Contains运算符检查集合中是否存在指定的元素，并返回布尔值。
Contains()扩展方法有以下两个重载。第一个重载方法需要一个值来检入集合，第二个重载方法需要使用IEqualityComparer类型的附加参数来进行自定义相等性比较。
Contains() 重载：
public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value); public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource value, IEqualityComparer<TSource> comparer);
如上所述，Contains()扩展方法需要一个值作为输入参数进行检查。值的类型必须与泛型集合的类型相同。下面的示例包含检查集合中是否存在10。请注意，int是泛型集合的一种类型。
IList<int> intList = new List<int>() { 1, 2, 3, 4, 5 };
bool result = intList.Contains(10); // 返回 falseDim intList As IList(Of Integer) = New List(Of Integer) From {1, 2, 3, 4, 5}
Dim result = intList.Contains(10) '返回 false上面的示例适用于原始数据类型。但是，它不适用于自定义类。看以下示例：
IList<Student> studentList = new List<Student>() {
new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
new Student() { StudentID = 2, StudentName = "Steve", Age = 15 } ,
new Student() { StudentID = 3, StudentName = "Bill", Age = 25 } ,
new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 }
};
Student std = new Student(){ StudentID =3, StudentName = "Bill"};
bool result = studentList.Contains(std); //returns false正如您在上面的示例中看到的，Contains返回false，即使studentList中存在“Bill”。这是因为Contains扩展方法只比较对象的引用，而不比较对象的实际值。所以要比较student对象的值，需要通过实现IEqualityComparer接口创建一个类，该接口比较两个student对象的值并返回boolean。
以下是StudentComparer类，它实现IEqualityComparer<Student>接口来比较两个Students对象的值：
class StudentComparer : IEqualityComparer<Student>
{
public bool Equals(Student x, Student y)
{
if (x.StudentID == y.StudentID &&
x.StudentName.ToLower() == y.StudentName.ToLower())
return true;
return false;
}
public int GetHashCode(Student obj)
{
return obj.GetHashCode();
}
}现在，你可以在Contains扩展方法的第二个重载方法中使用上面的StudentComparer类，该方法接受第二个参数为IEqualityComparer类型，如下所示:
示例: c # 中 Contains 与 Comparer 类
IList<Student> studentList = new List<Student>() {
new Student() { StudentID = 1, StudentName = "John", Age = 18 } ,
new Student() { StudentID = 2, StudentName = "Steve", Age = 15 } ,
new Student() { StudentID = 3, StudentName = "Bill", Age = 25 } ,
new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } ,
new Student() { StudentID = 5, StudentName = "Ron" , Age = 19 }
};
Student std = new Student(){ StudentID =3, StudentName = "Bill"};
bool result = studentList.Contains(std, new StudentComparer()); //returns true因此，必须使用Comparer类才能从自定义类的Contains扩展方法中获得正确的结果。
下面是 VB.Net 中的一个类似的实例:
public class Student
{
public int StudentID { get; set; }
public string StudentName { get; set; }
public int Age { get; set; }
}
Public Class StudentComparer Implements IEqualityComparer(Of Student)
Public Function Equals1(x As Student, y As Student) As Boolean Implements IEqualityComparer(Of Student).Equals
If (x.StudentID = y.StudentID And x.StudentName.ToLower() = y.StudentName.ToLower()) Then
Return True
End If
Return False
End Function
Public Function GetHashCode1(obj As Student) As Integer Implements IEqualityComparer(Of Student).GetHashCode
Return obj.GetHashCode()
End Function
End Class
Sub Main
Dim studentList = New List(Of Student) From {
New Student() With {.StudentID = 1, .StudentName = "John", .Age = 18},
New Student() With {.StudentID = 2, .StudentName = "Steve", .Age = 15},
New Student() With {.StudentID = 3, .StudentName = "Bill", .Age = 25},
New Student() With {.StudentID = 4, .StudentName = "Ram", .Age = 20},
New Student() With {.StudentID = 5, .StudentName = "Ron", .Age = 19}
}
Dim std As New Student With {.StudentID = 3, .StudentName = "Bill"}
Dim result = studentList.Contains(std, New StudentComparer()) ' returns true
End Sub在C#或VB.Net中，不支持限定符运算符。
All, Any & Contains是LINQ中的限定运算符。
All检查序列中的所有元素是否满足指定条件。
检查序列中是否有任何元素满足指定条件
Contains 操作符检查指定的元素是否存在于集合中。
使用派生 IEqualityOperator 和 Contains 的自定义类检查集合中的对象。
在C#或VB.Net的查询语法中不支持All, Any & Contains。
下一步了解聚合运算符。