public class TreeListChildrenLoadingOnDemandEventArgs :
    TreeListChildrenLoadingEventArgs

@inject FileSystemDataProvider FileSystemDataProvider

<DxTreeList Data="TreeListData"
            HasChildrenFieldName="HasChildren"
            ChildrenLoadingOnDemand="TreeList_ChildrenLoadingOnDemand"
            CustomizeCellDisplayText="TreeList_CustomizeCellDisplayText">
    <Columns>
        <DxTreeListDataColumn FieldName="Name" />
        <DxTreeListDataColumn FieldName="Type" Width="100" />
        <DxTreeListDataColumn FieldName="DateModified" Width="120" />
        <DxTreeListDataColumn FieldName="Size" Width="100" />
    </Columns>
</DxTreeList>

@code {
    object TreeListData { get; set; }

    protected override async Task OnInitializedAsync() {
        TreeListData = await FileSystemDataProvider.GetRootItemsAsync();
    }

    Task TreeList_ChildrenLoadingOnDemand(TreeListChildrenLoadingOnDemandEventArgs e) {
        var item = e.Parent as FileSystemDataItem;
        e.Children = item.Children;
        return Task.CompletedTask;
    }

    void TreeList_CustomizeCellDisplayText(TreeListCustomizeCellDisplayTextEventArgs e) {
        if(e.FieldName == "Size") {
            var item = (FileSystemDataItem)e.DataItem;
            e.DisplayText = GetSizeColumnDisplayText(item.Type, item.Size);
        }
    }

    string GetSizeColumnDisplayText(FileType fileType, long size) {
        if(fileType == FileType.Folder)
            return null;
        if(size >= 1024)
            return string.Format("{0:### ### ###} KB", size / 1024);
        return string.Format("{0} Bytes", size);
    }
}

using System.Collections.Generic;

public class FileSystemDataItem {
    public string Name { get; set; }
    public FileType Type { get; set; }
    public DateTime DateCreated { get; set; }
    public DateTime DateModified { get; set; }
    public long Size { get; set; }

    public List<FileSystemDataItem> Children { get; set; }
    public bool HasChildren => Children != null && Children.Count > 0;
}

public enum FileType { File, Folder }

public class FileSystemDataProvider {
    List<FileSystemDataItem> _rootItems;

    public FileSystemDataProvider(DataSourcesFileContentProvider fileContentProvider) {
        FileContentProvider = fileContentProvider;
    }

    DataSourcesFileContentProvider FileContentProvider { get; }

    public async Task<List<FileSystemDataItem>> GetRootItemsAsync() {
        if(_rootItems == null) {
            var json = await FileContentProvider.GetFileSystemDataItemsContentAsync();
            _rootItems = JsonSerializer.Deserialize<List<FileSystemDataItem>>(json);
        }
        return _rootItems;
    }
}

public class DataSourcesFileContentProvider{
    string _fileSystemDataItemsContent;

    public async Task<string> GetFileSystemDataItemsContentAsync() {
        if(_fileSystemDataItemsContent == null) {
            string pathToDataFile = Path.Combine(AppContext.BaseDirectory, "DataSources", "FileSystemDataItems.json");
            _fileSystemDataItemsContent = await File.ReadAllTextAsync(pathToDataFile);
        }
        return _fileSystemDataItemsContent;
    }
}

// ...
builder.Services.AddSingleton<FileSystemDataProvider>();
builder.Services.AddSingleton<DataSourcesFileContentProvider>();