Custom attributes with constructor

Yes, you read that correctly!

nanoFramework just got support for these. You can now have custom attributes with a constructor and access the value on the constructor.

Let’s see some code to illustrate this.

Consider these two classes defining attributes.

public class AuthorAttribute : Attribute
{
    private readonly string _author;

    public string Author => _author;

    public AuthorAttribute(string author)
    {
        _author = author;
    }
}
public class MaxAttribute : Attribute
{
    private readonly uint _max;

    public uint Max  => _max;

    public MaxAttribute(uint m)
    {
        _max = m;
    }
}

Now a class using the above attributes on a given field.

public class MyClass1
{
    [Attribute1]
    [Attribute3]
    public void MyMethod1(int i)
    {
        return;
    }

    [Ignore("I'm ignoring you!")]
    public void MyMethodToIgnore()
    {
    }

    private readonly int _myField;

    [Ignore("I'm ignoring you!")]
    public int MyField => _myField;

    [Max(0xDEADBEEF)]
    [Author("William Shakespeare")]
    public int MyPackedField;
}

And finally reaching those attributes.

var myClass = new MyClass1();

var myFieldAttributes = myClass.GetType().GetField("MyPackedField").GetCustomAttributes(true);

Console.WriteLine($"\nThe custom attributes of field 'MyPackedField' are:");

MaxAttribute attMax = (MaxAttribute)myFieldAttributes[0];
Console.WriteLine($"MaxAttribute value is: 0x{attMax.Max.ToString("X8")}");

AuthorAttribute attAuthor = (AuthorAttribute)myFieldAttributes[1];
Console.WriteLine($"AuthorAttribute value is: '{attAuthor.Author}'");

The above code will output something like this:

The custom attributes of field 'MyPackedField' are:
MaxAttribute value is: 0xDEADBEEF
AuthorAttribute value is: 'William Shakespeare'

Looks pretty simple and trivial doesn’t it? And can be very useful too!
Get the full project on our samples repo here.

And what are you waiting for? Go write some code and have fun with nanoFramework! 🙂

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