我该怎么做才能使这个 foreach 循环运行得更快，因为它需要很长时间才能执行？

Question

我该怎么做才能使这个循环运行得更快？

    private void accessVendorGridData()
    {
        try
        {
            foreach (var item in getAllVendorList)
            {
                item.CurrencyName = "USD";

                // Fetch Addresses in Vendor Grid
                var Addr = _vendorservice.GetAllVendorAdd().Where(x => x.vendorId == item.Id).ToList();
                if (Addr.Count > 0)
                {
                    item.VendorAddressLine = String.Format("{0}, {1}, {2}, {3}, {4}", Addr[0].Address, Addr[0].City, Addr[0].StateProvince, Addr[0].ZipPostalCode, Addr[0].CountryRegion);
                }

                // Fetch Payment terms in Vendor Grid
                var paymentTerm = _vendorservice.GetAllPaymentTerms().Where(x => x.Id == item.PaymentTermId).ToList().SingleOrDefault();
                if (paymentTerm != null)
                {
                    item.paymenttermitem = paymentTerm.Name;
                }

                // Fetch Tax Scheme in Vendor Grid
                var taxscheme = _vendorservice.GetAllTaxScheme().Where(x => x.Id == item.TaxschemeId).ToList().SingleOrDefault();
                if (taxscheme != null)
                {
                    item.TaxschemeName = taxscheme.TaxSchemaName;
                }
            }
        }
        catch (Exception ex)
        {
            _exLog.AddErrorLog(ex, "NewVendor, accessVendorGridData()");
            ModernDialog.ShowMessage(ex.Message, "Error!", MessageBoxButton.OK);
        }
    }

---

我能做些什么来让这个循环运行得更快？我尝试了 Parallel.ForEach 但在两者之间迷路了。有人可以帮忙吗？

private void accessVendorGridData()
    {
            foreach (var item in getAllVendorList)
            {
                var Addr = _vendorservice.GetAllVendorAdd().Where(x => x.vendorId == item.Id).ToList();
                var paymentTerm = _vendorservice.GetAllPaymentTerms().Where(x => x.Id == item.PaymentTermId).ToList().SingleOrDefault();
                var taxscheme = _vendorservice.GetAllTaxScheme().Where(x => x.Id == item.TaxschemeId).ToList().SingleOrDefault();
            }
    }

Answer 1

您可以从循环中提取vendors、paymentTerms 和allTaxSchemes 作为字典，而不是在每次迭代中查询：

private void accessVendorGridData() {
  var vendors = _vendorservice
    .GetAllVendorAdd()  
    .GroupBy(item => item.Id)
    .ToDictionary(chunk => chunk.Key, chunk => chunk.ToList());

  var paymentTerms = _vendorservice
    .GetAllPaymentTerms()
    .GroupBy(item => item.Id)
    .ToDictionary(chunk => chunk.Key, chunk => chunk.SingleOrDefault());

  var allTaxSchemes = _vendorservice
    .GetAllTaxScheme()
    .GroupBy(item => item.Id)
    .ToDictionary(chunk => chunk.Key, chunk => chunk.SingleOrDefault());

  foreach (var item in getAllVendorList) {
    var Addr = vendors.TryGetValue(item.Id, out var addrs) 
       ? addrs 
       : new List<Vendor>(); //TODO: put the right type instead of Vendor

    var paymentTerm = paymentTerms.TryGetValue(item.PaymentTermId, out var term) 
       ? term 
       : null;

    var taxscheme = allTaxSchemes.TryGetValue(item.PaymentTermId, out var scheme) 
       ? scheme 
       : null;
  } 
}

您当前的代码有

O(|getAllVendorList| * (|vendors| + |paymentTerms| + |allTaxSchemes|))

时间复杂度，这个有

O(|getAllVendorList| + |vendors| + |paymentTerms| + |allTaxSchemes|)

但是，如果_vendorservice.GetXXX() 是对服务、RDBMS 等的查询，并且您必须在每次迭代时调用它（因为可以更改数据），这不是解决方案