In this post Filter Data According to Month and Year

Important Namespaces Used
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using System.Data;
using System.Data.SqlClient;

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Filter Button Click Event Coding
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String mycon = "Data Source=HP-PC\\SQLEXPRESS; Initial Catalog=BankingRecord; Integrated Security=True";
        String myquery = "select transactionid, convert(varchar, cast(convert(varchar(10), transactiondate, 110) as datetime), 106) as transactiondate, transdetail, transamount from transactiondetail where month(transactiondate)=" + DropDownList1.SelectedItem.Value+" and year(transactiondate)="+TextBox1.Text;
        SqlConnection con = new SqlConnection(mycon);
        SqlCommand cmd = new SqlCommand();
        cmd.CommandText = myquery;
        cmd.Connection = con;
        SqlDataAdapter da = new SqlDataAdapter();
        da.SelectCommand = cmd;
        DataSet ds = new DataSet();
        da.Fill(ds);
        GridView1.DataSource = ds;
        GridView1.DataBind();
        con.Close();