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Issue with property not passing back value to text box on property change

I am working on a WPF application and i have a textbox bound (bidirectionally) to a property in my view model.
I am trying to prevent a user from typing more than 100 characters into this textbox (this is the max the database will store) so i have written this.
public abstract class AppBaseViewModel : ViewModelBase
{
private String _text;
public String Text
{
get { return _text; }
set
{
_text = CheckTextLength(value, _text);
OnPropertyChanged("Text");
}
}
private string CheckTextLength(string value, string text)
{
if (value.Length < 100)
{
return value;
}
else
{
return text;
}
}
}
All this code seems to do is save the first 100 characters to the field but it still allows the user to carry on typing past 100 characters... i would guess it is because the field value isn't being passed back to the textbox.
I don't understand why this doesn't work as i did something similar using MVVM Light's RaisePropertyChange() in a different application.
It is worth noting that i am unable to access the designer for the textbox so cannot set the .Net textbox property for max length.
Edit: Just for clarification i cannot view or edit the xaml as some are suggesting as i do not have access to the XAML file (i know, it's stupid). All the bindings we use are two way by default

Have you tried with TextBox.MaxLength ?
<TextBox MaxLength="100"/>
Gets or sets the maximum number of characters that can be manually entered into the text box.
If no access to the XAML, eventually get access to the XAML instead of parsing and verifying lengths of arrays and use substrings here and there. At least that's what i would do for this simple issue or talk to the designer to add that small piece of code.
Update 1
public static T GetChildOfType<T>(DependencyObject depObj) where T : DependencyObject
{
if (depObj == null) return null;
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
{
var child = VisualTreeHelper.GetChild(depObj, i);
var result = (child as T) ?? GetChildOfType<T>(child);
if (result != null) return result;
}
return null;
}
Go and get that child and set its MaxLength. This is just a slight modification on the View so it will not affect the MVVM pattern.

OK. I'm not at all sure that I'm proud of this, but am presenting it as an alternative.
You can change the UpdateSourceTrigger of the TextBox's Text property by applying a universal Style to all of the TextBoxes. This is only going to be practical in pretty weird arrangements, but the question is a little unusual in itself.
XAML codebehind:
//I'm using MVVM Light here - you need to be able to find an instance
//of your AppBaseViewModel somehow.
private ViewModelLocator _locator;
//View codebehind constructor, may need to change names as appropriate
public AppBaseView()
{
InitializeComponent();
//MVVM Light again
_locator = new ViewModelLocator();
//Create the binding
Binding binding = new Binding();
//Source = The instance of your ViewModel
binding.Source = _locator.AppBaseViewModel ;
binding.Path = new PropertyPath("Text");
binding.Mode = BindingMode.TwoWay;
binding.UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged;
//Create a Style with no Key - this will apply to *all* TextBoxes
//without their own explicit Style set.
Style style = new Style(typeof(TextBox));
style.Setters.Add(new Setter(TextBox.TextProperty, binding));
//Add the Style to the XAML's Resources:
Resources.Add(typeof(TextBox), style);
}

The view won't listen to the PropertyChanged notification if it's currently trying to change the property itself.
The only thing that comes to mind is launching an extra delayed PropertyChanged notification when you detect the constraint is not met...
private string CheckTextLength(string value, string text)
{
if (value.Length < 100)
{
return value;
}
else
{
MyDispatcher.BeginInvoke(new Action(() =>
OnPropertyChanged("Text")),
DispatcherPriority.Loaded);
return text;
}
}
Can't try the code, so sorry if it doesn't build righ away. MyDispatcher could be your Application.Current.Dispatcher, for instance.

The xaml view /the binding is only updated when the textbox has lost focus. if the text entered is <100 then the value is set otherwise _text is set. this means that initially _text has no value so null will be set upon the if statement being false. i also suggest yo use RaisePropertyChanged(); and when used within the property itself no parameter is needed.

cant figure out how to create a get set statement

I try to make a get set statement but it doesn't work. I have no idea why.
Basically I have a form named form1 that contains a combo-box with names and a button.
The button opens a new form form2 that has a text box. I want to be able to add the text in the text box to the combo box in form1. I searched the web a bit and found that in order to do so i need to create get and set statement instead of playing with the privacy of the controls.
The set and get statement doesn't work. It says only assignment call increment decrement and new object can be used as statement.
Much appreciate your help.
public string GuideNameFunc(){
Get { return GuideName.Text; }
set { GuideName.Text = value; }
}

Just remove it () from your code.
public string GuideNameFunc{
get { return GuideName.Text; }
set { GuideName.Text = value; }
}

public string GuideNameField{
get { return GuideName.Text; }
set { GuideName.Text = value; }
}

