问题
How would you implement paging in a LINQ query? Actually for the time being, I would be satisfied if the sql TOP function could be imitated. However, I am sure that the need for full paging support comes up sooner later anyway.
var queryResult = from o in objects
where ...
select new
{
A = o.a,
B = o.b
}
????????? TOP 10????????
回答1:
You're looking for the Skip and Take extension methods. Skip moves past the first N elements in the result, returning the remainder; Take returns the first N elements in the result, dropping any remaining elements.
See MSDN for more information on how to use these methods: http://msdn.microsoft.com/en-us/library/bb386988.aspx
For example:
int numberOfObjectsPerPage = 10;
var queryResultPage = queryResult
.Skip(numberOfObjectsPerPage * pageNumber)
.Take(numberOfObjectsPerPage);
回答2:
Using Skip and Take is definitely the way to go. If I were implementing this, I would probably write my own extension method to handle paging (to make the code more readable). The implementation can of course use Skip and Take:
static class PagingUtils {
public static IEnumerable<T> Page<T>(this IEnumerable<T> en, int pageSize, int page) {
return en.Skip(page * pageSize).Take(pageSize);
}
public static IQueryable<T> Page<T>(this IQueryable<T> en, int pageSize, int page) {
return en.Skip(page * pageSize).Take(pageSize);
}
}
The class defines two extension methods - one for IEnumerable and one for IQueryable, which means that you can use it with both LINQ to Objects and LINQ to SQL (when writing database query, the compiler will pick the IQueryable version).
Depending on your paging requirements, you could also add some additional behavior (for example to handle negative pageSize or page value). Here is an example how you would use this extension method in your query:
var q = (from p in products
where p.Show == true
select new { p.Name }).Page(10, pageIndex);
回答3:
Here is my performant approach to paging when using LINQ to objects:
public static IEnumerable<IEnumerable<T>> Page<T>(this IEnumerable<T> source, int pageSize)
{
Contract.Requires(source != null);
Contract.Requires(pageSize > 0);
Contract.Ensures(Contract.Result<IEnumerable<IEnumerable<T>>>() != null);
using (var enumerator = source.GetEnumerator())
{
while (enumerator.MoveNext())
{
var currentPage = new List<T>(pageSize)
{
enumerator.Current
};
while (currentPage.Count < pageSize && enumerator.MoveNext())
{
currentPage.Add(enumerator.Current);
}
yield return new ReadOnlyCollection<T>(currentPage);
}
}
}
This can then be used like so:
var items = Enumerable.Range(0, 12);
foreach(var page in items.Page(3))
{
// Do something with each page
foreach(var item in page)
{
// Do something with the item in the current page
}
}
None of this rubbish Skip and Take which will be highly inefficient if you are interested in multiple pages.
回答4:
( for o in objects
where ...
select new
{
A=o.a,
B=o.b
})
.Skip((page-1)*pageSize)
.Take(pageSize)
回答5:
Don't know if this will help anyone, but I found it useful for my purposes:
private static IEnumerable<T> PagedIterator<T>(IEnumerable<T> objectList, int PageSize)
{
var page = 0;
var recordCount = objectList.Count();
var pageCount = (int)((recordCount + PageSize)/PageSize);
if (recordCount < 1)
{
yield break;
}
while (page < pageCount)
{
var pageData = objectList.Skip(PageSize*page).Take(PageSize).ToList();
foreach (var rd in pageData)
{
yield return rd;
}
page++;
}
}
To use this you would have some linq query, and pass the result along with the page size into a foreach loop:
var results = from a in dbContext.Authors
where a.PublishDate > someDate
orderby a.Publisher
select a;
foreach(var author in PagedIterator(results, 100))
{
// Do Stuff
}
So this will iterate over each author fetching 100 authors at a time.
回答6:
EDIT - Removed Skip(0) as it's not necessary
var queryResult = (from o in objects where ...
select new
{
A = o.a,
B = o.b
}
).Take(10);
回答7:
var pages = items.Select((item, index) => new { item, Page = index / batchSize }).GroupBy(g => g.Page);
Batchsize will obviously be an integer. This takes advantage of the fact that integers simply drop decimal places.
I'm half joking with this response, but it will do what you want it to, and because it's deferred, you won't incur a large performance penalty if you do
pages.First(p => p.Key == thePage)
This solution is not for LinqToEntities, I don't even know if it could turn this into a good query.
回答8:
Similar to Lukazoid's answer I've created an extension for IQueryable.
public static IEnumerable<IEnumerable<T>> PageIterator<T>(this IQueryable<T> source, int pageSize)
{
Contract.Requires(source != null);
Contract.Requires(pageSize > 0);
Contract.Ensures(Contract.Result<IEnumerable<IQueryable<T>>>() != null);
using (var enumerator = source.GetEnumerator())
{
while (enumerator.MoveNext())
{
var currentPage = new List<T>(pageSize)
{
enumerator.Current
};
while (currentPage.Count < pageSize && enumerator.MoveNext())
{
currentPage.Add(enumerator.Current);
}
yield return new ReadOnlyCollection<T>(currentPage);
}
}
}
It is useful if Skip or Take are not supported.
