SqlConnection cn = new SqlConnection("Data Source = localhost; Integrated Security = True; Database = myDB;");
SqlDataAdapter adp = new SqlDataAdapter();

private void LoadSearch()
        {
            switch (cmbCategory.Text)
            {
                case "All":
                    adp.SelectCommand = new SqlCommand("SELECT * FROM tblCommunication WHERE LetterType LIKE '" + txSearch.Text.Trim() + "' OR LetterNumber LIKE '" + txSearch.Text.Trim() + "' OR LetterAmount LIKE '" + txSearch.Text.Trim() + "' OR LetterFrom LIKE '" + txSearch.Text.Trim() + "' OR LetterTo LIKE '" + txSearch.Text.Trim() + "' OR ReceivedBy LIKE '" + txSearch.Text.Trim() + "' OR Requisition LIKE '" + txSearch.Text.Trim() + "' OR LetterSubject LIKE '" + txSearch.Text.Trim() + "' OR LetterContent LIKE '" + txSearch.Text.Trim() + "' OR LetterRemarks LIKE '" + txSearch.Text.Trim() + "'", cn);
                    DataTable dtAll = new DataTable();
                    //cn.Open();
                    adp.Fill(dtAll);
                    dgCommunications.DataSource = dtAll;
                    //cn.Close();
                    break;

                case "Incoming Communications":
                    adp.SelectCommand = new SqlCommand("SELECT CommType = '" + cmbCategory.Text + "', LetterDate, LetterReceived, LetterType, LetterNumber, LetterAmount, LetterFrom, LetterTo, ReceivedBy, Requisition, LetterSubject, LetterContent, LetterRemarks FROM tblCommunication WHERE LetterType LIKE '" + txSearch.Text.Trim() + "' OR LetterNumber LIKE '" + txSearch.Text.Trim() + "' OR LetterAmount LIKE '" + txSearch.Text.Trim() + "' OR LetterFrom LIKE '" + txSearch.Text.Trim() + "' OR LetterTo LIKE '" + txSearch.Text.Trim() + "' OR ReceivedBy LIKE '" + txSearch.Text.Trim() + "' OR Requisition LIKE '" + txSearch.Text.Trim() + "' OR LetterSubject LIKE '" + txSearch.Text.Trim() + "' OR LetterContent LIKE '" + txSearch.Text.Trim() + "' OR LetterRemarks LIKE '" + txSearch.Text.Trim() + "'", cn);
                    DataTable dtInc = new DataTable();
                   // cn.Open();
                    adp.Fill(dtInc);
                    dgCommunications.DataSource = dtInc;
                    //cn.Close();
                    break;

                case "Inside Communications":
                    adp.SelectCommand = new SqlCommand("SELECT CommType = '" + cmbCategory.Text + "', LetterDate, LetterReceived, LetterType, LetterNumber, LetterAmount, LetterFrom, LetterTo, ReceivedBy, Requisition, LetterSubject, LetterContent, LetterRemarks FROM tblCommunication WHERE LetterType LIKE '" + txSearch.Text.Trim() + "' OR LetterNumber LIKE '" + txSearch.Text.Trim() + "' OR LetterAmount LIKE '" + txSearch.Text.Trim() + "' OR LetterFrom LIKE '" + txSearch.Text.Trim() + "' OR LetterTo LIKE '" + txSearch.Text.Trim() + "' OR ReceivedBy LIKE '" + txSearch.Text.Trim() + "' OR Requisition LIKE '" + txSearch.Text.Trim() + "' OR LetterSubject LIKE '" + txSearch.Text.Trim() + "' OR LetterContent LIKE '" + txSearch.Text.Trim() + "' OR LetterRemarks LIKE '" + txSearch.Text.Trim() + "'", cn); ;
                    DataTable dtIns = new DataTable();
                    //cn.Open();
                    adp.Fill(dtIns);
                    dgCommunications.DataSource = dtIns;
                    //cn.Close();
                    break;