Click a custom control and show some variables related to it in another control

in my Win Forms app I create an array of dynamic custom controls inside a loop. These, lets call them 'boxes', are like my basic pieces of information. I also create string arrays in other parts of the code that contain the information of this 'boxes', so that for example string[3] is a variable of box[3] and so does stringa[3], stringb[3], stringc[3]... all the arrays with the same index are related to the box with that index. Hope I make myself clear.
Only 2 of this strings are shown in 2 labels inside each custom control 'box' in the array, but the others are there because I want to make something so that when the user clicks one of these controls the other strings can be shown in another control. Sort of something like "More Information...". All the 'boxes' in the array need to have the same event handler because I create +100.
To put it more into context, each custom control 'box' in the array shows the Symbol and the Price of a stock and I want that when the user clicks on each stock more quote information is shown on another special control which is like a placeholder for "More info".
I am thinking of 2 ways to do it:
If I could "detect" the index of the clicked control (which is the same in the strings related to it), I could just set this to an int j and all I have to do is show all the strings a,b,c... with index j. Unfortunately I cannot find a way to do this, maybe it is not even possible.
The other way I have thought is to create some properties for my custom control which "store" this variables, and in my app instead of assigning strings I would set properties for each control, which I could later retrieve when the control is clicked. I haven't tryed this because I don't know exactly how to do it.
What do you think? Do you know how can I achieve this or do you have a different idea that will work? Please help! Thanks in advance.

It's kind of a broad implementation question since there are countless ways you could implement something like this.
If you are creating two collections, one with the buttons and one with the information, you potentially could just assign each of the buttons 'Tag' properties to point to the corresponding info and assign a generic OnClick event handler that displays the info.. something like:
infoControl.text = ((InfoClass)((Button)Sender.Tag)).pieceOfInformation;
But again there are many ways to do this, and the choice comes down to how you store your information.

For your first method, you could have a property of your custom control that is the index.
public class Box : Control
{
// ...existing code
private int index;
public int Index
{
get
{
return index;
}
set
{
index = value;
}
}
}
OR
For your second method, you could have a property of your custom control that is the additional info string.
public class Box : Control
{
// ...existing code
private string extraInfo;
public string ExtraInfo
{
get
{
return extraInfo;
}
set
{
extraInfo = value;
}
}
}
In either case, you could then access the proper information right in your click handler for the "box".

i don't know about the first way - got to noodle around more, but in the second way you can extended your custom or built-in control: for example:
public class ExtendedLabel: Label
{
public string[] MoreInfo { get; set; }
}
and initialize it
public TestForm()
{
InitializeComponent();
ExtendedLabel label = new ExtendedLabel();
label.MoreInfo = new string[] { "test" };
this.Controls.Add(label);
label.AutoSize = true;
label.Location = new System.Drawing.Point(120, 87);
label.Name = "label1";
label.Size = new System.Drawing.Size(35, 13);
label.TabIndex = 0;
label.Text = label.MoreInfo[0];
}
And later in your event handler you can use the inside information

Why is text in TextBox highlighted (selected) when form is displayed?

I have a form containing a TextBox in C# which I set to a string as follows:
textBox.Text = str;
When the form is displayed, why does the text in the texbox appear highlighted/selected?

The text box has a TabIndex of 0 and TabStop set to true. This means that the control will be given focus when the form is displayed.
You can either give another control the 0 TabIndex (if there is one) and give the text box a different tab index (>0), or set TabStop to false for the text box to stop this from happening.

The default behavior of a TextBox in Windows Forms is to highlight all of the text if it gets focused for the first time by tabbing into it, but not if it is clicked into. We can see this in Reflector by looking at the TextBox's OnGotFocus() override:
protected override void OnGotFocus(EventArgs e)
{
base.OnGotFocus(e);
if (!this.selectionSet)
{
this.selectionSet = true;
if ((this.SelectionLength == 0) && (Control.MouseButtons == MouseButtons.None))
{
base.SelectAll();
}
}
}
It's that if statement that is causing the behavior that we don't like. Furthermore, to add insult to injury, the Text property's setter blindly resets that selectionSet variable whenever the text is re-assigned:
public override string Text
{
get
{
return base.Text;
}
set
{
base.Text = value;
this.selectionSet = false;
}
}
So if you have a TextBox and tab into it, all the text will be selected. If you click into it, the highlight is removed, and if you re-tab into it, your caret position (and selection length of zero) is preserved. But if we programmatically set new Text, and tab into the TextBox again, then all of the text will be selected again.
If you are like me and find this behavior annoying and inconsistent, then there are two ways around this problem.
The first, and probably the easiest, is to simply trigger the setting of selectionSet by calling DeselectAll() on form Load() and whenever the Text changes:
protected override void OnLoad(EventArgs e)
{
base.OnLoad(e);
this.textBox2.SelectionStart = this.textBox2.Text.Length;
this.textBox2.DeselectAll();
}
(DeselectAll() just sets SelectionLength to zero. It's actually SelectionStart that flips the TextBox's selectionSet variable. In the above case, the call to DeselectAll() is not necessary since we are setting the start to the end of the text. But if we set it to any other position, like the start of the text, then calling it is a good idea.)
The more permanent way is to create our own TextBox with the desired behavior through inheritance:
public class NonSelectingTextBox : TextBox
{
// Base class has a selectionSet property, but its private.
// We need to shadow with our own variable. If true, this means
// "don't mess with the selection, the user did it."
private bool selectionSet;
protected override void OnGotFocus(EventArgs e)
{
bool needToDeselect = false;
// We don't want to avoid calling the base implementation
// completely. We mirror the logic that we are trying to avoid;
// if the base implementation will select all of the text, we
// set a boolean.
if (!this.selectionSet)
{
this.selectionSet = true;
if ((this.SelectionLength == 0) &&
(Control.MouseButtons == MouseButtons.None))
{
needToDeselect = true;
}
}
// Call the base implementation
base.OnGotFocus(e);
// Did we notice that the text was selected automatically? Let's
// de-select it and put the caret at the end.
if (needToDeselect)
{
this.SelectionStart = this.Text.Length;
this.DeselectAll();
}
}
public override string Text
{
get
{
return base.Text;
}
set
{
base.Text = value;
// Update our copy of the variable since the
// base implementation will have flipped its back.
this.selectionSet = false;
}
}
}
You maybe tempted to just not call base.OnGotFocus(), but then we would lose useful functionality in the base Control class. And you might be tempted to not mess with the selectionSet nonsense at all and simply deselect the text every time in OnGotFocus(), but then we would lose the user's highlight if they tabbed out of the field and back.
Ugly? You betcha. But it is what it is.

