WPF doesn't honor Textbox.MinLines for Auto height calculation - wpf

Question

I want to have a TextBox which Height grows as Iam entering lines of Text.
I've set the Height property to "Auto", and so far the growing works.
Now I want that the TextBox's Height should be at least 5 lines.
Now I've set the MinLines property to "5" but if I start the app the TextBox's height is still one line.

Answer 1

Try setting the MinHeight property.

Answer 2

A hack to make the MinLines property work
public class TextBoxAdv : TextBox
{
bool loaded = false;
/// <summary>
/// Constructor
/// </summary>
public TextBoxAdv()
{
Loaded += new RoutedEventHandler( Control_Loaded );
SetResourceReference( StyleProperty, typeof( TextBox ) );
}
void Control_Loaded( object sender, RoutedEventArgs e )
{
if( !loaded )
{
loaded = true;
string text = Text;
Text = "Text";
UpdateLayout();
Text = text;
}
}
}

Answer 3

I propose a different solution that properly respects the MinLines property, rather than forcing you to use MinHeight.
First, start with a convenience method to allow you to Post an action to the window loop. (I'm including both one where you need to pass state and one where you don't.)
public static class Globals {
public static void Post(Action callback){
if(SynchronizationContext.Current is SynchronizationContext currentContext)
currentContext.Post( _ => callback(), null);
else{
callback();
}
}
public static void Post<TState>(TState state, Action<TState> callback){
if(SynchronizationContext.Current is SynchronizationContext currentContext)
currentContext.Post(_ => callback(state), null);
else{
callback(state);
}
}
}
Next, create an extension method for TextBox to 'initialize' the proper size based on MinLines. I put this in a Hacks class because to me, that's what this is and it clearly identifies the code as such.
public static void FixInitialMinLines(this TextBox textBox) {
Globals.Post(() => {
var textBinding = textBox.GetBindingExpression(TextBox.TextProperty)?.ParentBinding;
if (textBinding != null) {
BindingOperations.ClearBinding(textBox, TextBox.TextProperty);
textBox.UpdateLayout();
BindingOperations.SetBinding(textBox, TextBox.TextProperty, textBinding);
}
else {
var lastValue = textBox.Text;
textBox.Text = lastValue + "a";
textBox.UpdateLayout();
textBox.Text = lastValue;
}
});
}
The above code handles both bound and unbound TextBox controls, but rather than simply changing the value like other controls which may cascade that change down through the bindings, it first disconnects the binding, forces layout, then reconnects the binding, thus triggering the proper layout in the UI. This avoids unintentionally changing your bound sources should the binding be two-way.
Finally, simply call the extension method for every TextBox where MinLines is set. Thanks to the Post call in the extension method, You can call this immediately after InitializeComponent and it will still be executed after all other events have fired, including all layout and the Loaded event.
public partial class Main : Window {
public Main() {
InitializeComponent();
// Fix initial MinLines issue
SomeTextBoxWithMinLines.FixInitialMinLines();
}
...
}
Add the above code to your 'library' of functions and you can address the issue with a single line of code in all of your windows and controls. Enjoy!