<TelerikComboBox Data="@myDdlData" 
                 TextField="MyTextField" 
                 ValueField="MyValueField" 
                 Value="selectedValue" 
                 ValueChanged="@((int? id) => ValueChangedHandler(id))" 
                 FilterOperator="StringFilterOperator.Contains"
                 Filterable="true">
</TelerikComboBox>

@code {
    private List<MyDdlModel> myDdlData { get; set; }

    private void ValueChangedHandler(int? id)
    {
        selectedValue = id;
    }

    public class MyDdlModel
    {
        public int? MyValueField { get; set; }
        public string MyTextField { get; set; }
    }

    int? selectedValue { get; set; }

    protected override void OnInitialized()
    {
        myDdlData = new List<MyDdlModel>()
        {
            new MyDdlModel()
            {
                MyValueField = 1,
                MyTextField = "John Smith"
            },
            new MyDdlModel()
            {
                MyValueField = 2,
                MyTextField = "John Smith"
            },
            new MyDdlModel()
            {
                MyValueField = 3,
                MyTextField = "John Smith"
            },
            new MyDdlModel()
            {
                MyValueField = 4,
                MyTextField = "Alice Jones"
            },
            new MyDdlModel()
            {
                MyValueField = 5,
                MyTextField = "Alice Jones"
            },
        };
    }
}

<div style="width: 500px; height: 300px; border: 1px solid red;">
 
    <TelerikSplitter @ref="ts" Width="100%" Height="100%" Orientation="@SplitterOrientation.Horizontal">
        <SplitterPanes>
 
            <SplitterPane Size="20%" Collapsible="true">
                <div>left</div>
            </SplitterPane>
 
            @if (bunique)
            {
            <SplitterPane Size="10%" Collapsible="true">
                <div>middle</div>
            </SplitterPane>
            }
 
            <SplitterPane>
                <div>right</div>
            </SplitterPane>
 
        </SplitterPanes>
    </TelerikSplitter>
 
</div>
 
@code{
    bool bunique = false;
    Telerik.Blazor.Components.TelerikSplitter ts;
 
    protected override void OnAfterRender(bool firstRender)
    {
        if (firstRender)
        {
            if (!bunique)
            {
                bunique = true;
                StateHasChanged();
            }
        }
    }
}

  public enum DeliveryMailOptions
        {
            Regular,
            [Display(Name ="FedEx Priority")]
            FedExPriority,
            [Display(Name ="FedEx Two-Day")]
            FedExTwoDay
        }

[Display(Name = "Mail Option")]
public DeliveryMailOptions MailOption { get; set; }

Selected value: @selectedValue
<br />

<TelerikComboBox Data="@myComboData"
                 TextField="MyTextField"
                 ValueField="MyValueField"
                 Value="selectedValue"
                 ValueChanged="@((int value) => ValueChangedHandler(value))"
                 Placeholder="Select an item..."
                 ClearButton="true"
                 Enabled="@isEnabled"
                 Filterable="true">
</TelerikComboBox>

@code {
    public bool isEnabled { get; set; } = true;
    IEnumerable<MyDdlModel> myComboData = Enumerable.Range(1, 20).Select(x => new MyDdlModel { MyTextField = "item " + x, MyValueField = x });

    int selectedValue { get; set; } = 3; //usually the current value should come from the model data

    public async Task ValueChangedHandler(int value)
    {
        isEnabled = false;
        await Task.Delay(4000); //simulate network delay

        selectedValue = value;

        isEnabled = true;
    }

    //in a real case, the model is usually in a separate file
    //the model type and value field type must be provided to the dropdpownlist
    public class MyDdlModel
    {
        public int MyValueField { get; set; }
        public string MyTextField { get; set; }
    }
}

@inject IJSRuntime _js

@*You should move this to a proper place for scripts,
 suppressing the error like this is just a hack to keep this sample code concise*@
<script suppress-error="BL9992">
    function uncheckGrid(parentElem) {
        var checkboxes = parentElem.querySelectorAll(".k-checkbox.k-grid-checkbox");
        for (var i = 0; i < checkboxes.length; i++) {
            checkboxes[i].checked = false;
        }
    }
</script>

<div @ref="@ParentElem">
    <TelerikGrid Data=@GridData
                 SelectionMode="@GridSelectionMode.Multiple"
                 SelectedItemsChanged="@((IEnumerable<Employee> employeeList) => OnSelect(employeeList))"
                 SelectedItems="@SelectedEmployees"
                 Pageable="true"
                 Height="400px">
        <GridColumns>
            <GridCheckboxColumn />
            <GridColumn Field=@nameof(Employee.Name) />
            <GridColumn Field=@nameof(Employee.Team) Title="Team" />
        </GridColumns>
    </TelerikGrid>
</div>
@if (SelectedEmployees != null)
{
    <ul>
        @foreach (Employee employee in SelectedEmployees)
        {
            <li>
                @employee.Name
            </li>
        }
    </ul>
}

@code {
    ElementReference ParentElem { get; set; }


    public List<Employee> GridData { get; set; }
    public IEnumerable<Employee> SelectedEmployees { get; set; } = Enumerable.Empty<Employee>();

    protected override void OnInitialized()
    {
        GridData = new List<Employee>();
        for (int i = 0; i < 15; i++)
        {
            GridData.Add(new Employee()
            {
                EmployeeId = i,
                Name = "Employee " + i.ToString(),
                Team = "Team " + i % 3
            });
        }
    }

    protected void OnSelect(IEnumerable<Employee> employees)
    {
        bool flag = true;
        foreach (Employee item in employees)
        {
            if(item.EmployeeId % 3 == 0)
            {
                flag = false;
            }
        }
        if (flag)
        {
            SelectedEmployees = employees;
        }
        else
        {
            SelectedEmployees = Enumerable.Empty<Employee>();
            _js.InvokeVoidAsync("uncheckGrid", ParentElem);
        }
    }

    public class Employee
    {
        public int EmployeeId { get; set; }
        public string Name { get; set; }
        public string Team { get; set; }
    }
}