The answers to this question helped me a lot with a similar problem, but the simple answer is only hinted at with a lot of other complex suggestions. Just set SelectionStart to 0 after setting your Text. Problem solved!
Example:
yourtextbox.Text = "asdf";
yourtextbox.SelectionStart = 0;

You can also choose the tab order for your form's controls by opening:
View->Tab Order
Note that this option is only available in "View" if you have the Form design view open.
Selecting "Tab Order" opens a view of the Form which allows you to choose the desired tab order by clicking on the controls.

To unhighlight a text field, with VS 2013, try init with:
myTextBox.GotFocus += new System.EventHandler(this.myTextBox_GotFocus);
And add the method:
public void myTextBox_GotFocus(object sender, EventArgs e)
{
myTextBox.SelectionLength=0;
}

I haven't tested this on C# but I ran into the same issue using a C++ WIN32 dialog box. Is seems like you can change the behavior by returning FALSE from OnInitDialog() or WM_INITDIALOG. Hope this helps.

Here is what worked for me
public void SetNotes(string notes)
{
notesTextBox.Text = notes;
notesTextBox.Select();
notesTextBox.SelectionLength = 0;
notesTextBox.SelectionStart = notes.Length;//place cursor at end of text
}

determine a textbox's previous value in its lost focused event? WPF

I have a textbox and have an onlostfocus event on it.
Inside the lostfocus method, is there a way I can determine if the user has actually changed the value in it?
i.e how do i get hold of any previous value in it?
Thanks

As with just about everything else in WPF, this is easier if you use data binding.
Bind the text box to a class property. By default, bindings update the source when the bound control loses focus, so you don't have to muck around with the LostFocus event. You then have access to both the new value and the value that the user entered in the property setter.
In the XAML it looks like this:
<TextBox Text="{Binding MyProperty, Mode=TwoWay}"/>
In the class it looks like this:
private string _MyProperty;
public string MyProperty
{
get { return _MyProperty; }
set
{
// at this point, value contains what the user just typed, and
// _MyProperty contains the property's previous value.
if (value != _MyProperty)
{
_MyProperty = value;
// assuming you've implemented INotifyPropertyChanged in the usual way...
OnPropertyChanged("MyProperty");
}
}

What comes to mind for me is a two stage approach. Handle the TextChanged event on the textbox and flag it. Then when the textbox OnLostFocus occurs you can simply check your flag to see if the text has been changed.
Here is a code snippet on how you could handle the tracking.
public class MyView
{
private bool _textChanged = false;
private String _oldValue = String.Empty;
TextChanged( ... )
{
// The user modifed the text, set our flag
_textChanged = true;
}
OnLostFocus( ... )
{
// Has the text changed?
if( _textChanged )
{
// Do work with _oldValue and the
// current value of the textbox
// Finished work save the new value as old
_oldValue = myTextBox.Text;
// Reset changed flag
_textChanged = false;
}
}
}

Store the original value somewhere. You could write a common component to store the value when it gets focus and compare the value when it loses focus. I've done this in ASP.NET and it works quite well.

Another way to solve this by databinding:
Bind the TextBox.Text to the property, that holds the inital value, but use a binding with
UpdateSourceTrigger=Explicit
Then, when the textbox loses focus, you can check the binding if source and target values differ, using this code snippet and evaluating the resulting BindingExpression:
BindingExpression be = tb.GetBindingExpression(TextBox.TextProperty);
Some more code can be found here:
http://bea.stollnitz.com/blog/?p=41