C# Language Extension methods can only see public (or internal) members of the extended class

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Example

public class SomeClass
{
    public void DoStuff()
    {
        
    }

    protected void DoMagic()
    {
        
    }
}

public static class SomeClassExtensions
{
    public static void DoStuffWrapper(this SomeClass someInstance)
    {
        someInstance.DoStuff(); // ok
    }

    public static void DoMagicWrapper(this SomeClass someInstance)
    {
        someInstance.DoMagic(); // compilation error
    }
}

Extension methods are just a syntactic sugar, and are not actually members of the class they extend. This means that they cannot break encapsulation—they only have access to public (or when implemented in the same assembly, internal) fields, properties and methods.

Extension methods dispatch based on static type