ASMX async System.InvalidOperationException：此时无法启动异步操作

Answer 1

ASMX 不支持async。在引入async 时，它已经有好几代了，所以它从未更新为支持async。

It does support APM-style asynchrony，所以你可以让它工作，但这会很尴尬。

首先，您需要确保您的目标是 .NET 4.5 或更高版本以及have httpRuntime.targetFramework set to 4.5 or higher。

然后，您可以使用 APM 方法定义您的 API，并包装更自然的 TAP 方法。包装可能有点痛苦，所以我建议使用我的AsyncEx library 中的ApmTaskFactory，如下所示：

[WebMethod(Description = "Return list of Countries")]
public IAsyncResult BeginLoadCountries(int ID, AsyncCallback callback, object state)
{
  var task = LoadCountriesAsync(ID);
  return ApmAsyncFactory.ToBegin(task, callback, state);
}

private static async Task<string> LoadCountriesAsync(int ID)
{
  var retval = new List<DDListItemLight>();
  retval = await DropDownData.GetCountriesListAsync();
  return JsonConvert.SerializeObject(retval);
}

public void EndLoadCountries(IAsyncResult asyncResult)
{
  var responseStr = ApmAsyncFactory.ToEnd<string>(asyncResult);
  Context.Response.Write(responseStr);
}

Answer 2

虽然接受的答案有效，但我决定尝试一些不同的东西并使用像 MVC 这样的控制器。 （最终将转换为 Blazor WASM，因此编写这些控制器并不是浪费时间）

这是控制器类：

public class DDServiceController : ApiController
{
    private static string DbConn;
    DropDownData dropDownData = new DropDownData();

    public DDServiceController()
    {
        DbConn = ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString;
        dropDownData.SetDBConnStr(DbConn);
    }

    [Route("api/LoadCountries")]
    [HttpGet]
    public async Task<IHttpActionResult> LoadCountries(int id)
    {
        var retval = new List<DDListItemLight>();
        retval = await dropDownData.GetCountriesListAsync();
        return Ok(retval);
    }

    [Route("api/LoadStateProvinces")]
    [HttpGet]
    public async Task<IHttpActionResult> LoadStateProvinces(int id)
    {
        var retval = new List<DDListItemLight>();
        retval = await dropDownData.GetStateProvincesByCountryListAsync(id);
        return Ok(retval);
    }

    [Route("api/LoadCityLocalities")]
    [HttpGet]
    public async Task<IHttpActionResult> LoadCityLocalities(int id)
    {
        var retval = new List<DDListItemLight>();
        retval = await dropDownData.GetCityLocalitiesByState_ProvinceListAsync(id);
        return Ok(retval);
    }

    [Route("api/loadsalutations")]
    [HttpGet]
    public async Task<IHttpActionResult> LoadSalutations(int id)
    {
        var retval = new List<DDListItemLight>();
        retval = await dropDownData.GetSalutationsListAsync();
        return Ok(retval);
    }

}

然后像这样从js调用它：

    function RetrieveDD(ddl, ddtype, id) {
    var callback = arguments.length <= 3 || arguments[3] === undefined ? false : arguments[3];

    var servicename;
    var hid;
    switch (ddtype) {
        case 'salutation':
            servicename = "LoadSalutations";
            hid = $("#hidSelectedSalutation").val();
            break;
        case 'country':
            servicename = "LoadCountries";
            hid = $("#hidSelectedCountry").val();
            break;
        case 'state':
            servicename = "LoadStateProvinces";
            hid = $("#hidSelectedState").val();
            break;
        case 'city':
            servicename = "LoadCityLocalities";
            hid = $("#hidSelectedCity").val();
            break;
        case 'salutation':
            servicename = "LoadSalutations";
            hid = $("#hidSelectedSalutation").val();
            break;
        default:
            hid = "";
            servicename = "";
    }
    if (servicename != "") {
        $.ajax({
            url: '../../api/' + servicename,
            dataType: 'json',
            method: 'get',
            data: { ID: id },
            success: function success(data) {
                ddl.empty();
                if (hid != "") {
                    $.each(data, function () {
                        if (this['V'] == hid) {
                            var newOption = new Option(this['T'], this['V'], true, true);
                            ddl.append(newOption);
                        } else {
                            var newOption = new Option(this['T'], this['V'], false, false);
                            ddl.append(newOption);
                        }
                    });
                } else {
                    $.each(data, function (index) {
                        if (index == 0) {
                            var newOption = new Option(this['T'], this['V'], true, true);
                            ddl.append(newOption);
                        } else {
                            var newOption = new Option(this['T'], this['V'], false, false);
                            ddl.append(newOption);
                        }
                    });
                };
                if (typeof callback === 'function') {
                    callback(data);
                };
            },
            error: function error(err) {
                console.log('Error (RetrieveDD): ' + JSON.stringify(err, null, 2));
                if (typeof callback === 'function') {
                    callback(err);
                }
            }
        });
    };
};