Compiler Error: Operator || cannot be applied to operands of type bool and string in ASP.Net

hi

I put below code in gridview:

                      
                         <asp:Label ID="Label14T" runat="server" Text='<%# Eval("GroupName")%>' CssClass="ViewNG" 
                               Visible = '<%#Eval("GroupName").ToString() == "فروشندگان ملک"||"مستاجرین" ? true : false %>'></asp:Label>

but below error happen:

Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. Compiler Error Message: CS0019: Operator '||' cannot be applied to operands of type 'bool' and 'string' Source Error:

Line 437:                    
Line 438:                             
Line 439:                         <asp:Label ID="Label14T" runat="server" Text='<%# Eval("GroupName")%>' CssClass="ViewNG" 
Line 440:                               Visible = '<%#Eval("GroupName").ToString() == "فروشندگان ملک"||"مستاجرین" ? true : false %>'></asp:Label>
Line 441:                                       </ItemTemplate>