Dynamic WHERE clause in LINQ query based on DropDownList selection using C# and VB.Net in ASP.Net

Hi nauna,

Use Swich case to build your query.

Check this example. Now please take its reference and correct your code.

Database

For this example I have used of Northwind database that you can download using the link given below.

Download Northwind Database

HTML

<asp:DropDownList runat="server" Width="200px" ID="ddlFilters">
    <asp:ListItem Text="All" Value="All" />
    <asp:ListItem Text="=" Value="=" />
    <asp:ListItem Text=">" Value=">" />
    <asp:ListItem Text="<" Value="<" />
    <asp:ListItem Text="&lt;=" Value="&lt;=" />
    <asp:ListItem Text="&gt;=" Value="&gt;=" />
</asp:DropDownList>
<asp:TextBox runat="server" ID="txtValue" />
<asp:Button ID="Button1" Text="Search" runat="server" OnClick="Search" />
<br />
<br />
<asp:GridView runat="server" ID="gvEmployees" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="EmployeeId" HeaderText="Id" />
        <asp:BoundField DataField="FirstName" HeaderText="Name" />
        <asp:BoundField DataField="City" HeaderText="City" />
        <asp:BoundField DataField="Country" HeaderText="Country" />
    </Columns>
</asp:GridView>

Code

C#

protected void Search(object sender, EventArgs e)
{
    NorthwindModel.NorthwindEntities entities = new NorthwindModel.NorthwindEntities();
    List<NorthwindModel.Employee> query = new List<NorthwindModel.Employee>();
    string filter = ddlFilters.SelectedValue;
    int value = Convert.ToInt32(!string.IsNullOrEmpty(txtValue.Text) ? txtValue.Text : "0");
    switch (filter)
    {
        case "=":
            query = (from c in entities.Employees
                        where c.EmployeeID == value
                        select c).ToList();
            break;
        case ">":
            query = (from c in entities.Employees
                        where c.EmployeeID > value
                        select c).ToList();
            break;
        case "<":
            query = (from c in entities.Employees
                        where c.EmployeeID < value
                        select c).ToList();
            break;
        case ">=":
            query = (from c in entities.Employees
                        where c.EmployeeID >= value
                        select c).ToList();
            break;
        case "<=":
            query = (from c in entities.Employees
                        where c.EmployeeID <= value
                        select c).ToList();
            break;
        default:
            query = (from c in entities.Employees
                        select c).ToList();
            break;
    }
    gvEmployees.DataSource = query;
    gvEmployees.DataBind();
}

VB.Net

Protected Sub Search(ByVal sender As Object, ByVal e As EventArgs)
    Dim entities As NorthwindModel.NorthwindEntities = New NorthwindModel.NorthwindEntities()
    Dim query As List(Of NorthwindModel.Employee) = New List(Of NorthwindModel.Employee)()
    Dim filter As String = ddlFilters.SelectedValue
    Dim value As Integer = Convert.ToInt32(If(Not String.IsNullOrEmpty(txtValue.Text), txtValue.Text, "0"))
    Select Case filter
        Case "="
            query = (From c In entities.Employees Where c.EmployeeID = value Select c).ToList()
        Case ">"
            query = (From c In entities.Employees Where c.EmployeeID > value Select c).ToList()
        Case "<"
            query = (From c In entities.Employees Where c.EmployeeID < value Select c).ToList()
        Case ">="
            query = (From c In entities.Employees Where c.EmployeeID >= value Select c).ToList()
        Case "<="
            query = (From c In entities.Employees Where c.EmployeeID <= value Select c).ToList()
        Case Else
            query = (From c In entities.Employees Select c).ToList()
    End Select
    gvEmployees.DataSource = query
    gvEmployees.DataBind()
End Sub

Screenshot