Detect or suppress invalid characters in TextBox

Hey there,

I have a TextBox with a list of filenames in it. The user is able to change these file names, but I don't want them to enter invalid characters, such as slashes or question marks.

I've tried two approaches, but they are not satisfactory:

1. Detect the key pressed with the KeyDown event:
Use e.KeyValue to detect keys like Keys.Oem5 (= backslash), then use 'e.SuppressKeyPress = True' to suppress the key.
Problem is, with different Keyboard layouts, The Oem5-key isn't always a backslash.

2. Filter the text:
Use the TextChanged event and do a Replace("\", "") on the text for every invalid character (there's 9 of them).
Of course this will create 'lag' with every keystroke, especially if there's a lot of text. Also the text-cursor and scrollbar will jump back to the top-left when using Replace.

In short: I'm looking for a way to detect the character that is being entered and suppress or remove it when invalid.

Any help is appreciated.

Best regards,
Alexander. :wave:

I do not know if this will be the perfect solution. This one prevents the user from typing backslash, star or character zero

vb.net Code:

Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
    TextBox1.Text = TextBox1.Text.Trim({"\"c, "*"c, Chr(0)})
    TextBox1.Select(TextBox1.Text.Length, 0)
End Sub

on a side note, this one will only allow alpha characters with space or backspace or delete. Technically there is a way to insert special characters but not directly with a single keyboard key. If you want to prevent those also include them in the string.

vb.net Code:

Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
    TextBox1.Text = TextBox1.Text.Trim("`1234567890-=~!@#$%^&*()_+[]\{}|;':"",./<>?".ToCharArray)
    TextBox1.Select(TextBox1.Text.Length, 0)
End Sub

You can check against these two arrays

IO.Path.GetInvalidFileNameChars
IO.Path.GetInvalidPathChars

@kaliman79912 That will only work if the invalid character is either the first or the last character. People will type in the middle of file names as well.

@kevinstructor Thanks for that, it may prove useful later!