Enumerable.Except Method (System.Linq) (original) (raw)
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Produces the set difference of two sequences by using the default equality comparer to compare values.
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generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
static System::Collections::Generic::IEnumerable<TSource> ^ Except(System::Collections::Generic::IEnumerable<TSource> ^ first, System::Collections::Generic::IEnumerable<TSource> ^ second);public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second);static member Except : seq<'Source> * seq<'Source> -> seq<'Source><Extension()>
Public Function Except(Of TSource) (first As IEnumerable(Of TSource), second As IEnumerable(Of TSource)) As IEnumerable(Of TSource)Type Parameters
TSource
The type of the elements of the input sequences.
Parameters
second
An IEnumerable whose elements that also occur in the first sequence will cause those elements to be removed from the returned sequence.
Returns
A sequence that contains the set difference of the elements of two sequences.
Exceptions
Examples
The following code example demonstrates how to use the Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method to compare two sequences of numbers and return elements that appear only in the first sequence.
double[] numbers1 = { 2.0, 2.0, 2.1, 2.2, 2.3, 2.3, 2.4, 2.5 };
double[] numbers2 = { 2.2 };
IEnumerable<double> onlyInFirstSet = numbers1.Except(numbers2);
foreach (double number in onlyInFirstSet)
Console.WriteLine(number);
/*
This code produces the following output:
2
2.1
2.3
2.4
2.5
*/
' Create two arrays of doubles.
Dim numbers1() As Double = {2.0, 2.0, 2.1, 2.2, 2.3, 2.3, 2.4, 2.5}
Dim numbers2() As Double = {2.2}
' Select the elements from the first array that are not
' in the second array.
Dim onlyInFirstSet As IEnumerable(Of Double) = numbers1.Except(numbers2)
Dim output As New System.Text.StringBuilder
For Each number As Double In onlyInFirstSet
output.AppendLine(number)
Next
' Display the output.
Console.WriteLine(output.ToString())
' This code produces the following output:
'
' 2
' 2.1
' 2.3
' 2.4
' 2.5
If you want to compare sequences of objects of some custom data type, you have to implement the IEquatable generic interface in a helper class. The following code example shows how to implement this interface in a custom data type and override the GetHashCode and Equals methods.
public class ProductA : IEquatable<ProductA>
{
public string Name { get; set; }
public int Code { get; set; }
public bool Equals(ProductA other)
{
if (other is null)
return false;
return this.Name == other.Name && this.Code == other.Code;
}
public override bool Equals(object obj) => Equals(obj as ProductA);
public override int GetHashCode() => (Name, Code).GetHashCode();
}
Public Class ProductA
Inherits IEquatable(Of ProductA)
Public Property Name As String
Public Property Code As Integer
Public Function Equals(ByVal other As ProductA) As Boolean
If other Is Nothing Then Return False
Return Me.Name = other.Name AndAlso Me.Code = other.Code
End Function
Public Overrides Function Equals(ByVal obj As Object) As Boolean
Return Equals(TryCast(obj, ProductA))
End Function
Public Overrides Function GetHashCode() As Integer
Return (Name, Code).GetHashCode()
End Function
End Class
After you implement this interface, you can use sequences of ProductA objects in the Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) method, as shown in the following example:
ProductA[] fruits1 = { new ProductA { Name = "apple", Code = 9 },
new ProductA { Name = "orange", Code = 4 },
new ProductA { Name = "lemon", Code = 12 } };
ProductA[] fruits2 = { new ProductA { Name = "apple", Code = 9 } };
// Get all the elements from the first array
// except for the elements from the second array.
IEnumerable<ProductA> except =
fruits1.Except(fruits2);
foreach (var product in except)
Console.WriteLine(product.Name + " " + product.Code);
/*
This code produces the following output:
orange 4
lemon 12
*/
Dim fruits1() As Product =
{New Product With {.Name = "apple", .Code = 9},
New Product With {.Name = "orange", .Code = 4},
New Product With {.Name = "lemon", .Code = 12}}
Dim fruits2() As Product =
{New Product With {.Name = "apple", .Code = 9}}
' Get all the elements from the first array
' except for the elements from the second array.
Dim except = fruits1.Except(fruits2)
For Each product In except
Console.WriteLine(product.Name & " " & product.Code)
Next
' This code produces the following output:
'
' apple 9
' orange 4
' lemon 12
Remarks
This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in C# or For Each in Visual Basic.
The default equality comparer, Default, is used to compare values of the types. To compare a custom data type, you need to override the Equals and the GetHashCode methods, and optionally implement the IEquatable generic interface in the custom type. For more information, see the Default property.