c# - 获取从抽象类继承并在属性中具有特定值的类的实例

Question

我正在一家工厂工作，根据两个标准创建具体实例:

1)类必须继承自具体的抽象类

2) 类必须在覆盖的属性中有一个特定的值

我的代码是这样的:

public abstract class CommandBase
{
    public abstract string Prefix { get; }
}

public class PaintCommand : CommandBase
{
    public override string Prefix { get; } = "P";
}

public class WalkCommand : CommandBase
{
    public override string Prefix { get; } = "W";
}

class Program
{
    static void Main(string[] args)
    {
        var paintCommand = GetInstance("P");
        var walkCommand = GetInstance("W");  

        Console.ReadKey();
    }

    static CommandBase GetInstance(string prefix)
    {
        try
        {            
            var currentAssembly = Assembly.GetExecutingAssembly();
            var concreteType = currentAssembly.GetTypes().Where(t => t.IsSubclassOf(typeof(CommandBase)) &&
                                                                     !t.IsAbstract &&
                                                                     t.GetProperty("Prefix").GetValue(t).ToString() == prefix).SingleOrDefault();

            if (concreteType == null)
                throw new InvalidCastException($"No concrete type found for command: {prefix}");

            return (CommandBase)Activator.CreateInstance(concreteType);
        }
        catch (Exception ex)
        {            
            return default(CommandBase);
        }
    }
}

我收到错误:

{System.Reflection.TargetException: Object does not match target type. at System.Reflection.RuntimeMethodInfo.CheckConsistency(Object target) at System.Reflection.RuntimeMethodInfo.InvokeArgumentsCheck(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)

Answer 1

更简洁的方法是定义您自己的属性来存储前缀。

[AttributeUsage(AttributeTargets.Class)]
public class CommandAttribute : Attribute
{
    public String Prefix { get; set; }

    public CommandAttribute(string commandPrefix)
    {
        Prefix = commandPrefix;
    }
}

然后像这样使用它们:

[CommandAttribute("P")]
public class PaintCommand : CommandBase
{}

[CommandAttribute("W")]
public class WalkCommand : CommandBase
{}

反射(reflection):

static CommandBase GetInstance(string prefix)
{       
    var currentAssembly = Assembly.GetExecutingAssembly();
    var concreteType = currentAssembly.GetTypes().Where(commandClass => commandClass.IsDefined(typeof(CommandAttribute), false) && commandClass.GetCustomAttribute<CommandAttribute>().Prefix == prefix).FirstOrDefault();

    if (concreteType == null)
        throw new InvalidCastException($"No concrete type found for command: {prefix}");

    return (CommandBase)Activator.CreateInstance(concreteType);
}