what is the equivalent of lblconteo.Text = Len (txtMessage.Text); in c #?

Use the property .Length that gives you the number of characters in a String :

lblconteo.Text = txtMensaje.Text.Length.ToString();

Note that you have to use ToString() since the property .Length is of type Int32 and lblconteo.Text accept String .

int conteo = txtMensaje.Text.Length;

Greetings

You can get the number of characters using the property Length and you can convert this value to String to use it in your label:

lblconteo.Text = txtMensaje.Text.Length.ToString();

Or also as another option:

lblconteo.Text = Convert.Tostring(txtMensaje.Text.Length);