How do I get an element by ID or Class in Blazor?

Blazor doesn’t manipulate the DOM directly at C# side. You can call the JavaScript method by using JavaScript Interop to get an element by ID or class. 

• The getElementsByClassName() method returns a collection of all elements in the document with the specified class names.
• The getElementById() method returns a collection of all elements in the document with the specified ID names.

[Index.razor]

@page "/"

@inject IJSRuntime JsRuntime

<h1 id="headingElement">Hello, world!</h1>

<p class="para-element">Welcome to your new app.</p>

@code {
    protected override async void OnAfterRender(bool firstRender)
    {
        await JsRuntime.InvokeVoidAsync("elementId");
    }
}

[_Host.cshtml]

<body>
      . . .
      . . .

      <script>
        function elementId() {
            // Get element with the specified ID name
            var idValue = document.getElementById("headingElement");
            console.log(idValue.innerHTML);
            // Get element with the specified Class name
            var classValue = document.getElementsByClassName("para-element");
            console.log(classValue[0].innerHTML);
        }
    </script>
</body>

Refer to “Call JavaScript functions from .NET methods in ASP.NET Core Blazor” for more details.