Winforms TextBox - Allow numeric or 1 decimal place only - winforms

Question

How can I prevent users from entering anything other than a numeric value or a decimal value with 1 decimal place?
The user should be allowed to enter any length of characters (if decimal value, before the decimal).

Answer 1

Try using Regex. This pattern should work: Regex match = new Regex(#"^[1-9]\d*(\.\d{1})?$"), put that in your validating event of the textbox. If its no match, Undo() or delete the Textbox.Text property.
Regex match = new Regex(#"^[1-9]\d*(\.\d{1})?$");
private void textBox1_Validating(object sender, CancelEventArgs e)
{
if (!match.IsMatch(textBox1.Text))
{
textBox1.Undo();
}
}
To actually undo the input immediatly, you have to use
private void textBox1_KeyUp(object sender, KeyEventArgs e)
{
if (!match.IsMatch(textBox1.Text))
{
textBox1.Undo();
}
}
Because if you use KeyDown, the TextBox has no Undo State.
2nd Edit: If you want both cases to match, you have to do the check in the Validating Event or a similar one. Since the regex uses "$" to make sure, no characters are added in the end, you cannot enter "." or else you'd end up having a number like 1. which would require additional checking.

Answer 2

Maybe a little late to the party on this one, but I extended a simple textbox to force the entry to always be formatted decimal.. Simple but effective
Imports System.Runtime.InteropServices
Imports System.Drawing.Imaging
Imports System.ComponentModel
Imports System.Text.RegularExpressions
<ToolboxBitmap(GetType(System.Windows.Forms.TextBox))> _
Public Class NumericTextBox
Inherits TextBox
Dim _TextBoxValue As String
Dim _CaretPosition As Integer
Protected Overrides Sub OnKeyDown(e As KeyEventArgs)
MyBase.OnKeyDown(e)
_TextBoxValue = Me.Text
_CaretPosition = Me.SelectionStart
End Sub
Protected Overrides Sub OnKeyUp(e As KeyEventArgs)
MyBase.OnKeyUp(e)
If (Me.Text.Length = 0) Or (Me.Text = _TextBoxValue) Then Exit Sub
If IsNumeric(Me.Text) Then
If Me.Text.EndsWith(".") Then
Me.Text = Convert.ToDecimal(Me.Text) & "."
Else
Me.Text = Convert.ToDecimal(Me.Text)
End If
Else
Me.Text = _TextBoxValue
End If
Me.SelectionStart = _CaretPosition + 1
End Sub
End Class

Answer 3

Regex match = new Regex(#"^[1-9]\d*(.\d{1})?$");
Working Correctly