回答9:
I use this extension method:
public static IQueryable<T> Page<T, TResult>(this IQueryable<T> obj, int page, int pageSize, System.Linq.Expressions.Expression<Func<T, TResult>> keySelector, bool asc, out int rowsCount)
{
rowsCount = obj.Count();
int innerRows = rowsCount - (page * pageSize);
if (innerRows < 0)
{
innerRows = 0;
}
if (asc)
return obj.OrderByDescending(keySelector).Take(innerRows).OrderBy(keySelector).Take(pageSize).AsQueryable();
else
return obj.OrderBy(keySelector).Take(innerRows).OrderByDescending(keySelector).Take(pageSize).AsQueryable();
}
public IEnumerable<Data> GetAll(int RowIndex, int PageSize, string SortExpression)
{
int totalRows;
int pageIndex = RowIndex / PageSize;
List<Data> data= new List<Data>();
IEnumerable<Data> dataPage;
bool asc = !SortExpression.Contains("DESC");
switch (SortExpression.Split(' ')[0])
{
case "ColumnName":
dataPage = DataContext.Data.Page(pageIndex, PageSize, p => p.ColumnName, asc, out totalRows);
break;
default:
dataPage = DataContext.vwClientDetails1s.Page(pageIndex, PageSize, p => p.IdColumn, asc, out totalRows);
break;
}
foreach (var d in dataPage)
{
clients.Add(d);
}
return data;
}
public int CountAll()
{
return DataContext.Data.Count();
}
回答10:
public LightDataTable PagerSelection(int pageNumber, int setsPerPage, Func<LightDataRow, bool> prection = null)
{
this.setsPerPage = setsPerPage;
this.pageNumber = pageNumber > 0 ? pageNumber - 1 : pageNumber;
if (!ValidatePagerByPageNumber(pageNumber))
return this;
var rowList = rows.Cast<LightDataRow>();
if (prection != null)
rowList = rows.Where(prection).ToList();
if (!rowList.Any())
return new LightDataTable() { TablePrimaryKey = this.tablePrimaryKey };
//if (rowList.Count() < (pageNumber * setsPerPage))
// return new LightDataTable(new LightDataRowCollection(rowList)) { TablePrimaryKey = this.tablePrimaryKey };
return new LightDataTable(new LightDataRowCollection(rowList.Skip(this.pageNumber * setsPerPage).Take(setsPerPage).ToList())) { TablePrimaryKey = this.tablePrimaryKey };
}
this is what i did. Normaly you start at 1 but in IList you start with 0. so if you have 152 rows that mean you have 8 paging but in IList you only have 7. hop this can make thing clear for you
回答11:
var results = (medicineInfo.OrderBy(x=>x.id)
.Skip((pages -1) * 2)
.Take(2));回答12:
There are two main options:
.NET >= 4.0 Dynamic LINQ:
- Add using System.Linq.Dynamic; at the top.
- Use:
var people = people.AsQueryable().OrderBy("Make ASC, Year DESC").ToList();
You can also get it by NuGet.
.NET < 4.0 Extension Methods:
private static readonly Hashtable accessors = new Hashtable();
private static readonly Hashtable callSites = new Hashtable();
private static CallSite<Func<CallSite, object, object>> GetCallSiteLocked(string name) {
var callSite = (CallSite<Func<CallSite, object, object>>)callSites[name];
if(callSite == null)
{
callSites[name] = callSite = CallSite<Func<CallSite, object, object>>.Create(
Binder.GetMember(CSharpBinderFlags.None, name, typeof(AccessorCache),
new CSharpArgumentInfo[] { CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null) }));
}
return callSite;
}
internal static Func<dynamic,object> GetAccessor(string name)
{
Func<dynamic, object> accessor = (Func<dynamic, object>)accessors[name];
if (accessor == null)
{
lock (accessors )
{
accessor = (Func<dynamic, object>)accessors[name];
if (accessor == null)
{
if(name.IndexOf('.') >= 0) {
string[] props = name.Split('.');
CallSite<Func<CallSite, object, object>>[] arr = Array.ConvertAll(props, GetCallSiteLocked);
accessor = target =>
{
object val = (object)target;
for (int i = 0; i < arr.Length; i++)
{
var cs = arr[i];
val = cs.Target(cs, val);
}
return val;
};
} else {
var callSite = GetCallSiteLocked(name);
accessor = target =>
{
return callSite.Target(callSite, (object)target);
};
}
accessors[name] = accessor;
}
}
}
return accessor;
}
public static IOrderedEnumerable<dynamic> OrderBy(this IEnumerable<dynamic> source, string property)
{
return Enumerable.OrderBy<dynamic, object>(source, AccessorCache.GetAccessor(property), Comparer<object>.Default);
}
public static IOrderedEnumerable<dynamic> OrderByDescending(this IEnumerable<dynamic> source, string property)
{
return Enumerable.OrderByDescending<dynamic, object>(source, AccessorCache.GetAccessor(property), Comparer<object>.Default);
}
public static IOrderedEnumerable<dynamic> ThenBy(this IOrderedEnumerable<dynamic> source, string property)
{
return Enumerable.ThenBy<dynamic, object>(source, AccessorCache.GetAccessor(property), Comparer<object>.Default);
}
public static IOrderedEnumerable<dynamic> ThenByDescending(this IOrderedEnumerable<dynamic> source, string property)
{
return Enumerable.ThenByDescending<dynamic, object>(source, AccessorCache.GetAccessor(property), Comparer<object>.Default);
}
来源：https://stackoverflow.com/questions/2380413/paging-with-linq-for-objects