                case "Outgoing Communications":
                    adp.SelectCommand = new SqlCommand("SELECT CommType = '" + cmbCategory.Text + "', LetterDate, LetterReceived, LetterType, LetterNumber, LetterAmount, LetterFrom, LetterTo, ReceivedBy, Requisition, LetterSubject, LetterContent, LetterRemarks FROM tblCommunication WHERE LetterType LIKE '" + txSearch.Text.Trim() + "' OR LetterNumber LIKE '" + txSearch.Text.Trim() + "' OR LetterAmount LIKE '" + txSearch.Text.Trim() + "' OR LetterFrom LIKE '" + txSearch.Text.Trim() + "' OR LetterTo LIKE '" + txSearch.Text.Trim() + "' OR ReceivedBy LIKE '" + txSearch.Text.Trim() + "' OR Requisition LIKE '" + txSearch.Text.Trim() + "' OR LetterSubject LIKE '" + txSearch.Text.Trim() + "' OR LetterContent LIKE '" + txSearch.Text.Trim() + "' OR LetterRemarks LIKE '" + txSearch.Text.Trim() + "'", cn); ;
                    DataTable dtOut = new DataTable();
                    //cn.Open();
                    adp.Fill(dtOut);
                    dgCommunications.DataSource = dtOut;
                   // cn.Close();
                    break;
            }
        }

using (SqlConnection connection = new SqlConnection(conString))
{
    try
    {
        //your switch case statement
    }
    catch (InvalidOperationException)
    {

    }
    catch (SqlException)
    {

    }
}

public void Command(string query,string conString)
{
    SqlDataAdapter adp = new SqlDataAdapter();
    using (SqlConnection connection = new SqlConnection(conString))
    {

        try
        {
            adp.SelectCommand = new SqlCommand(query,connection);
            DataTable dtOut = new DataTable();
            adp.Fill(dtOut);
            dgCommunications.DataSource = dtOut;
        }
        catch (InvalidOperationException)
        {

        }
        catch (SqlException)
        {

        }
    }
}

SqlDataAdapter adp = new SqlDataAdapter();
using (SqlConnection connection = new SqlConnection(conString))
{

    try
    {
        switch (cmbCategory.Text)
        {
            case "All":
                break;

        }
    }
    catch (InvalidOperationException)
    {

    }
    catch (SqlException)
    {

    }
}

class DAL
{
    private readonly string _connectionString;
    public DAL(string connectionString)
    {
        _connectionString = connectionString;
    }

    public DataTable SearchAll(string searchText)
    {
        var sql = "SELECT * FROM tblCommunication "+
                  "WHERE LetterType = @searchText "+
                  "OR LetterNumber = @searchText  "+
                  "OR LetterAmount = @searchText  "+
                  "OR LetterFrom  = @searchText  "+
                  "OR LetterTo  = @searchText  "+
                  "OR ReceivedBy  = @searchText  "+
                  "OR Requisition  = @searchText  "+
                  "OR LetterSubject  = @searchText  "+
                  "OR LetterContent  = @searchText  "+
                  "OR LetterRemarks  = @searchText";

        var parameter = new SqlParameter("@searchText", SqlDbType.VarChar);
        parameter.Value = searchText;
        return GetDataTable(sql, CommandType.Text, parameter);
    }

    public DataTable SearchIncomingCommunications(string caterogy, string searchText)
    {
        // implementation same as the example above
        throw new NotImplementedException();
    }

    public DataTable SearchInsideCommunications(string caterogy, string searchText)
    {
        // implementation same as the example above
        throw new NotImplementedException();
    }

    // add other methods as well

 private DataTable GetDataTable(string sql, CommandType commandType, params SqlParameter[] parameters)
    {
        var dt = new DataTable();
        using (var con = new SqlConnection(_connectionString))
        {
            using (var cmd = new SqlCommand(sql, con))
            {
                cmd.CommandType = commandType;
                foreach (var parameter in parameters)
                {
                    cmd.Parameters.Add(parameter);
                }
                using (var da = new SqlDataAdapter(cmd))
                {
                    da.Fill(dt);
                }
            }
        }
        return dt;
    }
}