Custom attribute on property - Getting type and value of attributed property

问题

I have the following custom attribute, which can be applied on properties:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class IdentifierAttribute : Attribute
{
}

For example:

public class MyClass
{
    [Identifier()]
    public string Name { get; set; }

    public int SomeNumber { get; set; }
    public string SomeOtherProperty { get; set; }
}

There will also be other classes, to which the Identifier attribute could be added to properties of different type:

public class MyOtherClass
{
    public string Name { get; set; }

    [Identifier()]
    public int SomeNumber { get; set; }

    public string SomeOtherProperty { get; set; }
}

I then need to be able to get this information in my consuming class. For example:

public class TestClass<T>
{
    public void GetIDForPassedInObject(T obj)
    {
        var type = obj.GetType();
        //type.GetCustomAttributes(true)???
    }
}

What's the best way of going about this? I need to get the type of the [Identifier()] field (int, string, etc...) and the actual value, obviously based on the type.

回答1:

Something like the following,, this will use only the first property it comes accross that has the attribute, of course you could place it on more than one..

    public object GetIDForPassedInObject(T obj)
    {
        var prop = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance)
                   .FirstOrDefault(p => p.GetCustomAttributes(typeof(IdentifierAttribute), false).Count() ==1);
        object ret = prop !=null ?  prop.GetValue(obj, null) : null;

        return ret;
    }

回答2:

public class TestClass<T>
{
    public void GetIDForPassedInObject(T obj)
    {
        PropertyInfo[] properties =
            obj.GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance);            

        PropertyInfo IdProperty = (from PropertyInfo property in properties
                           where property.GetCustomAttributes(typeof(Identifier), true).Length > 0
                           select property).First();

         if(null == IdProperty)
             throw new ArgumentException("obj does not have Identifier.");

         Object propValue = IdProperty.GetValue(entity, null)
    }
}

回答3:

A bit late but here is something I did for enums (could be any object also) and getting the description attribute value using an extension (this could be a generic for any attribute):

public enum TransactionTypeEnum
{
    [Description("Text here!")]
    DROP = 1,

    [Description("More text here!")]
    PICKUP = 2,

    ...
}

Getting the value:

var code = TransactionTypeEnum.DROP.ToCode();

Extension supporting all my enums:

public static string ToCode(this TransactionTypeEnum val)
{
    return GetCode(val);
}

public static string ToCode(this DockStatusEnum val)
{
    return GetCode(val);
}

public static string ToCode(this TrailerStatusEnum val)
{
    return GetCode(val);
}

public static string ToCode(this DockTrailerStatusEnum val)
{
    return GetCode(val);
}

public static string ToCode(this EncodingType val)
{
    return GetCode(val);
}

private static string GetCode(object val)
{
    var attributes = (DescriptionAttribute[])val.GetType().GetField(val.ToString()).GetCustomAttributes(typeof(DescriptionAttribute), false);

    return attributes.Length > 0 ? attributes[0].Description : string.Empty;
}

来源：https://stackoverflow.com/questions/3289198/custom-attribute-on-property-getting-type-and-value-of-attributed-property

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custom-attributes