C# LINQ Where in collection 子句
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LINQ Where in collection clause
提问by Jan W.
I've been looking on google but not finding anything that does the trick for me.
我一直在寻找谷歌，但没有找到任何对我有用的东西。
as you know SQL has a "where x in (1,2,3)" clause which allows you to check against multiple values. I'm using linq but I can't seem to find a piece of syntax that does the same as the above statement.
如您所知，SQL 有一个“where x in (1,2,3)”子句，它允许您检查多个值。我正在使用 linq，但似乎找不到与上述语句相同的语法。
I have a collection of category id's (List) against which I would like to check
我有一个类别 ID（列表）的集合，我想根据这些集合进行检查
I found something that uses the .contains method but it doesn't even build.
我发现了一些使用 .contains 方法的东西，但它甚至没有构建。
采纳答案by CMS
You have to use the Contains method on your id list:
您必须在您的 id 列表中使用 Contains 方法：
var query = from t in db.Table
where idList.Contains(t.Id)
select t;
回答by Darin Dimitrov
回答by David Hall
The syntax is below:
语法如下：
IEnumerable<int> categoryIds = yourListOfIds;
var categories = _dataContext.Categories.Where(c => categoryIds.Contains(c.CategoryId));
The key thing to note is that you do the contains on your list of ids - not on the object you would apply the in to if you were writing sql.
需要注意的关键是，您在 id 列表中执行 contains - 如果您正在编写 sql，则不在将应用 in 的对象上。
回答by Fragment
Here is my realization of WhereIn() Method, to filter IQueryable collection by a set of selected entities:
这是我对 WhereIn() 方法的实现，通过一组选定的实体过滤 IQueryable 集合：
public static IQueryable<T> WhereIn<T,TProp>(this IQueryable<T> source, Expression<Func<T,TProp>> memberExpr, IEnumerable<TProp> values) where T : class
{
Expression predicate = null;
ParameterExpression param = Expression.Parameter(typeof(T), "t");
bool IsFirst = true;
MemberExpression me = (MemberExpression) memberExpr.Body;
foreach (TProp val in values)
{
ConstantExpression ce = Expression.Constant(val);
Expression comparison = Expression.Equal(me, ce);
if (IsFirst)
{
predicate = comparison;
IsFirst = false;
}
else
{
predicate = Expression.Or(predicate, comparison);
}
}
return predicate != null
? source.Where(Expression.Lambda<Func<T, bool>>(predicate, param)).AsQueryable<T>()
: source;
}
And calling of this method looks like:
这个方法的调用看起来像：
IQueryable<Product> q = context.Products.ToList();
var SelectedProducts = new List<Product>
{
new Product{Id=23},
new Product{Id=56}
};
...
// Collecting set of product id's
var selectedProductsIds = SelectedProducts.Select(p => p.Id).ToList();
// Filtering products
q = q.WhereIn(c => c.Product.Id, selectedProductsIds);
