使用Xamarin.Forms和MvvmCross处理ICommand

我对使用MvvmCross和Xamarin.Forms简单处理ICommand有问题

我已经为VisualStudio创建了一个带有MvvmCross插件的项目，我得到了简单的MainView.xaml和MainViewModel.cs

我更改了ViewModel：只添加了一个字符串+一个ICommand

using System.Diagnostics;
using System.Windows.Input;
using Cirrious.MvvmCross.ViewModels;

namespace XamarinFormsMvvm.Core.ViewModels
{

    public class MainViewModel : BaseViewModel
    {

        private string sampleText = "Hello from the Ninja Coder!";

        private string sampleText2 = "Hello2";

        public MainViewModel()
        {

        }

        public string SampleText
        {
            get { return this.sampleText; }
            set { this.SetProperty(ref this.sampleText, value); }
        }

        public string SampleText2
        {
            get { return this.sampleText2; }
            set { this.SetProperty(ref this.sampleText2, value); }
        }

        public ICommand StartStopCommand
        {
            get
            {
                return new MvxCommand(() =>
                {
                    Debug.WriteLine("Test");
                });
            }
        }
    }
}

我的视图添加了标签+按钮

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="XamarinFormsMvvm.Forms.Views.MainView"
             xmlns:viewModels="clr-namespace:XamarinFormsMvvm.Core.ViewModels;assembly=XamarinFormsMvvm.Core"
             Title="MainView Page"
             >
  <ContentPage.BindingContext>
        <viewModels:MainViewModel />
    </ContentPage.BindingContext>
  <ContentView>
    <StackLayout>
        <Label Text='{Binding SampleText}' VerticalOptions='Center' HorizontalOptions='Center'/>
        <Label Text='{Binding SampleText2}' VerticalOptions='Center' HorizontalOptions='Center'/>
        <Button Command="{Binding StartStopCommand}" Text="asd" VerticalOptions="Center" HorizontalOptions="Center"/>

    </StackLayout>
  </ContentView>
</ContentPage>

运行该命令后，我遇到异常：

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object
03-05 18:32:16.898 I/MonoDroid( 2699):   at Cirrious.CrossCore.Mvx.TryResolve[IMvxCommandHelper] (IMvxCommandHelper& service) [0x00000] in <filename unknown>:0 
03-05 18:32:16.898 I/MonoDroid( 2699):   at Cirrious.MvvmCross.ViewModels.MvxCommandBase..ctor () [0x00000] in <filename unknown>:0 
03-05 18:32:16.898 I/MonoDroid( 2699):   at Cirrious.MvvmCross.ViewModels.MvxCommand..ctor (System.Action execute, System.Func`1 canExecute) [0x00000] in <filename unknown>:0 
03-05 18:32:16.898 I/MonoDroid( 2699):   at Cirrious.MvvmCross.ViewModels.MvxCommand..ctor (System.Action execute) [0x00000] in <filename unknown>:0 
03-05 18:32:16.898 I/MonoDroid( 2699):   at XamarinFormsMvvm.Core.ViewModels.MainViewModel.get_StartStopCommand () [0x00001] in c:\Users\Tomasz\Documents\Visual Studio 2013\Projects\XamarinFormsMvvm\XamarinFormsMvvm.Core\ViewModels\MainViewModel.cs:49 
...........

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这段代码有什么问题？

我刚刚用Ninja创建了一个项目，正如您所做的那样，在MainViewModel和MainView上通过了您的代码，一切都很好。 老实说，在忍者创造了解决方案结构之后，我对它做了一些修改

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你可以在这里找到我的NinjaMvxForms空白项目修改和工作

这里发生了什么：MainViewModel.cs:49？您提供的MainViewModel没有49行。。。