How to define a Color from a string constant

Question

I have a static class with statically defined HEX values of colors:

namespace Namespace1
{
    public static class MyColors
    {
        public const string COLOR_1 = "#AABBCC";
        // ...
    }
}

I want to have all these colors defined in a resource dictionary so that they can be used in XAML:

<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="Namespace2.Style.Colors"
    xmlns:n1="clr-namespace:Namespace1;assembly=Assembly1">
    <Color x:Key="Color1" x:FactoryMethod="FromHex">
        <x:Arguments>
            <x:String>{x:Static n1:MyColors.COLOR_1}</x:String>
        </x:Arguments>
    </Color>
    <!-- ... -->
</ResourceDictionary>

But this doesn't seem to work. There is no compile error, but the color itself is empty (transparent).

If I replace the {x:Static n1:MyColors.COLOR_1} part with the actual #AABBCC value, then it works.

I have also tested the {x:Static n1:MyColors.COLOR_1} part (with Label and Text property) and it works.

So what is the problem here? This seems like an internal Xamarin.Forms bug.

why do you need to define the color on the class. I would define it in the xaml... — Marco, Feb 03 '18 at 19:20
Because the color constants are defined in a common .NETStandard 2.0 assembly and is referenced by WPF applications too, not only Xamarin.Forms. — GregorMohorko, Feb 03 '18 at 19:21
Can you define both? The value as string and the value as Color? Something like this: https://stackoverflow.com/questions/10062376/creating-solidcolorbrush-from-hex-color-value — Marco, Feb 03 '18 at 19:38
I think it's not possible to do the way you want. It has been asked before. https://forums.xamarin.com/discussion/65684/setting-color-in-xaml-from-hex-string-declared-in-code — Marco, Feb 03 '18 at 19:39
Wow, I haven't found this discussion on the forum. So what is the alternative? Can I define the actual color in code and then statically reference it in XAML? — GregorMohorko, Feb 03 '18 at 19:41
You should create a custom type converter, more info here: https://msdn.microsoft.com/es-es/library/ee126043(v=vs.110).aspx — Gusman, Feb 03 '18 at 19:54
Here is an older answer: https://stackoverflow.com/a/12155892/3346583 But it strongly depends on the specific color structure/class you need. A simple Converter (string to XAML Color) should be usefull. — Christopher, Feb 03 '18 at 20:13
@Gusman And how do I then use that converter to define a color? I know how to create a type converter, I just don't know how to use it in this case. — GregorMohorko, Feb 03 '18 at 20:35

GregorMohorko · Accepted Answer · 2018-02-05T15:49:29.820

2

The problem itself was not solved, but the solution for what I was trying to achieve was found:

So, instead of declaring the colors in the Colors.xaml file, it is possible to declare them in the Colors.xaml.cs file instead, using the Add method:

namespace Namespace2.Style
{
    public partial class Colors : ResourceDictionary
    {
        public Colors()
        {
            InitializeComponent();

            Add("Color1", Color.FromHex(MyColors.COLOR_1));
            // ...
        }
    }
}

edited Feb 05 '18 at 15:49

answered Feb 03 '18 at 20:51

GregorMohorko

2,739
2
22
33

well, why not. Still, the type converter could be a nice way to get it done – Marco Feb 03 '18 at 20:53
Can you show me how to use the converter? If you do, I will accept your answer, because I would definitely prefer a XAML solution. – GregorMohorko Feb 03 '18 at 21:02

score 1 · Answer 2 · answered Feb 03 '18 at 21:47

1

Well, forget about the type converter. I got it working like this:

<SolidColorBrush x:Key="Color1" Color="{Binding Source={x:Static n1:MyColors.COLOR_1}}" />

Is this an alternative for you?

answered Feb 03 '18 at 21:47

Marco

984
10
18

This is how I did it in WPF, but this question is about Xamarin.Forms, where SolidColorBrush doesn't exist. – GregorMohorko Feb 03 '18 at 21:49
Then, i will stick to you solution – Marco Feb 03 '18 at 21:51

score 1 · Answer 3 · answered Feb 09 '18 at 22:44

Classes in an assembly named MyAssembly:

namespace MyNamespace
{
  public static class MyColors
  {
    public const string Blue = "#AABBCC";
  }

  public static class MyColor
  {
    public static Color MyBlue { get { return Color.FromHex(MyColors.Blue); } }
  }
}

XAML:

xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:c="clr-namespace:MyNamespace;assembly=MyAssembly"
...
<Label Text="MyColor.MyBlue example" TextColor="{x:Static c:MyColor.MyBlue}"/>

Please note that the c namespace might be named e.g. myColor to better identify it. The notation in the Label would then be myColor:MyColor.MyBlue.

This works, but I prefer my solution, where I can simply then use the colors as `StaticResource` without having to import a namespace etc.. — GregorMohorko, Feb 09 '18 at 22:55

score 0 · Answer 4 · answered Feb 03 '18 at 21:02

0

If you have the time to test the solution with a type converter:

namespace Namespace1
{
    using System.ComponentModel;
    using System.Windows.Media;

    public static class MyColors
    {
        [TypeConverter(typeof(ColorConverter))]
        public static string COLOR_1 { get; } = "#AABBCC";
    }
}

answered Feb 03 '18 at 21:02

Marco

984
10
18

And how do I now use this to define the color in XAML? Doing only this does nothing. – GregorMohorko Feb 03 '18 at 21:16
Well, it should have work automatically with the original code. So I will need to try it myself and check why the converter is not called in this case. I will get back to you if i get it working. I'm curious now ;-) – Marco Feb 03 '18 at 21:19
It expects a string, not a Color. The converter is used when you are using this string field and the color is expected. But like I said, in this case, a string is expected. – GregorMohorko Feb 03 '18 at 21:25

How to define a Color from a string constant

4 Answers4