Question

I am working on WPF app in which I have a textbox, Button, Label & a toggle Button. In my application, I should perform two operations as follows:

1. Set the input of textbox only between 0.0 to 5.0. Thus if any value written which exceeds 5.0, must be restricted to 5. Similarly any value written below 0.0 must be restricted and only 0.0 must be displayed.

2. My label should display content as “value” V. I.e. if value is 3.4 V as volts should be concatenated with it.

Here Goes The Xaml:

<TextBox Grid.Column="1" Text="{Binding VoltageText}" Name="VoltageBox" />        
<Label Grid.Column="2" Content="{Binding CurrentText}" Name="CurrentLabel" />

Model Class:

string voltageText = string.Empty;
    public string VoltageText
    {
        get
        {
            return voltageText;
        }

        set
        {
            voltageText = value;
            OnPropertyChanged("VoltageText");
        }
    }

This textbox must have input restrictions as mentioned in my first points 🙂 Here goes the textchanged method I wrote to allow only 0-9 and . in textbox:

// present in xaml.cs
private void VoltageBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        string txt = VoltageBox.Text;
        if (txt != "")
        {
            VoltageBox.Text = Regex.Replace(VoltageBox.Text, "[^0-9] [^.]", "");
            if (txt != VoltageBox.Text)
            {
                VoltageBox.Select(VoltageBox.Text.Length, 0);
            }
        }
    }

    string currentText = "0 V";
    public string CurrentText
    {
        get
        {
            return currentText;
        }

        set
        {
            currentText = value;
            OnPropertyChanged("CurrentText");
        }
    }     

You can notice on startup Currenttext = 0 V and by executing these statements I want to change the value of the CurrentText:

double tempval = 120.0;

string CurrentVal = Convert.ToString(tempval / 10);
CurrentText = CurrentVal; // here V as volts must be concatenated and value must be 12.0 V

Answer 1

Here is a rough way on how to do things.

First on your ViewModel, have one property that is your voltage effective value, and a calculated property that will add a “V” to it. It’s the ViewModel responsability to do these kind of thing, not the View one.

    private double voltage = 0.0D;
    public double Voltage
    {
        get
        {
            return voltage;
        }

        set
        {
            if (value > 5.0D || value < 0.0D)
                throw new InvalidOperationException();
            voltage = value;
            OnPropertyChanged("Voltage");
        }
    }

    public string VoltageText
    {
        get
        {
            return Voltage + " V";
        }
    }

Then for your control, don’t use TextChanged, prefer using PreviewTextInput

In your waml :

    <TextBox Grid.Column="1" Text="{Binding Voltage}" Name="VoltageBox" PreviewTextInput="TextBox_PreviewTextInput" />
    <Label Grid.Column="2" Content="{Binding VoltageText}" Name="CurrentLabel" />

In code behind

    private void TextBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var textBox = sender as TextBox;
        if (textBox == null)
            throw new ArgumentException("sender");

        double result;
        var insertedText = e.Text;
        if (!double.TryParse(insertedText, out result) && insertedText != ".")
        {
            e.Handled = true;
        }
        else
        {
            string textboxCurrentValue = textBox.Text;
            int selectionLength = textBox.SelectionLength;
            int selectionStart = textBox.SelectionStart;
            if (selectionLength != 0)
            {
                textboxCurrentValue = textboxCurrentValue.Remove(selectionStart, selectionLength);
            }
            textboxCurrentValue = textboxCurrentValue.Insert(selectionStart, insertedText);

            if (!double.TryParse(textboxCurrentValue, out result) || result > 5.0D || result < 0.0D)
            {
                e.Handled = true;
            }
        }
    }

This should get you started !