<script>
        function agreeForm(f)
        {
            if (f.UserName = "")
                f.Save.disabled = "1"
        }
</script>
<form asp-controller="Calculator" asp-action="Edit" method="post">
    @Html.HiddenFor(m => m.ErrorCode)
    @Html.HiddenFor(m => m.Id)

    <table>
        <tr>
            <th>Computer name:</th>
            <th>User Name:</th>
            <th>Expression:</th>
            <th>Result:</th>
            <th>Error:</th>
            <th>Date:</th>
        </tr>
        <tr>
            <td>@Html.EditorFor(m => m.ComputerName)</td>
            <td>@Html.EditorFor(m => m.UserName)</td>
            <td>@Html.EditorFor(m => m.Expression)</td>
            <td>@Html.EditorFor(m => m.Result)</td>
            <td>@Html.EditorFor(m => m.Description)</td>
            <td>@Html.EditorFor(m => m.Date)</td>
        </tr>
    </table>

    <input type="submit" name="Save" class="btn btn-default" value="Save" disabled="0" onchange="agreeForm(this.form)"/>
</form>