RelayCommand commandParameter为空

Question

我的用户控件上有一个按钮

<Button Grid.Row="13" Grid.Column="7" Content="Save" Name="btnSave" 
            CommandParameter="{Binding CompanyDetails}"
            Command="{Binding SaveCommand}" />

SaveCommand是在ViewModel中声明的RelayCommand。CompanyDetails是同一ViewModel中的属性。上面提到的ViewModel是我的UserControl的数据上下文。

    Public Class CompanyViewModel
    Inherits ViewModelBase
    Implements IDataErrorInfo

    Private _myData As DataFromDatabase      
    Private _companyDetails As New CompanyDetailsBase
    Private _saveCommand As RelayCommandWithParameter

    Public Property CompanyDetails() As CompanyDetailsBase
        Get
            Return _companyDetails
        End Get
        Set(ByVal value As CompanyDetailsBase)
            _companyDetails = value
            OnPropertyChanged("CompanyDetails")
        End Set
    End Property

    Public Property SaveCommand() As RelayCommandWithParameter
        Get
            If _saveCommand Is Nothing Then
                Dim saveAction As New Action(Of Object)(AddressOf Me.SaveIt)
                _saveCommand = New RelayCommandWithParameter(saveAction) ', Me.CanSave)
            End If
            Return _saveCommand
        End Get

        Set(ByVal value As RelayCommandWithParameter)
            _saveCommand = value
        End Set
    End Property

    Public Sub New()
        _myData = New DataFromDatabase(My.Settings.ConnectionString)
        _saveCommand = New RelayCommandWithParameter(New Action(Of Object)(AddressOf SaveIt))
    End Sub

    Public Function GetCompanyDetails(ByVal sap As String) As CompanyDetailsBase
        CompanyDetails = _myData.GetCompanyDetails(sap)
        Return CompanyDetails
    End Function

    Public Sub SaveIt(ByVal c As CompanyDetailsBase)
        MessageBox.Show("Save customer " & c.Sap)
    End Sub

    End Class

用户控件上的其他字段绑定到调用GetCompanyDetails函数的ObjectDataProvider。每次调用此函数时，都会将CompanyDetails设置为正确，直到我调用SaveIt子例程。当调用SaveIt时，其参数始终为Nothing。我该在哪里犯错呢？非常感谢你的回答。

Answer 1

在CompanyViewModel的构造器中，用GetCompanyDetails函数填充CompanyDetails。如下所示更改构造函数，

Public Sub New()
    _myData = New DataFromDatabase(My.Settings.ConnectionString)
    _saveCommand = New RelayCommandWithParameter(New Action(Of Object)(AddressOf SaveIt))
    _companyDetails = GetCompanyDetails()  
End Sub

这可能会有帮助。祝好运。

Answer 2

我做VB已经有一段时间了，我从来没有用过WPF，但我想我知道你的问题是什么了。

在初始化命令的方式中，实际上并没有传入参数。

当我在c#中这样做时，它看起来像这样的=>

_saveCommand = new RelayCommandWithParameter(
            (param) =>
            {
                SaveIt((int)param);
            },
               // this line is the same as your commented out Me.CanSave
            (param) => { return this.CanDisplaySelectedPolicy; }); 

您可以看到，在lamda表达式中，我传递了一个名为param的参数，然后将其放入SaveIt方法中。

在创建新操作时，尝试像上面那样在其中放入一个lamda表达式，这样就可以向SaveIt方法传递一个参数。