Set DropDownList Selected Value in GridView Row Edit using C# and VB.Net in ASP.Net

akhter says:

string selectcontainer = DataBinder.Eval(e.Row.DataItem, "Contnam").ToString();

Problem is in you are using FindByValue method and passing the CN_Name to find which should be CN_ID.

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

HTML

<asp:GridView ID="GridView1" runat="server" AllowSorting="True" DataKeyNames="CN_ID"
    HorizontalAlign="Center" AutoGenerateColumns="false" CssClass="mygrdContent"
    HeaderStyle-BackColor="#66ccff" HeaderStyle-CssClass="header" HeaderStyle-ForeColor="White"
    OnRowDataBound="GridView1_RowDataBound" 
    PagerStyle-CssClass="pager" RowStyle-CssClass="rows" Width="452px" ShowFooter="True"
    OnRowCancelingEdit="GridView1_RowCancelingEdit" OnRowEditing="GridView1_RowEditing"
    OnRowUpdating="GridView1_RowUpdating">
    <Columns>
        <asp:TemplateField HeaderText="ID" Visible="false">
            <ItemTemplate>
                <asp:Label ID="D_CID" runat="server" Text='<%#Bind("CN_ID")%>'></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Container">
            <ItemTemplate>
                <%#Eval("CN_Name")%>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:DropDownList ID="Contnam" runat="server" Width="150px">
                </asp:DropDownList>
            </EditItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="QTY">
            <ItemTemplate>
                <asp:Label ID="QTY" runat="server" Text='<%#Bind("QTY")%>'></asp:Label>
            </ItemTemplate>
            <EditItemTemplate>
                <asp:TextBox ID="QTY" runat="server" Text='<%#Eval("QTY") %>'></asp:TextBox>
            </EditItemTemplate>
            <FooterTemplate>
                <asp:Label ID="lblTotal1" runat="server"></asp:Label>
            </FooterTemplate>
        </asp:TemplateField>
        <asp:CommandField ShowEditButton="TRUE" ShowDeleteButton="True" ControlStyle-ForeColor="Blue" />
    </Columns>
    <HeaderStyle BackColor="#66CCFF" ForeColor="White" />
    <SelectedRowStyle BackColor="Yellow" />
</asp:GridView>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.AddRange(new DataColumn[3] { new DataColumn("CN_ID"), new DataColumn("CN_Name"), new DataColumn("QTY") });
        dt.Rows.Add(1, "Mango", 10);
        dt.Rows.Add(2, "Apple", 12);
        dt.Rows.Add(3, "Orange", 12);
        dt.Rows.Add(4, "Banana", 10);
        ViewState["dt"] = dt;
        this.BindGrid();
    }
}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow && GridView1.EditIndex == e.Row.RowIndex)
    {
        DropDownList ddlContnam = (DropDownList)e.Row.FindControl("Contnam");
        DataTable dt = ViewState["dt"] as DataTable;
        ddlContnam.DataSource = dt;
        ddlContnam.DataTextField = "CN_Name";
        ddlContnam.DataValueField = "CN_ID";
        ddlContnam.DataBind();
        string selectcontainer = DataBinder.Eval(e.Row.DataItem, "CN_ID").ToString();
        if (ddlContnam.Items.FindByValue(selectcontainer) != null)
        {
            ddlContnam.Items.FindByValue(selectcontainer).Selected = true;
        }
    }
}

protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
{
    GridView1.EditIndex = e.NewEditIndex;
    this.BindGrid();
}

protected void GridView1_RowUpdating(object sender, EventArgs e)
{
}

protected void GridView1_RowCancelingEdit(object sender, EventArgs e)
{
    GridView1.EditIndex = -1;
    this.BindGrid();
}

protected void BindGrid()
{
    GridView1.DataSource = ViewState["dt"] as DataTable;
    GridView1.DataBind();
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim dt As DataTable = New DataTable()
        dt.Columns.AddRange(New DataColumn(2) {New DataColumn("CN_ID"), New DataColumn("CN_Name"), New DataColumn("QTY")})
        dt.Rows.Add(1, "Mango", 10)
        dt.Rows.Add(2, "Apple", 12)
        dt.Rows.Add(3, "Orange", 12)
        dt.Rows.Add(4, "Banana", 10)
        ViewState("dt") = dt
        Me.BindGrid()
    End If
End Sub

Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow AndAlso GridView1.EditIndex = e.Row.RowIndex Then
        Dim ddlContnam As DropDownList = CType(e.Row.FindControl("Contnam"), DropDownList)
        Dim dt As DataTable = TryCast(ViewState("dt"), DataTable)
        ddlContnam.DataSource = dt
        ddlContnam.DataTextField = "CN_Name"
        ddlContnam.DataValueField = "CN_ID"
        ddlContnam.DataBind()
        Dim selectcontainer As String = DataBinder.Eval(e.Row.DataItem, "CN_ID").ToString()

        If ddlContnam.Items.FindByValue(selectcontainer) IsNot Nothing Then
            ddlContnam.Items.FindByValue(selectcontainer).Selected = True
        End If
    End If
End Sub

Protected Sub GridView1_RowEditing(ByVal sender As Object, ByVal e As GridViewEditEventArgs)
    GridView1.EditIndex = e.NewEditIndex
    Me.BindGrid()
End Sub

Protected Sub GridView1_RowUpdating(ByVal sender As Object, ByVal e As EventArgs)
End Sub

Protected Sub GridView1_RowCancelingEdit(ByVal sender As Object, ByVal e As EventArgs)
    GridView1.EditIndex = -1
    Me.BindGrid()
End Sub

Protected Sub BindGrid()
    GridView1.DataSource = TryCast(ViewState("dt"), DataTable)
    GridView1.DataBind()
End Sub

Screenshot