Only allowing up to three digit numeric characters

问题:

Is there a way to only allow a user to input a maximum number of characters into a text box? I want the user to input a mark/grade and only be able to input 0 - 100. Below I have code that monitors the keystroke and only allows for numbers to be input, but I want to find a way to only allow the user to input a number with a minimum value of 0 and a maximum of 100.

private void TxtMark4_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar < '0' || e.KeyChar > '9' || e.KeyChar == ' ')
    {
        e.Handled = true;
    }
    else
    {
        e.Handled = false;
    }
}

or I could use the following:

if (e.KeyChar >= 48 && e.KeyChar <= 57 || e.KeyChar == ' ')
{
    e.Handled = false;
}
else
{
    MessageBox.Show("You Can Only Enter A Number!");
    e.Handled = true;
}

But I would like to find a way to only allow three characters to be input maximum.

回答1:

I think it is as simple as:

textBox1.MaxLength = 3;

Then you handle the maximum value on the Leave event:

    private void textBox1_Leave(object sender, EventArgs e)
    {
        string s = (sender as TextBox).Text;
        int i = Convert.ToInt16(s);

        if (i > 100)
        {
            MessageBox.Show("Number greater than 100");
            (sender as TextBox).Focus();
        }
    }

or

You could also use System.Windows.Forms.NumericUpDown where you can easily setup minimum and maximum.

回答2:

This version set textBox1.Text to empty string if fail to parse

private void textBox1_TextChanged(object sender, EventArgs e) {
    int i;

    textBox1.Text=
        false==int.TryParse(textBox1.Text, out i)||0>i||i>100
            ?""
            :i.ToString();
}

If you want to keep the partial successfully parsed number then

String previousText="";

private void textBox1_TextChanged(object sender, EventArgs e) {
    var currentText=textBox1.Text;
    int i;

    textBox1.Text=
        int.TryParse(currentText, out i)
            ?0>i||i>99
                ?previousText
                :i.ToString()
            :""==currentText?currentText:previousText;

    previousText=textBox1.Text;
}

回答3:

I would do it like this with catch for every possible user error.

I'm assuming you textbox is named TxtMark4. Write txtMark4.Content() or what ever you need to read the content of the textbox in your framework in the if-test which does the TryParse

private void TxtMark4_KeyPress(object sender, KeyPressEventArgs e)
{
    int? tmp = null; //signed to allow it to be assigned the value of null
    if(int.TryParse(txtMark4.Text,out tmp)){
        if(tmp >=0 && tmp <= 100){
        //Here the number is between 0 and 100
        }
        else{//Number is below 0 or above 100
            if(tmp > 100){
                TxtMark4.Text = TxtMark4.Text.remove(2); //This will forcefully make it less or equal to 100
                DisplayLabel.text = "Numbers between 0-100 only";
            }
            else{
                TxtMark4.Text = ""; //and if its below 0 it will not be displayed
                DisplayLabel.text = "Numbers between 0-100 only";
            }
        }
    }
    else{//Not a number
        //Return some indicator to the user
        DisplayLabel.text = "numbers only";
    }
}