In this blog, I will explain about below point through C# code:
1. Bind all list in a dropdown
2 Load List items in a Gridview once a user changes list name in dropdown.

You need to create a visual webpart in Visual studio 2010.

Visual WebPart ASCX code:

<asp:DropDownList ID=”ddlList” AutoPostBack=”true” runat=”server” 
    onselectedindexchanged=”ddlList_SelectedIndexChanged”>
</asp:DropDownList>
<p>
    <asp:Label ID=”lblItemCount” runat=”server” Text=””></asp:Label>
</p>
<asp:GridView ID=”gvListItem” AutoGenerateColumns=”false” runat=”server”>
</asp:GridView>

Visual Webpart .ascx.cs code:

Add below name space:

using Microsoft.SharePoint;

 protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                var web = SPContext.Current.Web;
                ddlList.DataSource = web.Lists;
                ddlList.DataTextField = “Title”;
                ddlList.DataBind();
            }
        }

        protected void ddlList_SelectedIndexChanged(object sender, EventArgs e)
        {
            
            var web = SPContext.Current.Web;
            var list = web.Lists[ddlList.SelectedItem.Text];
            lblItemCount.Text = list.Items.Count.ToString();
            gvListItem.DataSource = list.Items.GetDataTable();
            gvListItem.DataBind();

        }