how to set error provide on prompt.

Question

hi..
I am using visual basic component in C#, now I want to set error provider message when input length is greater that four digits..

string str;
str = Interaction.InputBox("Please Enter Invoice No.", "Measurement Info", "", 260, 0xeb).Trim();
            if (str.Length > 4)
            {
              //  errorProvider1.SetError(str.ToString(), "Values is Greater");
               
            }

please help me..