C# basics (ATTRIBUTES )

Question

HELLO!!! I have custom attribute [AttributeUsage(AttributeTargets.Property)] public class TestAttribute : Attribute { public string S; public TestAttribute() { } } public class A { [Test] public string str { get; set; } } How can I get value of properti in my attribute where i use my attribute? For example i use my attribute for str properti how can i get str value in my TestAttribute Class?

Mario Scheeren · Answer

Well, you will have to do alot of reflection to get all this done. PropertyInfo[] props = typeof (Book).GetProperties(); foreach (PropertyInfo prop in props) { object [] attrs = prop.GetCustomAttributes( true ); foreach ( object attr in attrs) { AuthorAttribute authAttr = attr as AuthorAttribute; if (authAttr != null ) { string propName = prop.Name; string auth = authAttr.Name; // here you will have to deal with alle the instancea ... } } } But there is no way for an attribute to get the value of an instance.Its all done outside using Reflection - a real overhead.

Karlen Sahakyan · Answer

In this case how work [Requried] ? if this attribute not have any information about an instance it can not add any validation for property for example, i have [StringLength(50, MinimumLength=3)] public string Name { get; set; } how StringLengthAttribute class have information about length of Name Property?

Mario Scheeren · Answer

Well, I guess you misunderstood the usage of attributes. Although they target a class or property, they do not have any information about an instance. So basically you need to look at them from another perspective. Using Reflection you can determine if a class has an attribute set, and also get the values of that attribute, but they are the same for all instances because they are descriptive to the object. Hope this clears it up a little ;)