Display Name instead of Id in ASP.Net GridView using C#

I have a grid view like below, that I want to show the firstName and LastName of professor in the ProfessorId field:

                    <asp:GridView ID="GridView2" runat="server" AutoGenerateColumns="False" BackColor="White" BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px" CellPadding="3"
                        DataKeyNames="Id" DataSourceID="SsCourse" Visible="False" Width="845px" AutoGenerateDeleteButton="True" AutoGenerateEditButton="True" CssClass="auto-style19">
                        <Columns>                            
                            <asp:BoundField DataField="Code" HeaderText="Code" SortExpression="Code" />
                            <asp:TemplateField HeaderText=" Course Title" SortExpression="Title">
                                <EditItemTemplate>
                                    <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("Title") %>'></asp:TextBox>
                                </EditItemTemplate>
                                <ItemTemplate>
                                    <asp:Label ID="Label1" runat="server" Text='<%# Bind("Title") %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="Professor" SortExpression="ProfessorId">
                                <EditItemTemplate>
                                    <asp:DropDownList ID="DropDownList3" runat="server" DataSourceID="ShowProf" DataTextField="FirstName" DataValueField="FirstName" SelectedValue='<%# ShowProf(Eval("SubjectStudyId")) %>'>
                                    </asp:DropDownList>
                                    <asp:SqlDataSource ID="ShowProf" runat="server" ConnectionString="<%$ ConnectionStrings:KDUIS-v1ConnectionString %>" SelectCommand="SELECT IMS_Person.FirstName, IMS_Person.LastName, EDU_SubjectStudy_Professor.SubjectStudyId FROM EDU_Professor INNER JOIN EDU_SubjectStudy_Professor ON EDU_Professor.Id = EDU_SubjectStudy_Professor.ProfessorId INNER JOIN IMS_Person ON EDU_Professor.PersonId = IMS_Person.Id WHERE (EDU_SubjectStudy_Professor.SubjectStudyId = @SSId)">
                                        <SelectParameters>
                                            <asp:ControlParameter ControlID="LblSubjectStudyId" Name="SSId" PropertyName="Text" />
                                        </SelectParameters>
                                    </asp:SqlDataSource>
                                </EditItemTemplate>
                                <ItemTemplate>
                                    <asp:Label ID="Label4" runat="server" Text='<%# ShowProfessor(Eval("ProfessorId")) %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="Sexuality" SortExpression="Sexuality">
                                <EditItemTemplate>
                                    <asp:DropDownList ID="SexulityList" runat="server">
                                        <asp:ListItem Value="1">آقایان</asp:ListItem>
                                        <asp:ListItem Value="2">خانم ها</asp:ListItem>
                                        <asp:ListItem Value="3">مختلط</asp:ListItem>
                                    </asp:DropDownList>
                                </EditItemTemplate>
                                <ItemTemplate>
                                    <asp:Label ID="Label2" runat="server" Text='<%# ShowSexuality(Eval("Sexuality")) %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:TemplateField HeaderText="SessionNo" SortExpression="SessionNo">
                                <EditItemTemplate>
                                    <asp:DropDownList ID="DropDownList2" runat="server" DataSourceID="ShowProf" DataTextField="FirstName" DataValueField="FirstName">
                                    </asp:DropDownList>
                                    <asp:SqlDataSource ID="ShowProf" runat="server" ConnectionString="<%$ ConnectionStrings:KDUIS-v1ConnectionString %>" SelectCommand="SELECT IMS_Person.FirstName, IMS_Person.LastName FROM EDU_SubjectStudy_Professor INNER JOIN EDU_Professor ON EDU_SubjectStudy_Professor.ProfessorId = EDU_Professor.Id INNER JOIN IMS_Person ON EDU_Professor.PersonId = IMS_Person.Id WHERE (EDU_SubjectStudy_Professor.SubjectStudyId = @SSId)">
                                        <SelectParameters>
                                            <asp:ControlParameter ControlID="LblSubjectStudyId" Name="SSId" PropertyName="Text" />
                                        </SelectParameters>
                                    </asp:SqlDataSource>
                                </EditItemTemplate>
                                <ItemTemplate>
                                    <asp:Label ID="Label3" runat="server" Text='<%# ShowProfessor(Eval("SessionNo")) %>'></asp:Label>
                                </ItemTemplate>
                            </asp:TemplateField>

                            <asp:HyperLinkField DataNavigateUrlFields="Code" DataNavigateUrlFormatString="EditCourseSession.aspx?code={0}" HeaderText="مشاهده برنامه هفتگی" Text="مشاهده برنامه هفتگی" />
                        </Columns>                       
                    </asp:GridView>

 And I use this query to bind the data of the grid:

SELECT EDU_Lesson.Title, EDU_Course.ProfessorId, EDU_Course.Sexuality, EDU_Course.SessionNo, EDU_SubjectStudy_Lesson.SubjectStudyId,
 EDU_Course.Description, EDU_Course.Id, EDU_Course.Code FROM EDU_Course INNER JOIN EDU_Lesson ON EDU_Course.LessonId = EDU_Lesson.Id
 INNER JOIN EDU_SubjectStudy_Lesson ON EDU_Lesson.Id = EDU_SubjectStudy_Lesson.LessonId WHERE (EDU_SubjectStudy_Lesson.SubjectStudyId =
 @subject)

now for showing the firstName and LastName of professor, I use this code:

        protected string ShowProfessor(object i)
        {
            /// Get the PersonId of professor Table
            SqlCommand cm = new SqlCommand("select PersonId from EDU_Professor where Id Like N'%" + Convert.ToInt32(i) + "%'", con);
            SqlDataReader dr;
            con.Open();
            sqlcom.Connection = con;
            dr = cm.ExecuteReader();
            while (dr.Read())
            {
                LblPersonId.Text = dr["PersonId"].ToString();
            }
            sqlcom.Connection.Close();

            /// Get the PersonId of professor Table
            SqlCommand cm2 = new SqlCommand("select FirstName,LastName from IMS_Person where Id Like N'%" + LblPersonId.Text + "%'", con);
            SqlDataReader dr2;
            con.Open();
            sqlcom.Connection = con;
            dr2 = cm2.ExecuteReader();
            while (dr2.Read())
            {
                FirstName.Text = dr2["FirstName"].ToString();
                LastName.Text = dr2["LastName"].ToString();
            }
            sqlcom.Connection.Close();
            return FirstName.Text + "" + LastName.Text;
        }

but it returns the wrong name to me:

and I have this table for the info of person and professor:

how can I do this correctly?