C# TextBox that allows a maximum of one dot

Question

How can I make a TextBox in C# to allow a maximum of one . (dot)?

Thus, abcdef and abc.def would be valid inputs whereas ab.cd.ef wouldn't.

By allow I mean that the user should not be able to enter a dot if there is already one in the text field.

Java has DocumentFilters for that purpose, is there something similar in C#?

Answer 1

I guess this is for validating user inputs. You should create a button and tell the user when he is done, press it so that you can check whether there is only one . in the string.

Assumptions:

Let your text box be called tb. Let your button's Click event handler be BtnOnClick.

Now we can start to write code. First create the handler:

private void BtnOnClick (object sender, EventArgs e) {

}

In the handler, you need to loop through the string and check each character. You can use a foreach for this:

int dotCount = 0;
string s = tb.Text;
if (s.StartsWith(".")) //starts with .
    //code to handle invalid input
    return;
if (s.EndsWith(".")) //ends with .
    //code to handle invalid input
    return;
foreach (char c in s) {
    if (c == '.') {
        dotCount++;
    }
    if (dotCount >= 2) { //more than two .
        //code to handle invalid input
        return;
    }
}
// if input is valid, this will execute

Alternatively, you can use a query expression. But I think you are unlikely to know what that is.

string s = tb.Text;
if (s.StartsWith(".")) //starts with .
    //code to handle invalid input
    return;
if (s.EndsWith(".")) //ends with .
    //code to handle invalid input
    return;
var query = from character in s
            where character == '.'
            select character;
if (query.Count() > 1) {
    //code to handle invalid input
    return;
}
// if input is valid, this will execute