我假设，您需要某种MessageBox，但用于用户输入而不是通知。
有很多关于“如何创建自定义MessageBox 'es”的例子。对于目前的问题，我将使用我自己的想法。
因此，首先，您需要创建一个单独的窗口（视图），其中包含TextBox用于用户输入和OK/Cancel按钮。

UserTextInputWindow.xaml：

<Window x:Class="YourNamespace.Views.UserTextInputWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:viewModels="clr-namespace:YourNamespace.ViewModels"
        mc:Ignorable="d"
        Title="UserTextInputWindow" 
        Name="This"
        WindowStyle="None"
        WindowStartupLocation="CenterScreen"
        Height="200" 
        Width="400">
    <Window.DataContext>
        <viewModels:UserTextInputViewModel/>
    </Window.DataContext>
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="24"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="32"/>
        </Grid.RowDefinitions>
        <TextBlock Text="Please, input some text if you wish" 
                   FontWeight="Bold"
                   HorizontalAlignment="Center"
                   VerticalAlignment="Center"/>
        <TextBox Grid.Row="1" 
                 Margin="5" 
                 Text="{Binding UserText}"/>
        <StackPanel Grid.Row="2" 
                    Orientation="Horizontal" 
                    HorizontalAlignment="Center">
            <Button Content="OK" 
                    Width="100" 
                    Height="24" 
                    Margin="10,0"
                    Command="{Binding OkCommand}" 
                    CommandParameter="{Binding ElementName=This}"/>
            <Button Content="Cancel" 
                    Width="100" 
                    Height="24" 
                    Margin="10,0"
                    Command="{Binding CancelCommand}" 
                    CommandParameter="{Binding ElementName=This}"/>
        </StackPanel>

    </Grid>
</Window>

这一个看起来像下面。Ofc，你可以按照你的意愿定制它。

在XAML中可以看到，我使用UserTextInputViewModel类作为此窗口的ViewModel，其代码隐藏是：

UserTextInputViewModel.cs：

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Input;
using YourNamespace.Commands;

namespace YourNamespace.ViewModels
{
    public class UserTextInputViewModel : INotifyPropertyChanged
    {
        private string _userText;
        // Our binded to TextBox property, which stores user's input
        public string UserText
        {
            get => _userText;
            set
            {
                _userText = value;
                NotifyPropertyChanged();
            }
        }

        // A command for OK button
        // ParameterizedCommand is custom implementation of ICommand interface
        private ICommand _okCommand;
        public ICommand OkCommand => _okCommand ??= new ParameterizedCommand(commandParameter => (commandParameter as Window)?.Close() );

        // A command for Cancel button
        // ParameterizedCommand is custom implementation of ICommand interface
        private ICommand _cancelCommand;
        public ICommand CancelCommand => _cancelCommand ??= new ParameterizedCommand(commandParameter =>
        {
            // On Cancel we should "reset" user input
            UserText = string.Empty;
            (commandParameter as Window)?.Close();
        });

        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged([CallerMemberName] string propertyName = null) =>
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

ParameterizedCommand实现是对一些Action的简单 Package ，并以object作为参数，因此Action可以通过Command属性绑定到XAML中的Button。

parameterizedCommand.cs：

using System;
using System.Windows.Input;

namespace YourNamespace.Commands
{
    public class ParameterizedCommand : ICommand
    {
        private readonly Action<object> _action;

        public ParameterizedCommand(Action<object> action) => _action = action;

        public bool CanExecute(object parameter) => true;
        public void Execute(object parameter) => _action(parameter);

        public event EventHandler CanExecuteChanged
        {
            add => CommandManager.RequerySuggested += value;
            remove => CommandManager.RequerySuggested -= value;
        }
    }
}

因此，事实上，ViewModel只是存储用户输入，并通过关闭窗口（并在取消时重置用户输入）来处理OK/Cancel按钮单击。
最后，UserTextInputWindow本身的代码隐藏，我隐藏了默认构造函数，并将其调用替换为静态方法Show：

UserTextInputWindow.xaml.cs：

using System.Windows;
using YourNamespace.ViewModels;

namespace YourNamespace.Views
{
    public partial class UserTextInputWindow
    {
        // Hiding default constructor by making it private
        private UserTextInputWindow() => InitializeComponent();

        // Introducing static method, which should display window and return
        // user input string.
        // Note, that this is blocking call (because of ShowDialog inside).
        // Also note, that I add some useless parameter "topmost" just to avoid
        // conflicts with standart Window.Show method
        public static string Show(bool topmost = false)
        {
            // Securing ourselves by checking access from non-STA thread
            return Application.Current.Dispatcher.CheckAccess() 
                 ? ShowWindowAndGetUserInput() 
                 : Application.Current.Dispatcher.Invoke(ShowWindowAndGetUserInput);
        }
    
        private static string ShowWindowAndGetUserInput()
        {
            var userText = string.Empty;

            var  window = new UserTextInputWindow();
            window.Closing += delegate
            {
                if (window.DataContext is UserTextInputViewModel vm)
                    userText = vm.UserText;
            };

            window.ShowDialog();

            return userText;
        }
    }
}

我的这个例子的项目结构看起来像这样（与这个例子无关的项目被绘制）：

使用方法与MessageBox非常相似。它也可以安全地从任何线程。

var userInput = UserTextInputWindow.Show();
if (string.IsNullOrEmpty(userInput)) { /* User input nothing or cancel */ }

不确定这是纯粹和正确的MVVM方法，但它的工作。你可以纠正我。

您可以使用SemaphoreSlim异步等待，直到窗口关闭，以不阻塞UI线程：

public async Task OpenWindowAsync()
{
    using (SemaphoreSlim semaphoreSlim = new SemaphoreSlim(0, 1))
    {
        string enteredText = null;

        TextboxAndButtons textBoxWindow = new TextboxAndButtons();
        textBoxWindow.Owner = System.Windows.Application.Current.Windows.Cast<Window>().LastOrDefault(x => x.IsActive) ?? System.Windows.Application.Current.MainWindow;
        textBoxWindow.SetText(setValue);

        textBoxWindow.Closed += (sender, e) =>
        {
            enteredText = textBoxWindow.GetText();
            semaphoreSlim.Release();
        };

        textBoxWindow.Show();

        await semaphoreSlim.WaitAsync();

        if (!string.IsNullOrEmpty(enteredText))
        {
            string a = enteredText;
        }
    }
}