Check Spaces

Hi all,
I am using below code for check space but it generate errors.
my Code::
string str = TextBox1.Text;
        int l = str.Length;
        int i;
        for (i = 0; i < l; i++)
        {
            if (str[i] == " ")
            {
                Response.Write("Invalid");
            }
            else
            {
              & ...