Queryable.Select
Method
Definition
Overloads
| Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) |
Projects each element of a sequence into a new form by incorporating the element's index. |
| Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) |
Projects each element of a sequence into a new form. |
Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>)
Projects each element of a sequence into a new form by incorporating the element's index.
public static System.Linq.IQueryable<TResult> Select<TSource,TResult> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,int,TResult>> selector);
- TSource
The type of the elements of source.
- TResult
The type of the value returned by the function represented by selector.
- source
- IQueryable<TSource>
A sequence of values to project.
- selector
- Expression<Func<TSource,Int32,TResult>>
A projection function to apply to each element.
An IQueryable<T> whose elements are the result of invoking a projection function on each element of source.
source or selector is null.
Examples
The following code example demonstrates how to use Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) to project over a sequence of values and use the index of each element in the projected form.
string[] fruits = { "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.
var query =
fruits.AsQueryable()
.Select((fruit, index) =>
new { index, str = fruit.Substring(0, index) });
foreach (var obj in query)
Console.WriteLine("{0}", obj);
/*
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 }
*/
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.
MsgBox(output.ToString())
' 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 }
Remarks
This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.
The Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,Int32,TResult>>) method generates a MethodCallExpression that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<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<TSource,TResult>(IQueryable<TSource>, Expression<Func<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.
Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>)
Projects each element of a sequence into a new form.
public static System.Linq.IQueryable<TResult> Select<TSource,TResult> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,TResult>> selector);
- TSource
The type of the elements of source.
- TResult
The type of the value returned by the function represented by selector.
- source
- IQueryable<TSource>
A sequence of values to project.
- selector
- Expression<Func<TSource,TResult>>
A projection function to apply to each element.
An IQueryable<T> whose elements are the result of invoking a projection function on each element of source.
source or selector is null.
Examples
The following code example demonstrates how to use Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) to project over a sequence of values.
List<int> range =
new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
// Project the square of each int value.
IEnumerable<int> squares =
range.AsQueryable().Select(x => x * x);
foreach (int num in squares)
Console.WriteLine(num);
/*
This code produces the following output:
1
4
9
16
25
36
49
64
81
100
*/
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.
MsgBox(output.ToString())
' This code produces the following output:
' 1
' 4
' 9
' 16
' 25
' 36
' 49
' 64
' 81
' 100
Remarks
This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.
The Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<TSource,TResult>>) method generates a MethodCallExpression that represents calling Select<TSource,TResult>(IQueryable<TSource>, Expression<Func<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<TSource,TResult>(IQueryable<TSource>, Expression<Func<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.