Unable to cast object of type 'System.Windows.Data.Binding' to type 'System.String

Question

Hello I am receiving this error

The invocation of the constructor on type 'System.Windows.Data.Binding' that matches the specified binding constraints threw an exception. {"Unable to cast object of type 'System.Windows.Data.Binding' to type 'System.String'."}

I have a class below for radio buttons

public class GroupedList
{
    public string Group { get; set; }

    public bool IsSelected { get; set; }        
}

Then In my view model I have a observable collection of this class

/// <summary>
/// The <see cref="GroupList" /> property's name.
/// </summary>
public const string GroupListPropertyName = "GroupList";

private ObservableCollection<GroupedList> _groupList = new ObservableCollection<GroupedList>();

/// <summary>
/// Sets and gets the GroupList property.
/// Changes to that property's value raise the PropertyChanged event. 
/// </summary>
public ObservableCollection<GroupedList> GroupList
{
    get { return _groupList; }
    set { Set(GroupListPropertyName, ref _groupList, value); }
}

In my view I have a combo box with a radio button as a template. The ComboBox ItemSource is set to the Collection. The Content and IsCheckedProperties are set to the two properties in the class.

<ComboBox Name="groupCombo" ItemsSource="{Binding GroupList}">
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">               
              <RadioButton x:Name="GroupButton" 
                          GroupName="Options1" 
                          Content="{Binding Group}"  
                          IsChecked="{Binding IsSelected}">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="Checked">
                        <command:EventToCommand Command="{Binding 
                        {Binding RelativeSource={RelativeSource FindAncestor, 
                                                                           AncestorType={x:Type UserControl}}, Path=DataContext.GroupChecked}}"
                        CommandParameter="{Binding Content,ElementName=GroupButton}" />
                    </i:EventTrigger>
                </i:Interaction.Triggers>
            </RadioButton>                                
        </StackPanel>
    </DataTemplate>
</ComboBox.ItemTemplate>

I added the event trigger to try and access the content property on the checked event in the below relay command in the view model.

private RelayCommand<string> _groupChecked;

/// <summary>
/// Gets the GroupChecked.
/// </summary>
public RelayCommand<string> GroupChecked
{
    get
    {
        return _groupChecked
            ?? (_groupChecked = new RelayCommand<string>(
            x =>
            {
                if (!string.IsNullOrWhiteSpace(x))
                {
                    //DoStuff
                }
            }));
    }
}

After adding the command the error is happening and I think, but not sure that it is throwing becuase of this line CommandParameter="{Binding Content,ElementName=GroupButton}" /> I was wondering if I can accomplish this with the relay command as I am trying. Thanks

Answer 1

Your relay command is a generic of type <string> , hence it is trying to cast the binding from command parameter to a string.

The content here would be a framework element which is determined by available DataTemplates at that point.

Make relay command a RelayCommand<object> . In the anonymous method for relay command the x.GetType will tell you whats the content set there.

Hope this clears your doubts.