Enumerable.Except Method

Definition

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.

C#
public static System.Collections.Generic.IEnumerable<TSource> Except<TSource>(this System.Collections.Generic.IEnumerable<TSource> first, System.Collections.Generic.IEnumerable<TSource> second);

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

IEnumerable<TSource>

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.

C#
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
*/

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.

C#
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();
}

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:

C#
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
*/

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

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 2.0, 2.1
UWP 10.0

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.

C#
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);
C#
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);

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

IEnumerable<TSource>

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.

C#
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;
    }
}

After you implement this comparer, you can use sequences of Product objects in the Except method, as shown in the following example:

C#
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
*/

Remarks

If comparer is null, the default equality comparer, Default, is used to compare values.

See also

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 2.0, 2.1
UWP 10.0