Hi Lewis,

I changed the code a little. Because when you press the button, projects will come according to that category id. I do not come id that way. 

Also how to get projects by category. Isn't jquery right?

Index.cshtml

                <div id="Projeler" class="filter-items">
                    <div class="row">
                        @foreach (var item in ViewBag.Proje)
                        {
                            <div id="catid" class="mix col-lg-4 col-md-4 col-sm-12 col-12 @item.CatId">
                                <div class="box-item">
                                    <a id="catresim" class="resim" href="~/resimler/@item.ResimYolu" asp-append-version="true" 
                                       data-rel="lightcase">
                                        <img class="img-fluid resim" src="~/resimler/@item.ResimYolu" alt="item1" />
                                    </a>
                                    <a class="resim" href="~/resimler/@item.ResimYolu" data-rel="lightcase">
                                        <h3 id="catitem" class="item-name mb-0">@item.Baslik</h3>
                                    </a>
                                    <span id="catname" class="cat-name">@item.İcerik</span>
                                </div>
                            </div>

                        }

                    </div>
                </div>

Jquery,

    <script type="text/javascript">
            $(document).ready(function () {
                $(".but").click(function () {
                    function GetProje(id) {
                        $.ajax({
                            type: 'GET',
                            url: '/Home/GetProje/',
                            data: { id: id },
                            contentType: "application/json; charset=utf-8",
                            dataType: 'json',
                            success: function (data) {
                               
                                $.each(data, function (i, item) {

                                    $("#catid").append(item.catId);
                                    $("#catresim").append(item.resimYolu);
                                    $(".resim").append(item.resimYolu);
                                    $("#catitem").append(item.baslik);
                                    $("#catname").append(item.İcerik);

                                });

                            },
                            error: function (response) {
                                alert(response.responseText);
                            }
                        });
                    };
                });
            });
        
        </script>