Default value at design time XAML

Update: Visual Studio 2019 v16.7

You can now do:

<TextBlock Text="{Binding MyText}" d:Text="Design time value"/>

If you would prefer a less verbose version of Ian Bamforth's answer, you can just do

<TextBlock Text="{Binding MyText, FallbackValue=None}"/>

Adapting an example from this question.

This works for me - the text "None" is shown in the designer:

 <TextBlock>    <TextBlock.Text>        <Binding ElementName="root" Path="blah" FallbackValue="None" />    </TextBlock.Text></TextBlock>

Hope that helps

Using FallbackValue is wrong, because it also affects the runtime behavior (the fallback value is used if the binding fails to obtain a value from the source).

I came up with a custom markup extension that mimics Binding (ideally I would have preferred to inherit from Binding, but the ProvideValue method is not virtual...):

using System;using System.ComponentModel;using System.Windows;using System.Windows.Data;using System.Windows.Markup;namespace MyNamespace{    public class BindingEx : MarkupExtension    {        private readonly Binding _binding;        public BindingEx()        {            _binding = new Binding();        }        public BindingEx(string path)        {            _binding = new Binding(path);        }        public PropertyPath Path        {            get => _binding.Path;            set => _binding.Path = value;        }        public BindingMode Mode        {            get => _binding.Mode;            set => _binding.Mode = value;        }        public RelativeSource RelativeSource        {            get => _binding.RelativeSource;            set => _binding.RelativeSource = value;        }        public string ElementName        {            get => _binding.ElementName;            set => _binding.ElementName = value;        }        public IValueConverter Converter        {            get => _binding.Converter;            set => _binding.Converter = value;        }        public object DesignValue { get; set; }        public override object ProvideValue(IServiceProvider serviceProvider)        {            var target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));            if (target.TargetObject is DependencyObject d && DesignerProperties.GetIsInDesignMode(d))                return DesignValue;            return _binding.ProvideValue(serviceProvider);        }    }}

You can use it just like Binding, with the addition of the DesignValue property:

<TextBlock Text="{my:BindingEx Name, DesignValue=John Doe}" />

Note that BindingEx doesn't have all the properties from Binding, but you can easily add them if necessary.