用绑定更新文本框时限制其字符串长度

Question

I know that you can limit the input characters of TextBox from user by setting MaxLength property.

Is there a similar way to limit the number of characters shown in Text when the Text is updated with Binding ? For example, when it is updated from Binding just show the first 5 characters and leave the rest?

Update: Thanks for all the info, I got inspired by your recommendation and in the end did it with a converter. Here is how I did it, if someone wants to use it later.

    public class StringLimiter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            string val = value.ToString();
            if (val.Length < 5)
                return val;
            else
                return val.Substring(0, 5);
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            string val = value.ToString();
            if (val.Length < 5)
                return val;
            else
                return val.Substring(0, 5);
        }
    }

Answer 1

This should work:

Xaml:

<TextBox Text="{Binding TextToDisplay}" />

Code:

    private const int maxLength = 5;
    private string _textToDisplay = "Hello SO";
    public string TextToDisplay
    {
        get
        {
            if(_textToDisplay.Length > maxLength)
            {
                return _textToDisplay.Substring(0, maxLength);
            }
            return _textToDisplay;
        }
        set
        {
            _textToDisplay = value;
            RaiseProperyChanged();
        }
    }

Answer 2

I hope to understand you right. You could create a new Property in the ViewModel that returns only the first 5 chars of the text and set your binding to that property. You might need to call PropertyChanged for the new Property when the text changes.

Answer 3

A simple but very flexible way of doing it would be to introduce a projected property in your Viewmodel that returns the first 5 characters of the original property and then bind your control to this property. Since you're only showing part of the property value, I assume that you don't want to write to this property from that TextBox. So make you projected property read-only too.