I am trying to override the ContentTemplate for the Button, but nothing happens.

[homov]

I am trying to override the ContentTemplate for the Button, but nothing happens.

Template definition:

<DataTemplate x:Class="Namespace.StringTemplate"
              xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              x:DataType="x:String">

    <TextBlock FontSize="18"
               Text="{Binding .}" />
</DataTemplate>

Using the template:
<Button ContentTemplate="{Binding t:StringTemplate}" xmlns:t="clr-namespace:Namespace.StringTemplate" />

The StringTemplate property is defined in the ViewModel.

This is a simple example. In reality, the template is more complex - it’s not just a string.

[maxkatz6]

Can you open a bug with repro? It's not a common usecase, but seems like something is broken.

[Vituartzz]

Two things in that snippet would make the button render nothing even if the template itself is fine.

1. The button has no Content. A ContentTemplate is a template for the content. With Content unset, ContentPresenter.CreateChild still builds your template, but with null as the data. So the TextBlock binds {Binding .} to null and shows an empty string, which looks exactly like "nothing happens". (DataTemplate.Match returns true for any data when DataType isn't set, so the template does get applied — just to null.)

<Button Content="Hello" ContentTemplate="{Binding StringTemplate}" />

2. The binding path. {Binding t:StringTemplate} isn't a property path. It should be just the property name on the view model, {Binding StringTemplate}, where StringTemplate returns an IDataTemplate instance. With the t: prefix the binding fails and ContentTemplate stays null; you should see a binding error in the debug output.

A quick way to rule out the view-model side is to put the template in resources and reference it directly:

<Window.Resources>
  <DataTemplate x:Key="StringTemplate" DataType="x:String">
    <TextBlock FontSize="18" Text="{Binding}" />
  </DataTemplate>
</Window.Resources>

<Button Content="Hello" ContentTemplate="{StaticResource StringTemplate}" />

If that renders and the bound version still doesn't (with Content set and the path fixed), then it's worth the repro @maxkatz6 asked for.