WPF Binding when a button is pressed (Explicit binding) has always BindingExpression at null?

Question

I have a textbox that contain a string that must be binded only when the user press the button. In XAML:

        <Button Command="{Binding Path=PingCommand}" Click="Button_Click">Go</Button>
        <TextBox x:Name="txtUrl" Text="{Binding Path=Url,UpdateSourceTrigger=Explicit, Mode=OneWay}"></TextBox>

In the code-behind:

    private void Button_Click(object sender, RoutedEventArgs e)
    {

        BindingExpression be = this.txtUrl.GetBindingExpression(TextBox.TextProperty);
        be.UpdateTarget();

    }

"be" is always NULL. Why?

Update:

Alright, here is some update after a lot of try.

IF, I set the Mode the OneWay with Explicit update. I have a NullReferenceException in the "be" object from the GetBindingExpression.

IF, I set the Mode to nothing (default TwoWay) with Explicit update. I have the binding getting the value (string.empty) and it erases every times everything in the textbox.

IF, I set the Mode to OneWay, with PropertyChanged I have nothing raised by the property binded when I press keys in the textbox and once I click the button, I have a NULLReferenceException in the "be" object.

IF, I set the Mode to nothing (default TwoWay), with PropertyChanged I have the property that raise everytime I press (GOOD) but I do not want to have the property change everytime the user press a key... but only once the user press the Button.

Answer 1

It should work, may be you are calling functions that delete the binding before the lines you provide in event handler? Assigning value to text property of text box will remove binding

Answer 2

Have you tried your data-binding without the UpdateSourceTrigger? If the binding failed, I would expect null to be the result of GetBindingExpression.

One cause of this could be that Url was not implemented as a DependencyProperty.

Answer 3

Alright, after thinking a little more I noticed that :

 BindingExpression be = this.txtUrl.GetBindingExpression(TextBox.TextProperty);
 be.UpdateTarget();

Has something illogical because I do not want to update the Target but the source. I have simply changed be.Updatetarget() to be.UpdateSource(); and everything worked with this XAML:

 <TextBox x:Name="txtUrl" Text="{Binding Path=Url, UpdateSourceTrigger=Explicit}">...

Thank you to everybody who have helped me in the process to solve this problem. I have added +1 to everybody! Thanks