C# , do not execute the method when the attribute's method not return true

Question

1.50/5 (2 votes)

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perhaps there is a attribute

C#

[AttributeUsage(AttributeTargets.Method)]
class AuthAttribute : Attribute
{
    public AuthAttribute)
    {
        Do();
    }

    private void Do()
    {
        do some check...
        //What's here ?
        // if the current logon user has permission ,the calling method (ShowA ,ShowB) will execute ,else will not execute
    }
}

and a test class

C#

class TauthTest
{
    [Auth]
    public void ShowA()
    {
        Debug.WriteLine("ShowA");
    }

    [Auth]
    public void ShowB()
    {
        Debug.WriteLine("ShowB");
    }
}

i know [PrincipalPermissionAttribute] can do the same thing,but i want do it myself,and i don't know how .please help.

Posted 31-Jul-15 18:39pm

goldli88

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Sergey Alexandrovich Kryukov 1-Aug-15 1:46am

What are you trying to achieve with that?
—SA

2 solutions

Solution 2

Thanks for answer.
for additional,the sense looks like below

C#

    public class EmailService : AuthObj
    {

        public EmailService()
        {
            var mailServer = new MailServer
            {
                Host = "",
                Port = 25,
                UserName = "",
                Password = ""
            };
            EmailHelper.Instance.Server = mailServer;
        }

        private bool CheckAuth()
        {
            var result = ServiceAuth.IsAvailable();
            if (!result)
            {
                LogHelper.Instance.GetLogger().Info("RtxService=> 认证失败!");
            }
            return result;
        }

        [WebMethod]
        [SoapHeader("ServiceAuth")]
        public void Send(MailUser revicer,string subject,string body)
        {
            if (!CheckAuth()) return;
            EmailHelper.Instance.GetMail().Send(revicer, subject, body);
        }
	......
}

and every class inherited from AuthObj should have on private method CheckAuth(),and every method marked with [WebMethod] has a [SoapHeader] attribute;
what i want is ,when the logon user calls method Send,then the attribute (perhaps Permit,then no need marked [SoapHeader]) will check the user's permission.if the user has no permission,the method Send will not be fired.
i know i should user reflection,but i don't know how.

Posted 31-Jul-15 21:36pm

goldli88

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Sergey Alexandrovich Kryukov · Accepted Answer · 2015-07-31T20:14:00

First, in the line "public AuthAttribute)", brackets are unbalanced.

Unfortunately, the question makes little to no sense; and something tells me that you won't add much sense when you answer my question I asked you in my comment to the question. However, it wakes up my curiosity. So, let me answer to the question as it is, even without your explanation (which are normally important).

Of course you can use such attribute to mark some methods as having some special property. And of course you can have some instance method of an attribute class and call it from its constructor. But what can it do? Pretty much nothing, if it is a parameterless method. Of course, this is so if you don't misuse the attribute, that is if you use it only to apply to some some declaration, from the level of an assembly to the level of a method parameter and don't instantiate such class in your runtime code directly.

You just need to understand what CLI and CLR do to the attribute. If an attribute is applied to some declaration, even if that declaration is never used in code, for each such declaration, the attribute type is statically instantiated. By the way, your code sample would be a great answer to the question which are asked from time to time: "Is it possible to write code fragment which is executed before the entry-point method (Main)?" Of course it is possible; this would be, for example, your method Do! When it happens, nothing in the application is yet called. (By the way, even the System.Console is not initialized, so in my example, writing to the console from Do screws up the whole output!) Your methods ShowA and ShowB are not yet called, moreover, your attribute instance "knows" nothing about them. If you thought that you can control access to these method, it would be quite a naive thought.

This is the reason why non-constructor methods of attribute types are rarely used: there is just not enough work for them. In your example, it is also useless. :-)

But what code can "know" about these methods and attributes applied to them? Of course, this is reflection. That said, you can affect the use of your ShowA and ShowB only if they are called not directly, but through reflection, more exactly, through one of the System.Reflection.MethodInfo.Invoke methods:
https://msdn.microsoft.com/en-us/library/system.reflection.methodinfo.invoke%28v=vs.110%29.aspx[^].

During runtime, you can obtain the instances of System.Reflection.MethodInfo and apply one of the reflection's GetCustomAttribute methods. This way, you can figure out if your attribute is applied to this method and then decide if you want to invoke the method or not, in what order, with what parameters, and so on. The whole chain starts from the type System.Type, in your case, obtained from the type TauthTest or its instance. This chain can be relatively long, but extremely trivial; you only need to look at the MSDN documentation on the reflection types you face down the road.

That's all.

—SA