ReactiveCommand.Create抛出“NotSupportedException”：“索引表达式仅支持常量。”

Question

以下行抛出一个运行时异常：

Accept = ReactiveCommand.Create(this.WhenAnyValue(x => x.Canexecute())); 

下面的代码：

public class InstructionsViewModel : ReactiveObject 
{ 
    public InstructionsViewModel() 
    { 
     Accept = ReactiveCommand.Create(this.WhenAnyValue(x => x.CanExecute)); 
     Accept.Subscribe(x => 
      { 
      Debug.Write("Hello World"); 
      }); 
    } 

     public ReactiveCommand<object> Accept { get; } 

    bool _canExecute; 
    public bool CanExecute { get { return _canExecute; } set { this.RaiseAndSetIfChanged(ref _canExecute, value); } } 
} 

错误：

Cannot convert lambda expression to type 'IObserver' because it is not a delegate type

我也试过如下：

public InstructionsViewModel() 
    { 
     Accept = ReactiveCommand.Create(this.WhenAnyValue(x => x.Canexecute())); 
     Accept.Subscribe(x => 
     { 
      Debug.Write("Hello World"); 
     }); 
    } 

    public ReactiveCommand<object> Accept { get; } 


    public bool Canexecute() => true; 

我收到以下错误：

An exception of type 'System.NotSupportedException' occurred in ReactiveUI.dll but was not handled in user code

Additional information: Index expressions are only supported with constants.

这甚至支持在Windows Phone上10？

Answer 1

我想你的问题不是ReactiveCommand，而是WhenAnyValue。

WhenAnyValue接受一个属性，而你用一个方法提供它，这会导致运行时异常(see the sourcecode)。

检查，如果这个工程（我改变CanExecute是一个属性，而不是一种方法）：

public InstructionsViewModel() 
{ 
    Accept = ReactiveCommand.Create(this.WhenAnyValue(x => x.CanExecute)); 
    Accept.Subscribe(x => 
    { 
     Debug.Write("Hello World"); 
    }); 
} 

public ReactiveCommand<object> Accept { get; } 

private bool _canExecute; 
public bool CanExecute { get { return _canExecute; } set { this.RaiseAndSetIfChanged(ref _canExecute, value); } } 

此外，作为一般的建议 - 不要窝您的来电，这使得调试困难。你应该将创建命令分成两行：

var canExecute = this.WhenAnyValue(x => x.CanExecute) 
Accept = ReactiveCommand.Create(canExecute);