如何从多个属性的自定义属性中获取特定数据

Question

我是属性新手，所以我试图实现的目标可能是不可能的。据我了解，属性是用于在编译时将数据绑定到特定的方法、属性、类等。

我的属性很简单，定义如下：

public class NowThatsAnAttribute : Attribute
{
    public string HeresMyString { get; set; }
}

我有两个属性使用相同的自定义属性，如下所示：

public class MyClass
{
    [NowThatsAnAttribute(HeresMyString = "A" )]
    public string ThingyA
    {
        get;
        set;
    }

    [NowThatsAnAttribute(HeresMyString = "B" )]
    public string ThingyB
    {
        get;
        set;
    }
}

这一切都很好，很花哨，但我正在尝试从另一种方法访问这些属性的属性，并根据属性的值设置某些数据。

这是我现在正在做的事情：

private void MyMethod()
{
    string something = "";

    var props = typeof (MyClass).GetProperties().Where(prop => Attribute.IsDefined(prop, typeof(NowThatsAnAttribute)));

    //this is where things get messy
    //here's sudo code of what I want to do
    //I KNOW THIS IS NOT VALID SYNTAX
    foreach(prop in props)
    {
        //my first idea was this
        if(prop.attribute(NowThatsAnAttribute).HeresMyString == "A")
        {
            something = "A";
        } 
        else if(prop.attribute(NowThatsAnAttribute).HeresMyString == "B")
        {
            something = "B";
        }

        //my second idea was this
        if(prop.Name == "ThingyA")
        {
            something = "A";
        } 
        else if(prop.Name == "ThingyB")
        {
            something = "B";
        }
    }

    //do stuff with something
}

问题是something 总是被设置为“B”，我知道这是由于循环属性。我只是不确定如何访问与特定属性的属性关联的值。我有一种感觉，有一种我没有看到的明显的痛苦方式。

我尝试过使用StackFrame，但在我现在所在的位置附近结束。

注意：使用 .Net 4.0。

注意2：MyMethod 是我无法编辑的界面的一部分，因此我无法传递任何参数或任何东西。

感谢任何帮助。

Answer 1

如果您传递特定属性的名称，您将能够获取正确的属性值：

private string MyMethod(string propName)
{
    PropertyInfo pi = typeof (MyClass).GetProperty(propName);
    if (pi == null)
        return null;
    var a = (NowThatsAnAttribute)pi.GetCustomAttribute(typeof(NowThatsAnAttribute));
    if (a!=null)
        return a.HeresMyString;
    return null;
}

MyMethod("ThingyA") 返回A

Answer 2

反射还为您提供属性的名称。

Type t = typeof(MyClass);
PropertyInfo[] props = t.GetProperties();
foreach (PropertyInfo p in props) {
    NowThatsAnAttribute attr = p.GetCustomAttributes(false)
        .OfType<NowThatsAnAttribute>()
        .FirstOrDefault();
    if (attr != null) {
        string propName = p.Name;
        string attrValue = attr.HeresMyString;
        switch (propName) {
            case "ThingyA":
                //TODO: Do something for ThingyA
                break;
            case "ThingyB":
                //TODO: Do something for ThingyB
                break;
            default:
                break;
        }
    }
}

您还可以将信息添加到字典中：

var dict = new Dictionary<string, string();
...
if (attr != null) {
    string propName = p.Name;
    string attrValue = attr.HeresMyString;
    dict.Add(propName, attrValue);
}
...

获取一些道具的价值

string value = dict["ThingyA"];

Answer 3

你可以做一个这样的通用方法：

    static object GetAttributeValue<T, A>(string attribName, string propName = "") where A : Attribute
    {
        object[] attribs = null;

        if (string.IsNullOrEmpty(propName))
            attribs = typeof(T).GetCustomAttributes(true);

        else
        {
            PropertyInfo pi = typeof(T).GetProperty(propName);
            if (pi == null)
                return null;
            attribs = pi.GetCustomAttributes(true);
        }

        A a = null;
        foreach (object attrib in attribs)
        {
            if (attrib is A)
            {
                a = attrib as A;
                break;
            }
        }

        if (a != null)
        {
            var prop = a.GetType().GetProperty(attribName);
            return prop.GetValue(a, null);
        }
        return null;
    }

然后你可以这样称呼它：

object value = GetPropertyAttributeValue<MyClass,NowThatsAnAttribute>("HeresMyString", "ThingyA");

所以你的“MyMethod”可能看起来像这样：

private void MyMethod()
{
    string something = "";

    var props = typeof (MyClass).GetProperties().Where(prop => Attribute.IsDefined(prop, typeof(NowThatsAnAttribute)));

    //this is where things get messy
    //here's sudo code of what I want to do
    //I KNOW THIS IS NOT VALID SYNTAX
    foreach(prop in props)
    {
        string attribVal = object value = GetPropertyAttributeValue<MyClass,NowThatsAnAttribute>("HeresMyString", prop.Name);
        //my first idea was this
        if(attribVal == "A")
        {
            something = "A";
        } 
        else if(attribVal == "B")
        {
            something = "B";
        }

    }

    //do stuff with something
}

这将允许您重用相同的方法来获取不同类的不同属性（或通过跳过第二个参数直接在类上的属性）上的不同属性的属性值。