Restrict users from Entering in TextBox

Masked textbox supports a declarative syntax for accepting or rejecting user input. Masked textbox may be used to specify input characters, input position in the mask, mask literals and some more operations can also be done using masked textbox without any custom validation logic.

Textbox is used to allow the user to input text information to be used by the programmer. To use our custom logic we can restrict numbers/characters to be inputted by the user. We can restrict any key to be inputted by the user.

Textbox’s Key Press event occurs when the control has focus and user press and releases a key. It has following format that is in C# language:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
}

The parameter e provides the data for this event which is of type KeyPressEventArgs. Char class has some in-built functions for checking the pressed and released key by user.

char.IsDigit(): whether the character is decimal digit or not.
char.IsLetter(): whether the character is a letter.
Char.IsLetterOrDigit(): whether the character is letter or digit.

There are many more method to check each type of data, we can read more here.
To restrict the user from entering the digit in textbox

if (char.IsDigit(e.KeyChar))
e.Handled = true;

Here e.Handled will tell the debugger that I have handled this event. We can use desired method to prevent specified type of key by the user.

To restrict both the letter and digit to be inputted by the user in c# language:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (char.IsLetterOrDigit(e.KeyChar) || char.IsDigit(e.KeyChar))
e.Handled = true;
}

Now when we run this form and try to enter anything between a-z, A-Z and 0-9, we can’t enter anything among these values.

Validating Input Controls in windows forms C#

Data entered by user, should be valid and properly formatted, Otherwise a lot of resources could be wasted in fixing the problems that could be arise due to incorrect data format. Data should be properly checked according to the input criteria. Windows forms provides some ways to validate input in your application.

Masked TextBox Validation

We often requires some data like contact number in a well-defined format and also correct no of digits. This problem can easily be solved by using MaskedTextBox in windows forms. MaskedTextBox displays some formats to the user. If user types a wrong entry then this textbox automatically reject that input.

Just add a MaskedTextBox control and look out the mask property. There are approx. 9 predefined masks such as Phone number, zip code, Numeric digits etc. as in following screenshot:

There is no mask for mobile no in the above masks, so we have to create a new mask. Use the last mask i.e. custom mask and insert ten zero’s in the mask textbox just below the list of mask because mobile no is of ten digits, and our new mask has been created.

Event-Driven Validation

Each input control has a Validating event that occur whenever the control requires data validation. Validating event enables you to prevent from shifting focus from the control being validated to another control until validation has been completed.

To check whether the textbox is blank or not, we will simply check this by using the following line of codes in validating event of textbox in C# language.

private void textBox1_Validating(object sender, CancelEventArgs e)

{

if (textBox1.Text == string.Empty)

{

MessageBox.Show("Textbox is empty");

}

}

Just like above code we can validate any other control using some lines of code. There is an argument e of type CancelEventArgs, which can be used to cancel the validating event if required input has been entered by user.

As in above screenshot if the textbox left empty by the user then a message has been displayed written above. This message can be displayed by any of the message box, status strip etc.