Changing a binding in code

If you have ever tried to modify a binding in code like this DetailsQty.Binding.StringFormat = "0" you have probably seen the following error...

Binding cannot be changed after it has been used

Previously I discovered this problem with styles and the solution there is to create a new style based on the original one, modify it, then assign it back to the UIElement replacing the original one. Not elegant, but it works.

Dim Style As System.Windows.Style
Style = New System.Windows.Style(GetType(TextBox), DetailsQty.EditingElementStyle)
Style.Setters.Add(New Setter(TextBox.TagProperty, "8N"))
DetailsQty.EditingElementStyle = Style

Unfortunately there is no Binding constructor that takes an existing binding object so we are going to have to work harder. The Binding class does have a Clone method but it is private and we would have to use reflection to call it. I don't want to do that. I wrote a Clone method for bindings. It's very simple. The only tricky bit is that Source, RelativeSource and ElementName are mutually exclusive so I only copy them if they have values.

    Public Shared Function CloneBinding(OldBinding As Binding) As Binding
        Dim NewBinding = New Binding
        If OldBinding.Source IsNot Nothing Then NewBinding.Source = OldBinding.Source
        NewBinding.AsyncState = OldBinding.AsyncState
        NewBinding.BindingGroupName = OldBinding.BindingGroupName
        NewBinding.BindsDirectlyToSource = OldBinding.BindsDirectlyToSource
        NewBinding.Converter = OldBinding.Converter
        NewBinding.ConverterCulture = OldBinding.ConverterCulture
        NewBinding.ConverterParameter = OldBinding.ConverterParameter
        If OldBinding.ElementName IsNot Nothing Then NewBinding.ElementName = OldBinding.ElementName
        NewBinding.FallbackValue = OldBinding.FallbackValue
        NewBinding.IsAsync = OldBinding.IsAsync
        NewBinding.Mode = OldBinding.Mode
        NewBinding.NotifyOnSourceUpdated = OldBinding.NotifyOnSourceUpdated
        NewBinding.NotifyOnTargetUpdated = OldBinding.NotifyOnTargetUpdated
        NewBinding.NotifyOnValidationError = OldBinding.NotifyOnValidationError
        NewBinding.Path = OldBinding.Path
        If OldBinding.RelativeSource IsNot Nothing Then NewBinding.RelativeSource = OldBinding.RelativeSource
        NewBinding.StringFormat = OldBinding.StringFormat
        NewBinding.TargetNullValue = OldBinding.TargetNullValue
        NewBinding.UpdateSourceExceptionFilter = OldBinding.UpdateSourceExceptionFilter
        NewBinding.UpdateSourceTrigger = OldBinding.UpdateSourceTrigger
        NewBinding.ValidatesOnDataErrors = OldBinding.ValidatesOnDataErrors
        NewBinding.ValidatesOnExceptions = OldBinding.ValidatesOnExceptions
        NewBinding.XPath = OldBinding.XPath

        Return NewBinding
    End Function

Now changing an active binding in code is much easier. To change the FormatString on a DataGridTextColumn called DetailsQty we would use this code...

Dim Binding As Binding
Binding = CloneBinding(DetailsQty.Binding)
Binding.StringFormat = "0"
DetailsQty.Binding = Binding