TextBox Allow Decimal and minus sign

If you're going to do this in the Text_Changed event, then the invalid characters will already have been inserted and so you'll need to remove them.

The following should do that.

As well as removing invalid characters, it should also ensure that:

1. There's no more than one decimal point (only the last one entered is retained).

2. If there is a minus sign it's at the start of the string.

3. There's always at least one zero before a decimal point.

private void textBox1_TextChanged(object sender, EventArgs e)

{

textBox1.Text = textBox1.Text.Trim();

if (textBox1.Text.Length == 0) return;

StringBuilder sb = new StringBuilder(textBox1.Text);

bool dotExists = false;

for (int i = sb.Length - 1; i >= 0; i--)

{

if (!Char.IsDigit(sb[i]) && sb[i] != '.' && sb[i] != '-')

{

sb.Remove(i, 1);

}

else if (sb[i] == '.')

{

if (dotExists)

{

sb.Remove(i, 1);

}

else

{

dotExists = true;

}

else if (sb[i] == '-' && i > 0)

{

sb.Remove(i, 1);

}

if (sb[0] == '.') sb.Insert(0, '0');

if (sb[0] == '-' && (sb.Length > 1 && sb[1] == '.')) sb.Insert(1, '0');

textBox1.Text = sb.ToString();

textBox1.SelectionStart = textBox1.Text.Length + 1;

}

1 Reply

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Vulpes13y ago
If you're going to do this in the Text_Changed event, then the invalid characters will already have been inserted and so you'll need to remove them.

The following should do that.

As well as removing invalid characters, it should also ensure that:

1. There's no more than one decimal point (only the last one entered is retained).

2. If there is a minus sign it's at the start of the string.

3. There's always at least one zero before a decimal point.

private void textBox1_TextChanged(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text.Trim();
if (textBox1.Text.Length == 0) return;
StringBuilder sb = new StringBuilder(textBox1.Text);
bool dotExists = false;

for (int i = sb.Length - 1; i >= 0; i--)
{
if (!Char.IsDigit(sb[i]) && sb[i] != '.' && sb[i] != '-')
{
sb.Remove(i, 1);
}
else if (sb[i] == '.')
{
if (dotExists)
{
sb.Remove(i, 1);
}
else
{
dotExists = true;
}
}
else if (sb[i] == '-' && i > 0)
{
sb.Remove(i, 1);
}
}

if (sb[0] == '.') sb.Insert(0, '0');
if (sb[0] == '-' && (sb.Length > 1 && sb[1] == '.')) sb.Insert(1, '0');
textBox1.Text = sb.ToString();
textBox1.SelectionStart = textBox1.Text.Length + 1;
}