System.Data.SqlClient.SqlException: Incorrect syntax near 0

I am working on the simple project in Search of SQL Database. In the project I have:  combox for display table names, listboxes for display of column names, data types, data view and Sql Query statments, and textbox with button for search any keyword.

Every time I type a word in the textbox like "rio" I am getting an error 

ERROR: System.Data.SqlClient.SqlException: 'Incorrect syntax near '0'.'

I debugged that line of code but i see the SQL statement shows:  SELECT 1 FROM 0 WHERE 1 LIKE %rio% ???

Please see the code below:

Form1.cs

        private void TextSearchDB()
        {
            lstBoxDataViewInfo.Items.Clear();
            int tablecount = cmbBoxTableName.Items.Count;
            for(int i=0;i<tablecount;i++)
            {
                cmbBoxTableName.SelectedIndex = i;
                int columncount = lstBoxColumnName.Items.Count;
                for(int j=0;j<columncount;j++)
                {
                    lstBoxColumnName.SelectedIndex = j;
                    lstBoxDataType.SelectedIndex = j;

                    string tablename = cmbBoxTableName.SelectedIndex.ToString();
                    string columnname = lstBoxColumnName.SelectedIndex.ToString();
                    string datatype = Parseinput(textBoxSearchFullDB.ToString());
                    if (lstBoxDataType.SelectedItem.ToString() == datatype)
                    {
                        if (datatype != "int")
                        {
                            sqlcmd.CommandText = "SELECT " + columnname.ToString() + " FROM " + tablename.ToString() + " WHERE " + columnname.ToString() + " LIKE '%" + textBoxSearchFullDB.Text.ToString() + "%'";
                        }
                        if (datatype == "int")
                        {
                            sqlcmd.CommandText = "SELECT " + columnname.ToString() + " FROM " + tablename.ToString() + " WHERE " + columnname.ToString() + " LIKE '%" + textBoxSearchFullDB.Text.ToString() + "%'";
                         }

                        sqlconn.Open();

                        Error --->
                        sqldr = sqlcmd.ExecuteReader();     //ERROR: System.Data.SqlClient.SqlException: 'Incorrect syntax near '0'.'

                        if (sqldr.HasRows)
                        {
                            while (sqldr.Read())
                            {
                                lstBoxDataViewInfo.Items.Add(sqldr[0].ToString() + " Column Name: " + columnname.ToString() + " FROM TABLE:" + tablename.ToString());
                                Application.DoEvents();
                                lstBoxSQLQueryStatement.Items.Add(sqlcmd.CommandText.ToString());
                            }
                        }
                        sqlconn.Close();                     
                    }
                }
            }

Your help is much appreciated.  Thanks.