asp.net C# get textbox lenght from backend

Question

I would like to inform user if he is adding more character than he needs for a certain field.

my code behind :

if (txt1.txt.??? < X)
{
   alert(You exceed to the max amount of characters, max is X number);
}

Answer 1

To get the length of a textbox value from behind code.

if(txt1.Text.Length < 11)
{
 //Your code here
}
else
{
  //Your Code here 
}

However, why don't you used a textbox masked extender like Ajax or a RegularExpressionValidator validation?

<asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" ErrorMessage="Enter valid Phone number" ControlToValidate="txt1" ValidationExpression="\d{10}" ValidChars="()-" ></asp:RegularExpressionValidator>