Enumerable.Except Method
Definition
Important
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Produces the set difference of two sequences.
Overloads
Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>) |
Produces the set difference of two sequences by using the default equality comparer to compare values. |
Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>) |
Produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values. |
Remarks
The set difference of two sets is defined as the members of the first set that don't appear in the second set.
This method returns those elements in first
that don't appear in second
. It doesn't return those elements in second
that don't appear in first
.
Only unique elements are returned.
Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>)
- Source:
- Except.cs
- Source:
- Except.cs
- Source:
- Except.cs
Produces the set difference of two sequences by using the default equality comparer to compare values.
public:
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
- first
- IEnumerable<TSource>
An IEnumerable<T> whose elements that are not also in second
will be returned.
- second
- IEnumerable<TSource>
An IEnumerable<T> 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
first
or second
is null
.
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<T> 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<T> generic interface in the custom type. For more information, see the Default property.
Applies to
Except<TSource>(IEnumerable<TSource>, IEnumerable<TSource>, IEqualityComparer<TSource>)
- Source:
- Except.cs
- Source:
- Except.cs
- Source:
- Except.cs
Produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values.
public:
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, System::Collections::Generic::IEqualityComparer<TSource> ^ comparer);
public static System.Collections.Generic.IEnumerable<TSource> Except<TSource> (this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource> comparer);
public static System.Collections.Generic.IEnumerable<TSource> Except<TSource> (this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second, System.Collections.Generic.IEqualityComparer<TSource>? comparer);
static member Except : seq<'Source> * seq<'Source> * System.Collections.Generic.IEqualityComparer<'Source> -> seq<'Source>
<Extension()>
Public Function Except(Of TSource) (first As IEnumerable(Of TSource), second As IEnumerable(Of TSource), comparer As IEqualityComparer(Of TSource)) As IEnumerable(Of TSource)
Type Parameters
- TSource
The type of the elements of the input sequences.
Parameters
- first
- IEnumerable<TSource>
An IEnumerable<T> whose elements that are not also in second
will be returned.
- second
- IEnumerable<TSource>
An IEnumerable<T> whose elements that also occur in the first sequence will cause those elements to be removed from the returned sequence.
- comparer
- IEqualityComparer<TSource>
An IEqualityComparer<T> to compare values.
Returns
A sequence that contains the set difference of the elements of two sequences.
Exceptions
first
or second
is null
.
Examples
If you want to compare sequences of objects of some custom data type, you have to implement the IEqualityComparer<T> generic interface in a helper class. The following code example shows how to implement this interface in a custom data type and provide GetHashCode and Equals methods. The following example shows how to implement an equality comparer that can be used in the Except method.
public class Product
{
public string Name { get; set; }
public int Code { get; set; }
}
// Custom comparer for the Product class
class ProductComparer : IEqualityComparer<Product>
{
// Products are equal if their names and product numbers are equal.
public bool Equals(Product x, Product y)
{
//Check whether the compared objects reference the same data.
if (Object.ReferenceEquals(x, y)) return true;
//Check whether any of the compared objects is null.
if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
return false;
//Check whether the products' properties are equal.
return x.Code == y.Code && x.Name == y.Name;
}
// If Equals() returns true for a pair of objects
// then GetHashCode() must return the same value for these objects.
public int GetHashCode(Product product)
{
//Check whether the object is null
if (Object.ReferenceEquals(product, null)) return 0;
//Get hash code for the Name field if it is not null.
int hashProductName = product.Name == null ? 0 : product.Name.GetHashCode();
//Get hash code for the Code field.
int hashProductCode = product.Code.GetHashCode();
//Calculate the hash code for the product.
return hashProductName ^ hashProductCode;
}
}
Public Class Product
Public Property Name As String
Public Property Code As Integer
End Class
' Custom comparer for the Product class
Public Class ProductComparer
Implements IEqualityComparer(Of Product)
Public Function Equals1(
ByVal x As Product,
ByVal y As Product
) As Boolean Implements IEqualityComparer(Of Product).Equals
' Check whether the compared objects reference the same data.
If x Is y Then Return True
'Check whether any of the compared objects is null.
If x Is Nothing OrElse y Is Nothing Then Return False
' Check whether the products' properties are equal.
Return (x.Code = y.Code) AndAlso (x.Name = y.Name)
End Function
Public Function GetHashCode1(
ByVal product As Product
) As Integer Implements IEqualityComparer(Of Product).GetHashCode
' Check whether the object is null.
If product Is Nothing Then Return 0
' Get hash code for the Name field if it is not null.
Dim hashProductName =
If(product.Name Is Nothing, 0, product.Name.GetHashCode())
' Get hash code for the Code field.
Dim hashProductCode = product.Code.GetHashCode()
' Calculate the hash code for the product.
Return hashProductName Xor hashProductCode
End Function
End Class
After you implement this comparer, you can use sequences of Product
objects in the Except method, as shown in the following example:
Product[] fruits1 = { new Product { Name = "apple", Code = 9 },
new Product { Name = "orange", Code = 4 },
new Product { Name = "lemon", Code = 12 } };
Product[] fruits2 = { new Product { Name = "apple", Code = 9 } };
// Get all the elements from the first array
// except for the elements from the second array.
IEnumerable<Product> except =
fruits1.Except(fruits2, new ProductComparer());
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, New ProductComparer())
For Each product In except
Console.WriteLine(product.Name & " " & product.Code)
Next
' This code produces the following output:
'
' orange 4
' lemon 12
Remarks
If comparer
is null
, the default equality comparer, Default, is used to compare values.
See also
Applies to
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