在 Asp.net Core 中，要执行控制器操作并渲染视图，您可以使用 View Components。如果要使用 RenderAction 方法，则必须添加 HtmlHelperViewExtensions，并在 Html Helper 中添加 @Html.RenderAction() 方法，更多详细信息可以查看this thread。
在您的场景中，我建议您可以使用 JQuery Ajax 调用 action 方法并从数据库中获取数据，然后在 Ajax 成功方法中更新页面内容。您可以参考以下示例：
控制器中的代码：
public IActionResult GetMessage()
{
//query database, and get the data.
return Content("10000$");
}
Layout.cshtml 中的代码：
添加<span> 显示返回数据：
<div id="txt_navbar_output">
<h4 class="col-4 text-center" style="font-size:16px;font-weight:bold;color:forestgreen;border-style:inset;">
<img src="~/Images/Coin.jpg" alt="Site Logo" style="height:30px; width: 30px" />
Current Balance : <span id="returndata">3000$</span>
</h4>
</div>
页面加载后，使用JQuery Ajax调用action方法获取数据，然后更新内容。
<script>
$(function () {
$.ajax({
type: "Get",
url: "/Home/GetMessage",
success: function (data) {
//update the page content.
$('#returndata').html(""); //clear the content
$('#returndata').html(data); //add the latest data.
},
error: function (response) {
console.log(response.responseText);
}
});
});
</script>
输出如下：值已更改为“10000$”。
编辑：
关于“Uncaught ReferenceError: $ is not defined at (index)”错误，好像是JQuery引用没有添加成功，尝试在JQuery脚本前添加JQuery引用：
<script src="~/lib/jquery/dist/jquery.min.js"></script>
<script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js"></script>
<script src="~/js/site.js" asp-append-version="true"></script>
<script>
$(function () {
$.ajax({
type: "Get",
url: "/Home/GetMessage",
success: function (data) {
//update the page content.
$('#returndata').html(""); //clear the content
$('#returndata').html(data); //add the latest data.
},
error: function (response) {
console.log(response.responseText);
}
});
});
</script>
您也可以使用 CDN 参考：
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
_layout.csthml 资源如下：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>@ViewData["Title"] - Test</title>
<link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.min.css" />
<link rel="stylesheet" href="~/css/site.css" />
</head>
<body>
<header>
<nav class="navbar navbar-expand-sm navbar-toggleable-sm navbar-light bg-white border-bottom box-shadow mb-3">
<div class="container">
<a class="navbar-brand" asp-area="" asp-controller="Home" asp-action="Index">Test</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target=".navbar-collapse" aria-controls="navbarSupportedContent"
aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="navbar-collapse collapse d-sm-inline-flex flex-sm-row-reverse">
<ul class="navbar-nav flex-grow-1">
<li class="nav-item">
<a class="nav-link text-dark" asp-area="" asp-controller="Home" asp-action="Index">Home</a>
</li>
<li class="nav-item">
<a class="nav-link text-dark" asp-area="" asp-controller="Home" asp-action="Privacy">Privacy</a>
</li>
</ul>
</div>
<div id="txt_navbar_output">
<h4 class="col-4 text-center" style="font-size:16px;font-weight:bold;color:forestgreen;border-style:inset;">
<img src="~/Images/Coin.jpg" alt="Site Logo" style="height:30px; width: 30px" />
Current Balance : <span id="returndata">3000$</span>
</h4>
</div>
</div>
</nav>
</header>
<div class="container">
<main role="main" class="pb-3">
@RenderBody()
</main>
</div>
<footer class="border-top footer text-muted">
<div class="container">
© 2020 - Test - <a asp-area="" asp-controller="Home" asp-action="Privacy">Privacy</a>
</div>
</footer>
<script src="~/lib/jquery/dist/jquery.min.js"></script>
<script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js"></script>
<script src="~/js/site.js" asp-append-version="true"></script>
@RenderSection("Scripts", required: false)
<script>
$(function () {
$.ajax({
type: "Get",
url: "/Home/GetMessage",
success: function (data) {
//update the page content.
$('#returndata').html(""); //clear the content
$('#returndata').html(data); //add the latest data.
},
error: function (response) {
console.log(response.responseText);
}
});
});
</script>
</body>
</html>