Join multiple table using Linq and display in GridView using C# in ASP.Net

Your code will be like below. As per your design bind data to datalist.

SQL

CREATE PROC GetMessage
	@UserName VARCHAR(50)
AS
BEGIN
	SELECT * FROM dbo.USERPost 
	WHERE UserName = @UserName
END


CREATE PROC GetShared
	@UserName VARCHAR(50)
AS
BEGIN
	SELECT * FROM dbo.SharePost
	WHERE UserName1 = @UserName
END

HTML

<div>
    <asp:GridView runat="server" ID="GetMessage" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="UserName" HeaderText="UserName" />
            <asp:BoundField DataField="FriendUserName" HeaderText="FriendUserName" />
            <asp:BoundField DataField="ContentPost" HeaderText="ContentPost" />
            <asp:BoundField DataField="SendDate" HeaderText="SendDate" />
            <asp:BoundField DataField="ImageName1" HeaderText="ImageName1" />
            <asp:BoundField DataField="ImageName2" HeaderText="ImageName2" />
            <asp:BoundField DataField="ImageName3" HeaderText="ImageName3" />
            <asp:BoundField DataField="UserName1" HeaderText="UserName1" />
            <asp:BoundField DataField="FriendUserName1" HeaderText="FriendUserName1" />
            <asp:BoundField DataField="ContentPost1" HeaderText="ContentPost1" />
            <asp:BoundField DataField="SendDate1" HeaderText="SendDate1" />
            <asp:BoundField DataField="ImageName01" HeaderText="ImageName01" />
            <asp:BoundField DataField="ImageName02" HeaderText="ImageName02" />
            <asp:BoundField DataField="ImageName03" HeaderText="ImageName03" />
            <asp:BoundField DataField="ImageName04" HeaderText="ImageName04" />
        </Columns>
    </asp:GridView>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (Session["UserName"] != null && Session["UserName"].ToString() != string.Empty)
    {
        string username = Session["UserName"].ToString();
        if (!IsPostBack)
        {
            SqlParameter[] paramUserpost = new SqlParameter[]{
            new SqlParameter("@UserName",username)
        };
            DataTable userpost = GetDataTable("GetMessage", paramUserpost);

            SqlParameter[] paramShare = new SqlParameter[]{
            new SqlParameter("@UserName",username)
        };
            DataTable share = GetDataTable("GetShared", paramShare);

            var result = from usp in userpost.AsEnumerable()
                            join shr in share.AsEnumerable() on usp.Field<string>("UserName") equals shr.Field<string>("UserName1")
                            where usp.Field<string>("UserName") == username
                            select new
                            {
                                UserName = usp.Field<string>("UserName"),
                                FriendUserName = usp.Field<string>("FriendUserName"),
                                ContentPost = usp.Field<string>("ContentPost"),
                                SendDate = usp.Field<DateTime>("SendDate"),
                                ImageName1 = usp.Field<string>("ImageName1"),
                                ImageName2 = usp.Field<string>("ImageName2"),
                                ImageName3 = usp.Field<string>("ImageName3"),
                                UserName1 = shr.Field<string>("UserName1"),
                                FriendUserName1 = shr.Field<string>("FriendUserName1"),
                                ContentPost1 = shr.Field<string>("ContentPost1"),
                                SendDate1 = shr.Field<DateTime>("SendDate1"),
                                ImageName01 = shr.Field<string>("ImageName01"),
                                ImageName02 = shr.Field<string>("ImageName02"),
                                ImageName03 = shr.Field<string>("ImageName03"),
                                ImageName04 = shr.Field<string>("ImageName04")
                            };
            this.GetMessage.DataSource = result;
            this.GetMessage.DataBind();
        }
    }
}

private DataTable GetDataTable(string query, SqlParameter[] param)
{
    DataTable dt = new DataTable();
    using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings[1].ConnectionString))
    {
        using (SqlCommand cmd = new SqlCommand(query, con))
        {
            con.Open();
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.AddRange(param);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt);
            con.Close();
        }
    }
    return dt;
}

Screenshot

StoredProcedure OutPut

Page OutPut