Example of Header Template in ASP.NET Core Tree Grid Control
This sample demonstrates the TreeGrid header template feature. In this sample, we have shown custom icons in the column headers.
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            The TreeGrid provides a way to define a custom element in header element. Columns->HeaderTemplate property accepts either string or HTML element`s ID value, which will be used as the template for the header cell.
        
In this demo, we are rendering customized template for all column headers.
More information about Header template can be found in this documentation section.