"You must add a reference to assembly" compiler error in weird case

I have two applications App1 and App2. App1 added a reference to App2 and App2 added a reference to System.Drawing.

In App2 there's a function with two overloads, the first one is protected and has a parameter from System.Drawing.Bitmap and the second one is public has an integer parameter.

namespace App1Namespace
{
    class Program
    {
        static void Main(string[] args)
        {
            App2Namespace.StaticClass.Func(4);

namespace App2Namespace
{
    public class StaticClass
    {
        protected static void Func(System.Drawing.Bitmap bitmapParam) { }
        public static void Func(int intParam) { }

When the App1 is built, the following compile time error will be thrown:

Error 1 T开发者_开发问答he type 'System.Drawing.Bitmap' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Although Func which has Bimap param is protected, the App1 needs having a reference to System.Drawing.

The questions are that why does App1 need System.Drawing? and why this error appears only when the second overload is called and doesn't appear when another overload (for example with two parameters) is called?

Note: When the first overload becomes private the error will disappear.

The reason is likely because you are declaring StaticClass as a regular class. So it is possible to override the class in any referencing assemblies (thus why using protected would almost make sense for a static method). If all the methods are truly static methods, then make the class itself static and change the scope from protected to internal or private.

It may simply be that it needs a reference, because since your App2Namespace.StaticClass.Func could be inherited from (since it's protected) in a class in the App1 namespace. If that were to happen, it would also need a reference to System.Drawing.