I am performing server side validation on Business Loging Layer (BLL) using 3 tier architecture in asp.net. The problem I am facing is : I don't know what is this "ModelState" and what libararies/dll, I have to add for getting it. I can perform server side validation on product.aspx.cs file but I need to do it in BLL. I read each and every word and link of this question but got zero help. Kindly help me to perform validation in ProductBL.cs file by guiding me how to get ModelState. Here is my Product.aspx file code: <asp:Label ID="lbProductName" AssociatedControlID="txtProductName" runat="server">Product Name</asp:Label> <asp:TextBox ID="txtProductName" runat="server"></asp:TextBox> <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txtProductName" CssClass="field-validation-error" ErrorMessage="Required"> </asp:RequiredFieldValidator> My Product.aspx.cs file code: protected void btnSaveProduct_Click(object sender, EventArgs e) { ProductBL produc

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