在接口中定义方法

Question

首先让我为提出这么多愚蠢的问题而道歉。我刚刚开始使用c＃并试图了解类/接口和继承。在这个例子中，我有三个共享接口的类。像这样;

public  class Platform : IGraphic
{
    public GraphicType Type;        
    public string Name;
    public Texture2D Icon;

    public string GetName()
    {
        return Name;
    }

    public Texture2D GetIcon(GraphicType a_type)
    {
        if (a_type == Type || a_type == GraphicType.CHARACTERS)
            return Icon;
        else
            return null;
    }

}

public class Decal : IGraphic
{
    public GraphicType Type;        
    public string Name;
    public Texture2D Icon;

    public string GetName()
    {
        return Name;
    }

    public Texture2D GetIcon(GraphicType a_type)
    {
        if (a_type == Type)
            return Icon;
        else
            return null;
    }

}

public  class Character : IGraphic
{
    public GraphicType Type;        
    public string Name;
    public Texture2D Icon;

    public string GetName()
    {
        return Name;
    }

    public Texture2D GetIcon(GraphicType a_type)
    {
        if (a_type == Type || a_type == GraphicType.CHARACTERS)
            return Icon;
        else
            return null;
    }

}

public interface IGraphic
{
    string GetName();
    Texture2D GetIcon(GraphicType a_type);
}

当我调用GetName（）时，它工作正常，但当我尝试调用GetIcon（）时，我收到一条错误消息; 错误1无法隐式转换类型＆＃39; bool＆＃39;到＆＃39; Microsoft.Xna.Framework.Graphics.Texture2D＆＃39; 我在这做错了什么？