In order to not set the default string as "0" I used the following code in my usercontrol:
public double txt
{
get { return double.Parse(txtInput.Text); }
set { txtInput.Text = value.ToString(); }
}
public string text
{
get { return txtInput.Text; }
set { txtInput.Text = value; }
}
And in the designer class of the form I replaced
this.uscontrTextbox.txt = "";
with
this.uscontrTextbox.text = "";