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Restrictions on Using Accessibility Levels (C# Reference)

When you declare a type, make sure that you see whether that type has to be at least as accessible as another member or type. For example, the direct base class must be at least as accessible as the derived class. The following declarations will cause a compiler error because the base class BaseClass is less accessible than MyClass:

class BaseClass {...}
public class MyClass: BaseClass {...} // Error

The following table summarizes the restrictions on using declared accessibility levels.

Context

Remarks

Classes

The direct base class of a class type must be at least as accessible as the class type itself.

Interfaces

The explicit base interfaces of an interface type must be at least as accessible as the interface type itself.

Delegates

The return type and parameter types of a delegate type must be at least as accessible as the delegate type itself.

Constants

The type of a constant must be at least as accessible as the constant itself.

Fields

The type of a field must be at least as accessible as the field itself.

Methods

The return type and parameter types of a method must be at least as accessible as the method itself.

Properties

The type of a property must be at least as accessible as the property itself.

Events

The type of an event must be at least as accessible as the event itself.

Indexers

The type and parameter types of an indexer must be at least as accessible as the indexer itself.

Operators

The return type and parameter types of an operator must be at least as accessible as the operator itself.

Constructors

The parameter types of a constructor must be at least as accessible as the constructor itself.

The following example contains erroneous declarations of different types. The comment following each declaration indicates the expected compiler error.

// Restrictions on Using Accessibility Levels
// CS0052 expected as well as CS0053, CS0056, and CS0057
// To make the program work, change access level of both class B
// and MyPrivateMethod() to public.

using System;

// A delegate:
delegate int MyDelegate();

class B
{
    // A private method:
    static int MyPrivateMethod()
    {
        return 0;
    }
}

public class A
{
    // Error: The type B is less accessible than the field A.myField.
    public B myField = new B();

    // Error: The type B is less accessible
    // than the constant A.myConst.
    public readonly B myConst = new B();

    public B MyMethod()
    {
        // Error: The type B is less accessible 
        // than the method A.MyMethod.
        return new B();
    }

    // Error: The type B is less accessible than the property A.MyProp
    public B MyProp
    {
        set
        {
        }
    }

    MyDelegate d = new MyDelegate(B.MyPrivateMethod);
    // Even when B is declared public, you still get the error: 
    // "The parameter B.MyPrivateMethod is not accessible due to 
    // protection level."

    public static B operator +(A m1, B m2)
    {
        // Error: The type B is less accessible
        // than the operator A.operator +(A,B)
        return new B();
    }

    static void Main()
    {
        Console.Write("Compiled successfully");
    }
}

For more information, see the following sections in the C# Language Specification:

  • 3.5.1 Declared Accessibility

  • 3.5.4 Accessibility Constraints

  • 10.3.5 Access Modifiers

  • 10.3.8.2 Declared Accessibility

  • 10.3.8.5 Access to private and protected members of the containing type

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