
    public static void tm()
    { }
    }

class Program
    {
    public static void Main(string[] args)
        {
        ClassA obj = new ClassA(1);
        //obj.tm();
        }
    }