weird The tag '' does not exist in XML namespace 'clr-namespace:'. error

Question

Ok so I've added this code for Binding Color converter inside Application.Resources in App.xaml:

xmlns:converter="clr-namespace:FtpUploader2.Converters"
<converter:MessageTypeEnumToColor x:Key="MessageTypeEnumToColor"/>

I get no error messages and the program compiles just fine and it changes the different colors on my message element until I added the PrintMessage() function at a line in my code.

<Label Content="{Binding Text}" 
Background="{Binding MessageType, Converter={StaticResource MessageTypeEnumToColor}}"/>

Print message to wpf viewlist:

public void PrintMessage(string a_message, Model.Message.MsgType a_msgType = Model.Message.MsgType.Information)
{
                    Model.Message message = new Model.Message(a_message, a_msgType);    
                    Messages.Insert(0, message);                
}

I've used the PrintMessage funtion just fine many times in the application. Now comes the part I can't figure out why it happens.

In a function called HandleUploadTabSelected()

PrintMessage("Failed to upload one or more accounts", Model.Message.MsgType.Error);

causes the error message in title:

Error 1 The tag 'MessageTypeEnumToColor' does not exist in XML namespace 'clr-namespace:FtpUploader2.Converters'. Line 7 Position 10.

EDIT: After testing Mike's suggestion it doesn't compile at all. Adding ;assembly=FtpUploader2

Here's the code for MessageTypeEnumToColor:

using FtpUploader2.Model;
namespace FtpUploader2.Converters
{
    class MessageTypeEnumToColor : IValueConverter
    {
        #region IValueConverter Member

        object IValueConverter.Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            Message.MsgType tempMsg = (Message.MsgType)value;
            Brush retval;

            switch (tempMsg)
            {
                case Message.MsgType.Ok:
                    retval = Brushes.Green;
                    break;

                case Message.MsgType.Warning:
                    retval = Brushes.Yellow;
                    break;

                case Message.MsgType.Error:
                    retval = Brushes.Red;
                    break;              
                default:
                    retval = Brushes.White;
                    break;
            }

            return retval;
        }

        object IValueConverter.ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }

        #endregion
    }
}

Answer 1

确保应用程序的目标框架 （属性>应用程序>目标框架）与目标程序集的框架相同（例如：.NET Framework 4.7.2）