ModernWpfPlayground/PixelsToGridLengthConverter.cs

using System;
using System.Globalization;
using System.Windows;
using System.Windows.Data;

namespace ModernWpfPlayground
{
    public class PixelsToGridLengthConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return value is double d ? new GridLength(d) : new GridLength(1.0, GridUnitType.Auto);
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}