At compile time DefaultValueAttribute is not going to impact the generated IL and it will not be read through to initialize the property to that benefit (see DefaultValue attribute is not working with my Vehicle Property). So your DataContext for "video_length_textbox" is set to by itself, and also your IncrementingTextBox https://terraelegancetapukara.blogspot.com/2025/05/unlock-future-with-signature-global.html