第一步是在父帐户中包含导航属性：

public virtual Account ParentAccount { get; set; }

然后包括父帐户，并按父AccountName分组：

var accountsGrouped = db.Accounts
    .Include(x => x.ParentAccount) // Include parent accounts
    .Where(x => x.Status == true && x.CompanyID == CompanyID)
    .OrderBy(x => x.ParentAccount.AccountName)
    .GroupBy(x => x.ParentAccount.AccountName) // Group by parent account name
    .ToList();

ViewBag.AccountsGrouped = accountsGrouped;

然后，您必须手动创建select元素以及option和optgroup元素：

<div class="col-sm-8">
    <select id="AccountTypes" name="CurrentAccountID" class="form-select acctype" required oninvalid="this.setCustomValidity('Please Select the Account Type')" oninput="this.setCustomValidity('')">
        <option value="">--Select Account Type--</option>
        @foreach (var group in ViewBag.AccountsGrouped)
        {
            <optgroup label="@group.Key"> <!-- group.Key is the parent AccountName -->
                @foreach (var item in group)
                {
                    <option value="@item.ID">@item.AccountName</option>
                }
            </optgroup>
        }
    </select>
</div>