Recently I got a chance to work with Cascading ComboBox using Telerik. We can implemented this functionalities as given below.

<p>Category</p>
<TelerikComboBox Value="@Currentproduct.CategoryId"
                 Data="@Categories"
                 Placeholder="Select Category"
                 TextField="CategoryName" ValueField="CategoryId" Filterable="true"
                 ValueChanged="@( (int? c) => CategorySelected(c) )">
</TelerikComboBox>
<br />
<br />
<p>Product</p>
<TelerikComboBox Value="@Currentproduct.ProductId" Data="@CurrentProducts"
                 Placeholder="Select Product" Filterable="true"
                 TextField="ProductName" ValueField="ProductId"
                 Enabled="@( Currentproduct.CategoryId > 0 )">
</TelerikComboBox>

<p>Selected CategoryId:@Currentproduct.CategoryId </p> 
<p>Selected CategoryName:@Currentproduct.CategoryName</p>


@code {
    // data sources
    List<Category> Categories { get; set; }
    List<Product> AllProducts { get; set; }
    List<Product> CurrentProducts { get; set; }
    List<int> Quantities { get; set; }
    // model
    Product Currentproduct { get; set; } = new Product();

    protected override void OnInitialized()
    {
        base.OnInitialized();

        Categories = Enumerable.Range(1, 6).Select(x => new Category
            {
                CategoryId = x,
                CategoryName = $"Category {x}"
            }).ToList();

        AllProducts = Enumerable.Range(1, 50).Select(x => new Product
            {
                ProductId = x,
                ProductName = $"Product {x}",
                CategoryId = (int)Math.Ceiling((double)x % 7)
            }).ToList();
    }

    //ValueChanged handlers - for cascading comboBox
    void CategorySelected(int? category)
    {
        // the default value - the user selected the default item == deselected the current item
        if (category == null) 
        {
            //reset the "form" / process
            Currentproduct = new Product();
            return;
        }

        // cascade the selection by filtering the data for the next dropdown
        CurrentProducts = AllProducts.Where(p => p.CategoryId == category).ToList();

        // get the selected model from the data source
        Category SelectedCategory = Categories.Where(c => c.CategoryId == category).First();

        // business logic
        Currentproduct.CategoryId = SelectedCategory.CategoryId;
        Currentproduct.CategoryName = SelectedCategory.CategoryName;
    }

    public class Category
    {
        public int CategoryId { get; set; }
        public string CategoryName { get; set; }
    }

    public class Product
    {
        public int CategoryId { get; set; }
        public string CategoryName { get; set; }
        public int ProductId { get; set; }
        public string ProductName { get; set; }
    }
}

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