Question

I don’t really understand attributes. I’ve read all kinds of books & posts on them, but I just don’t get it.

Since I don’t understand them, I also don’t understand how to effectively use them.

1) Can you give me a good definition of what an attribute is & what it’s used for?

2) Can you give me a good code example in C# of how to make and consume a custom attribute?

Answer 1

Say you’ve got a class with a series of properties that you are going to walk through using reflection. Any that are strings may need to be validated to check they are not longer than a certain amount.

You could then create a TextLength attribute, with a default integer constructor and integer property/field. You could then read your attribute on each string property in your class and compare the length of the property value to the number specified in the attribute.

Code:

public class TextLengthAttribute : Attribute
{
    private int length;
    public int Length { get { return length; } set { length = value; } }

    public TextLengthAttribute(int num) { this.length = num; }
}

public class MyClass
{
    [TextLength(10)]
    public string Property1;
    [TextLength(20)]
    public string Property2;
}

public class ClassReader
{
     public static void Main()
     {
          MyClass example = MyClass.GetTestData();

          PropertyInfo[] props = typeof(MyClass).GetProperties();
          foreach (PropertyInfo prop in props)
          {
               if (prop.ValueType == typeof(String) 
               {
                    TextLengthAttribute[] atts = 
                      (TextLengthAttribute)[]prop.GetCustomAttributes(
                           typeof(TextLengthAttribute), false);
                    if (prop.GetValue(example, null).ToString().Length > 
                         atts[0].Length) 
                        throw new Exception(prop.name + " was too long");
               }
          }
     }
}

Note: untested