I guess you want to delete all rows from table so correct the command first as
<pre lang="c#">protected void Delete_Click(object sender, EventArgs e)
{
SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["registrationConnectionString"].ConnectionString);
string sqlStatement = "DELETE * from userdata";
SqlCommand com = new SqlCommand(sqlStatement, conn);
cmd.ExecuteNonQuery();//have to execute above command query
conn.Close();
}</pre>
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