Implement an internal interface in an internal class in C#

Question

Why am I not allowed to implement an internal interface in an internal class? Is there a specific reason for this restriction or is it just a language design decision?

internal interface IDefinition
{
    string GetValueAsString(string property);
}

internal sealed class DefinitionArray : IDefinition
{
    internal string GetValueAsString(string property)
    {
        return m_definitionRows
            .Select(o => o.GetValueAsString(property))
            .FirstOrDefault();
    }
}

Answer 1

§3.5.6 of the C# 6.0 Language Specification states:

Interface members implicitly have public declared accessibility. No access modifiers are allowed on interface member declarations.

So what you 'theoretically' have is

internal interface IDefinition
{
    public string GetValueAsString(string property);
}

But this is not a problem, since (§3.5.2):

The accessibility domain of a nested member M declared in a type T within a program P is defined as follows (noting that M itself may possibly be a type):

• If the declared accessibility of M is public, the accessibility domain of M is the accessibility domain of T.

So the accessibility of the member is equivalent as it would have been declared as internal.