How to filter characters in an EditText

Issue

I am trying to create an IInputFilter for an EditText in my Xamarin Android app. I created the class below to do this. It works fine until I enter an invalid character. It filters the invalid character as expected, but after that the source includes all valid characters entered since the invalid one.

Key "a" - source = "a"

Key "b" - source = "b"

Key "," - source = "," (this is an invalid character so we return empty string)

Key "c" - source = "c"

Key "d" - source = "cd" (why not just "d"?)

Key "e" - source = "cde" (and so on)

public class AlphaFilter : Java.Lang.Object, IInputFilter
{
    public Java.Lang.ICharSequence FilterFormatted(Java.Lang.ICharSequence source, int start, int end, ISpanned dest, int dstart, int dend)
    {
        Java.Lang.String strResult;
        bool blnValidated = source.ToString() == string.Empty || source.ToString().All(Char.IsLetterOrDigit);

        if (blnValidated)
        {
            strResult = new Java.Lang.String(source.ToString());
        }
        else
        {
            strResult = new Java.Lang.String(string.Empty);
        }
        return strResult;
    }
}

Solution

Found the problem. When the character is valid you just return null. Here is the working code:

    public class AlphanumericFilter : Java.Lang.Object, IInputFilter
{
    public Java.Lang.ICharSequence FilterFormatted(Java.Lang.ICharSequence source, int start, int end, ISpanned dest, int dstart, int dend)
    {
        Java.Lang.String strResult;
        bool blnValidated = source.ToString() == string.Empty || source.ToString().All(Char.IsLetterOrDigit);

        if (blnValidated)
        {
            strResult = null;
        }
        else
        {
            strResult = new Java.Lang.String(string.Empty);
        }
        return strResult;
    }
}

Answered By - P. Mancini