Question

When I type a LINQ query, intellisense in Visual Studio 2010 shows that the where clause expects < condition > . As in

From anItem in listOfItems where ... Intellisense shows it expects a < condition >

What does this < condition > refer to? Is it a type? A predicate? A boolean expression? For instance, if you type…

listOfItems.RemoveAll(

…intellisense shows that it is expecting a predicate. What is the analog for a < condition > in linq? (I get how to use a where condition, I want to know what it means). In the long run, I want to start writing complex where clauses and want to understand what LINQ is actually doing/expecting so I can understand what is going on.

I’ve googled around a little but nothing I find on msdn seems to fit:

• http://msdn.microsoft.com/en-us/library/ms606907.aspx (seems wrong)

• http://msdn.microsoft.com/en-us/library/7szfhaft.aspx (seems wrong)

Answer 1

It pretty much just asks for a Boolean expression that can be evaluated for every anItem in your query expression.

The first paragraph of where clause (C# Reference) sums it up nicely:

The where clause is used in a query expression to specify which elements from the data source will be returned in the query expression. It applies a Boolean condition (predicate) to each source element (referenced by the range variable) and returns those for which the specified condition is true. A single query expression may contain multiple where clauses and a single clause may contain multiple predicate subexpressions.

And this is what Where Clause (Visual Basic) says:

condition

Required. An expression that determines whether the values for the current item in the collection are included in the output collection. The expression must evaluate to a Boolean value or the equivalent of a Boolean value. If the condition evaluates to True, the element is included in the query result; otherwise, the element is excluded from the query result.