C#

<asp:Panel ID="PanelAddCategory" runat="server">
                                    <asp:Table ID="TableAddCategory" runat="server">

                                        <asp:TableRow>
                                            <asp:TableCell>Name:</asp:TableCell>
                                            <asp:TableCell>
                                                <asp:TextBox ID="TextBoxNewCategoryName" runat="server" ValidationGroup="GroupAddCategory"></asp:TextBox>
                                            </asp:TableCell>
                                            <asp:TableCell>
                                                <asp:RequiredFieldValidator ID="RequiredFieldValidatorNewCategoryName" runat="server" ErrorMessage="*Required" ValidationGroup="GroupAddCategory" ControlToValidate="TextBoxNewCategoryName"></asp:RequiredFieldValidator>
                                                <asp:CustomValidator ID="CustomValidatorNewCategoryName" runat="server" ErrorMessage="This name already exists." ValidationGroup="GroupAddCategory" ControlToValidate="TextBoxNewCategoryName" OnServerValidate="TextBoxNewCategoryName_ServerValidate"></asp:CustomValidator>
                                            </asp:TableCell>
                                        </asp:TableRow>
                                        <asp:TableRow>
                                            <asp:TableCell>Description:</asp:TableCell>
                                            <asp:TableCell>
                                                <asp:TextBox ID="TextBoxNewDescription" runat="server" Rows="10" TextMode="MultiLine" ValidationGroup="GroupAddCategory"></asp:TextBox>
                                            </asp:TableCell>
                                            <asp:TableCell>
                                                <asp:RequiredFieldValidator ID="RequiredFieldValidatorNewDescription" runat="server" ErrorMessage="*Required" ValidationGroup="GroupAddCategory" ControlToValidate="TextBoxNewDescription" Display="Dynamic"></asp:RequiredFieldValidator>

                                            </asp:TableCell>
                                        </asp:TableRow>
                                        <asp:TableRow>
                                            <asp:TableCell> </asp:TableCell>
                                        </asp:TableRow>
                                        <asp:TableRow>
                                            <asp:TableCell>
                                                <asp:Button ID="ButtonAddCategory" runat="server" Text="Add" ValidationGroup="GroupAddCategory" OnClick="ButtonAddCategory_Click" />
                                            </asp:TableCell>
                                        </asp:TableRow>
                                    </asp:Table>
                                    <br />
                                </asp:Panel>

############################################################################################################

protected void TextBoxNewCategoryName_ServerValidate(object source, ServerValidateEventArgs args)
    {
        BLPasteCategorie blPasteCategorie = new BLPasteCategorie();

        if (TextBoxNewCategoryName.Text != blPasteCategorie.GetPasteCategorieByName(TextBoxNewCategoryName.Text).Naam)
        {
            args.IsValid = true;
        }
        else
        {
            args.IsValid = false;
        }
    }