In this article I will explain with an example, how to make TextBox required in Windows Forms (WinForms) Application using C# and VB.Net.
 
 
Form Design
The Form consists of a Label, a TextBox, two ErrorProvider controls i.e. one for displaying error message and one for displaying success message and a Button control.
Note: For more details on using ErrorProvider, please refer my article Using ErrorProvider control in Windows Forms with C# and VB.Net.
 
Make TextBox required in Windows Forms in C# and VB.Net
 
Inside the Properties window, the Icon for ErrorProvider i.e. epSuccess has been set with a tick mark icon in the Icon property.
Make TextBox required in Windows Forms in C# and VB.Net
 
 
Validating TextBox on Button Click
Inside the Properties window, the TextBox is assigned with Validating event handler as shown below.
Make TextBox required in Windows Forms in C# and VB.Net
 
Inside this Validating event handler, the TextBox is validated for empty (blank) and white space and if the validation fails i.e. if the TextBox is empty (blank) an error message is set in the ErrorProvider control (epError) which will make the error icon visible.
And if the TextBox is not empty i.e. if it has valid value, then the ErrorProvider control (epError) is set with a blank error message which will hide the error icon.
C#
private void txtName_Validating(object sender, CancelEventArgs e)
{
    if (!string.IsNullOrEmpty(txtName.Text.Trim()))
    {
        epError.SetError(txtName, string.Empty);
    }
    else
    {
        epError.SetError(txtName, "Name is required.");
    }
}
 
VB.Net
Private Sub txtName_Validating(sender As Object , e As System.ComponentModel.CancelEventArgs) Handles txtName.Validating
    If Not String.IsNullOrEmpty(txtName.Text.Trim) Then
        epError.SetError(txtName, String.Empty)
    Else
        epError.SetError(txtName, "Name is required.")
    End If
End Sub
 
 
Displaying Success Icon when the TextBox is filled
Inside the Properties window, the TextBox is assigned with KeyUp event handler as shown below.
Make TextBox required in Windows Forms in C# and VB.Net
 
Inside this KeyUp event handler, in the very first line, the ErrorProvider control (epError) is set with an empty error message so that the error icon is hidden.
And then, a check is made whether the TextBox is empty (blank) or not, if the TextBox has value then, the ErrorProvider control (epSuccess) is set with the success message which displays the check icon.
C#
private void txtName_KeyUp(object sender, KeyEventArgs e)
{
    epError.SetError(txtName, string.Empty);
    if (!string.IsNullOrEmpty(txtName.Text.Trim()))
    {
        epSuccess.SetError(txtName, "Valid");
    }
    else
    {
        epSuccess.SetError(txtName, string.Empty);
    }
}
 
VB.Net
Private Sub txtName_KeyUp(sender As Object, e As KeyEventArgs) Handles txtName.KeyUp
    epError.SetError(txtName, String.Empty)
    If Not String.IsNullOrEmpty(txtName.Text.Trim) Then
        epSuccess.SetError(txtName, "Valid")
    Else
        epSuccess.SetError(txtName, String.Empty)
    End If
End Sub
 
 
Screenshot
Make TextBox required in Windows Forms in C# and VB.Net
 
 
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