Why is the WPF application not using the custom TTF font?

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There could be a few reasons why the WPF application is not using the custom TTF font:

1. Wrong Font Family Name: Ensure that the Font Family Name used in the XAML code matches the actual name of the TTF font file.

2. Font Not Embedded: Confirm that the font is embedded within the application. To do this, go to the properties of the TTF file and confirm that the ‘Build Action’ for the file is set to ‘Content’ and ‘Copy to Output Directory’ is set to ‘Copy if newer’.

3. Font Not installed: Confirm that the custom TTF font is installed on the system where the application is running.

4. Global Resource File Not Configured: Ensure that the corresponding <resourcedictionary> tag is included in the App.xaml file, listing the path to the custom font file.

5. Font Not Applied to the Control: Confirm that the custom TTF font is applied to the control correctly. Check if the fontFamily property is set correctly in XAML code.

By checking and correcting the above issues, the WPF application should be able to use the custom TTF font without any issues.