Queryable.Select(Of TSource, TResult) Method (IQueryable(Of TSource), Expression(Of Func(Of TSource, 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.
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, 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, 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, TResult))) method generates a MethodCallExpression that represents calling Select(Of TSource, TResult)(IQueryable(Of TSource), Expression(Of Func(Of TSource, 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, TResult))) depends 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, TResult))) to project over a sequence of values.
Dim range As New List(Of Integer)(New Integer() {1, 2, 3, 4, 5, 6, 7, 8, 9, 10})
' Project the square of each int value.
Dim squares As IEnumerable(Of Integer) = _
range.AsQueryable().Select(Function(x) x * x)
Dim output As New System.Text.StringBuilder
For Each num As Integer In squares
output.AppendLine(num)
Next
' Display the output.
outputBlock.Text &= output.ToString() & vbCrLf
' This code produces the following output:
' 1
' 4
' 9
' 16
' 25
' 36
' 49
' 64
' 81
' 100