Queryable.Select(Of TSource, TResult) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, TResult)))
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Projects each element of a sequence into a new form by incorporating the element's index.
Assembly: System.Core (in System.Core.dll)
'Declaration <ExtensionAttribute> _ Public Shared Function Select(Of TSource, TResult) ( _ source As IQueryable(Of TSource), _ selector As Expression(Of Func(Of TSource, Integer, TResult)) _ ) As IQueryable(Of TResult)
Type Parameters
- TSource
The type of the elements of source.
- TResult
The type of the value returned by the function represented by selector.
Parameters
- source
- Type: System.Linq.IQueryable(Of TSource)
A sequence of values to project.
- selector
- Type: System.Linq.Expressions.Expression(Of Func(Of TSource, Int32, TResult))
A projection function to apply to each element.
Return Value
Type: System.Linq.IQueryable(Of TResult)An IQueryable(Of T) whose elements are the result of invoking a projection function on each element of source.
Usage Note
In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(Of TSource). When you use instance method syntax to call this method, omit the first parameter.| Exception | Condition |
|---|---|
| ArgumentNullException | source or selector is Nothing. |
This method has at least one parameter of type Expression(Of TDelegate) whose type argument is one of the Func(Of T, TResult) types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression(Of TDelegate).
The Select(Of TSource, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, TResult))) method generates a MethodCallExpression that represents calling Select(Of TSource, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, TResult))) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.
The query behavior that occurs as a result of executing an expression tree that represents calling Select(Of TSource, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, TResult))) depend on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into a different form.
The following code example demonstrates how to use Select(Of TSource, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, Int32, TResult))) to project over a sequence of values and use the index of each element in the projected form.
Dim fruits() As String = {"apple", "banana", "mango", "orange", _
"passionfruit", "grape"}
' Project an anonymous type that contains the
' index of the string in the source array, and
' a string that contains the same number of characters
' as the string's index in the source array.
Dim query = _
fruits.AsQueryable() _
.Select(Function(fruit, index) New With {index, .str = fruit.Substring(0, index)})
Dim output As New System.Text.StringBuilder
For Each obj In query
output.AppendLine(obj.ToString())
Next
' Display the output.
outputBlock.Text &= output.ToString() & vbCrLf
' This code produces the following output:
' { index = 0, str = }
' { index = 1, str = b }
' { index = 2, str = ma }
' { index = 3, str = ora }
' { index = 4, str = pass }
' { index = 5, str = grape }