DialogContext.razor

<CascadingValue Value="@ModalDialogService">
    @ChildContent
    <div class="modal @ModalClasses" tabindex="-1" role="dialog" style="@ModalStyles">
        <div class="modal-dialog" role="document">
            <div class="modal-content">
                <div class="modal-header">
                    <h5 class="modal-title">
                        @Title
                    </h5>
                    <button class="close" type="button" @onclick="HideDialog"></button>
                </div>
                <div class="modal-body">
                    <p>@Message</p>
                </div>
                <div class="modal-footer">
                    <button type="button" class="btn btn-secondary" @onclick="ModalDialogService.Confirm">Yes</button>
                    <button type="button" class="btn btn-secondary" @onclick="ModalDialogService.Cancel">No</button>
                </div>
            </div>
        </div>
    </div>
    <div class="modal-backdrop @ModalClasses" style="@ModalStyles"></div>
</CascadingValue> 

ModalDialogService.cs

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Components;

namespace Promenade.Client.Services
{
    public class ModalDialogService
    {
        private readonly Action<string, string> _showDialogHandler;
        private readonly Action _hideDialogHandler;

        public ModalDialogService(Action<string, string> showDialogHandler, Action hideDialogHandler)
        {
            _showDialogHandler = showDialogHandler;
            _hideDialogHandler = hideDialogHandler;
        }
        public EventCallback Confirmed { get; set; } = EventCallback.Empty;
        public EventCallback Cancelled { get; set; } = EventCallback.Empty;

        public async Task Confirm()
        {
            _hideDialogHandler.Invoke();
            await Confirmed.InvokeAsync(null);
        }

        public async Task Cancel()
        {
            _hideDialogHandler.Invoke();
            await Cancelled.InvokeAsync(null);
        }

        public void Show(string title, string message)
        {
            _showDialogHandler.Invoke(title, message);
        }
    }
}